EP4630842A1

EP4630842A1 - Cw coherent lidar system for detecting and imaging aerial vehicles

Info

Publication number: EP4630842A1
Application number: EP23821243.5A
Authority: EP
Inventors: Peter John Rodrigo; Christian Pedersen
Original assignee: Danmarks Tekniske Universitet
Current assignee: Danmarks Tekniske Universitet
Priority date: 2022-12-09
Filing date: 2023-12-07
Publication date: 2025-10-15
Also published as: WO2024121314A1

Abstract

One aspect of the present disclosure relates to a system for detecting aerial vehicles, the system comprising a continuous-wave coherent lidar unit selectively operable in at least a detection mode and an imaging mode. The continuous-wave coherent lidar unit is configured, when operated in the detection mode: to emit and/or scan a continuous-wave laser detection beam towards a detection target region, and to receive a return laser signal from the detection target region by coherent detection. The system is configured to process the return laser signal, received by the continuous-wave coherent lidar unit when operated in the detection mode, so as to detect one or more signal features indicative of the presence of an aerial vehicle in the detection target region, and to determine one or more attributes of the said aerial vehicle in the detection target region from the detected one or more signal features. The continuous‐wave coherent lidar unit is further configured, when operated in the imaging mode to scan a focused continuous‐wave imaging laser beam across an imaging target region, and to receive respective return laser signals from respective positions within the imaging target region by coherent detection. The system is configured to process the return laser signals, received from respective positions by the continuous‐wave coherent lidar unit when operated in the detection mode, to create at least one two‐dimensional image of the imaging target region.

Description

CW COHERENT LIDAR SYSTEM FOR DETECTING AND IMAGING AERIAL VEHICLES

TECHNICAL FIELD

The present disclosure relates to the detection of aerial vehicles.

BACKGROUND

A variety of aerial vehicles exist, including manned aerial vehicles and unmanned aerial vehicles (UAVs).

Unmanned aerial systems (UAS), sometimes also called unmanned aircraft systems generally include an unmanned aerial vehicle (UAV) and, optionally, one or more associated components, such as a remote electronic controller. A UAS may also comprise a command and control data link connecting an UAV with a remote controller so that they can communicate. An unmanned aerial vehicle, commonly also known as a drone, is an aircraft without any human pilot, crew, or passengers on board.

Manned aerial vehicles generally have a pilot. Examples of manned aerial vehicles include helicopters, dual-mode vehicles that can drive and fly, airplanes, or other types of manned aircrafts, etc.

UAVs are used for many different purposes and there are many different types of UAVs. UAVs have many different shapes and sizes, including UAVs that are small enough to be man portable. Recently, palm-sized to meter-wide drones have become more and more accessible to average consumers. Such UAVs are sometimes also referred to as miniature UAVs or micro drones.

Despite their positive applications, drones, including micro drones, have also posed an increasing threat to safety and security. For example, airports or large industrial installations are commonly affected areas when it comes to drone misuse. To address this problem, considerable efforts have been made and continue to be made to develop drone countermeasure solutions.

Examples of counter-UAS (cUAS) architectures combine a plurality of air surveillance sensors given that a cUAS system often is desired to perform a number of tasks. Such tasks may include one or more of the following: detection, ranging, tracking, classification, imaging, identification, and ultimately deployment of an effective interdiction.

For example WO 2020/236238 discloses a counter drone system, which comprises a plurality of sensor systems, a counter drone, and a processor. A sensor system of the plurality of sensor systems comprises one or more sensors that are connected to a network. The counter drone is connected to the network. The processor is configured to receive an indication of a potential target from the plurality of sensor systems; generate a fused data set for the potential target; determine whether the potential target comprises the threat drone based at least in part on the fused data set; and in response to determining that the potential target comprises the threat drone, provide counter drone instructions to the counter drone. The above document mentions a variety of possible technologies that may be employed by a counter drone system, including light detection and ranging (lidar).

Indeed, various technologies for detecting drones have been suggested in the past. The article "Multi-sensor field trials for detection and tracking of multiple small unmanned aerial vehicles flying at low altitude," by M. Laurenzis et al., in Proc. SPIE 10200, 102001A, (International Society for Optics and Photonics, 2017), discloses a Time-of- Flight (ToF) pulsed lidar (Velodyne VLP-16) originally developed and commercialized for the automobile industry that was used for detection and tracking of airborne drones at a limited range of less than 100 m). Similarly, ToF lidars for cUAS were also proposed by B. H. Kim, et al. using higher power pulsed lasers in order to increase drone detection range up to 2 km, see e.g.: B. H. Kim et al., "Ladar data generation fused with virtual targets and visualization for small drone detection system," Proceedings of the Technologies for Optical Countermeasures XV, Berlin, Germany, 10-13 September 2018. SPIE 10797, 1079701 (2018); or B. H. Kim et al., "V-RBNN Based Small Drone Detection in Augmented Datasets for 3D LADAR System," Sensors 18(11), 3825 (2018).

Despite previous efforts, it remains desirable to provide a system for detecting UAVs and/or other aerial vehicles that facilitates a reliable detection of the aerial vehicles while keeping the costs for manufacturing and/or operation of the system low.

It is further desirable to provide a system for detecting UAVs and/or other aerial vehicles that facilitates reliable threat assessment, such as to reduce the number of false alarms or false positives, thereby avoiding unnecessary deployment of interdiction systems.

It is further desirable to provide a system for detecting UAVs and/or other aerial vehicles that facilitates distinction between aerial vehicles and other flying objects, such as birds, and/or distinction between different types of aerial vehicles, e.g. between different types of UAVs.

It is further desirable to provide a system for detecting UAVs and/or other aerial vehicles that provides additional information about the detected aerial vehicles, thereby facilitating an improved threat assessment.

It is further desirable to provide a system for detecting UAVs and/or other aerial vehicles that is capable of detecting miniature UAVs and/or UAVs that fly at low altitude and/or low speed. It is further desirable to provide a system having a large detection range and/or low sensitivity to noise, weather conditions and/or other variations of environmental factors.

It is further desirable to provide a system that is capable of detecting autonomous UAVs and/or other aerial vehicles that do not communicate with a remote controller.

SUMMARY

It is an object of the present disclosure to provide a system for detecting manned and/or unmanned aerial vehicles that solves one or more of the problems identified above and/or other problems of known systems, or that at least may serve as an alternative to known systems.

According to a first aspect, disclosed herein are embodiments of a system for detecting aerial vehicles, wherein the system comprises a continuous-wave coherent lidar unit and wherein the system is configured to obtain position data indicative of a detection target region in which presence of an aerial vehicle is suspected and, responsive to the obtained position data, to operate the continuous-wave coherent lidar unit in an imaging mode; wherein the continuous-wave coherent lidar unit is configured, when operated in the imaging mode: to focus a continuous-wave imaging laser beam at the detection target region and to scan the focused continuous-wave imaging laser beam across an imaging target region, the image target region including at least a portion of the detection target region, wherein the continuous-wave imaging laser beam is a narrow line-width, continuous-wave imaging laser beam; and to receive respective return laser signals from respective positions within the imaging target region by coherent detection, and wherein the system is configured to process the return laser signals, received from respective positions by the continuous-wave coherent lidar unit, to create at least one two-dimensional image of the imaging target region. Accordingly, the system is capable of obtaining high-resolution images of the target objects even at a large distance. In particular, focusing the imaging laser beam provides images having small pixel dimensions. Using narrow line-width continuous-wave imaging laser beam provides a high signal-to-noise ratio.

The laser source may provide a laser output of a suitable wavelength, e.g. between 1000 nm and 2000nm, such as between 1000 nm and 1800 nm, such as between 1400 nm and 1600 nm. The term narrow line-width continuous-wave imaging laser beam is intended to refer to a laser beam having a line width of no more than 10 MHz at a wavelength between 1000 nm and 2000 nm. The line-width of the laser output is preferably between 0.1 kHz and 10 MHz, such as between 0.1 kHz and 5 MHz, such as between 0.1 kHz and 1 MHz, thereby providing for an efficient laser source with a high signal-to-noise ratio, thereby facilitating the capture of high-resolution images of small objects at large distances.

Inn some embodiments, the system comprises:

- a laser source,

- a coherent detector, and

- an optical transceiver system configured to receive laser light from the laser source and to emit the imaging beam, the imaging beam being focused towards the imaging target region, and wherein the optical transceiver system comprises a scanner module for scanning the imaging beam across the imaging target region.

Preferably, the optical transceiver system is configured to focus the imaging beam so as to illuminate a portion of the imaging target region and to scan the focused imaging beam across the imaging target region so as to successively illuminate respective portions of the imaging target region, e.g. a raster of respective positions within the imaging target region. To this end, the optical transceiver system may comprise a scanner module for scanning at least the imaging beam across the imaging target region, thereby allowing the lidar unit to capture a raster image of the entire imaging target region, or of at least a portion of the imaging target region.

The imaging beam may be a focused beam having a beam waist at the imaging target region. For example, the imaging beam may have a beam cross section at the imaging target region defined by a radius of the imaging beam cross section at the imaging target region. In some embodiments, the radius of the imaging beam cross section may be between 1 mm and 200 mm, e.g. between 1 mm and 10 mm, or between 10 mm and 20 mm, or between 20 mm and 200 mm. For example, when the imaging beam is focused at a distance between 10 m and 1000 m or more, a radius of the imaging beam cross section between 1 mm and 20 mm, such as between 1 mm and 10 mm may allow resolution of millimeter to centimeter sized features of the target, at least when a sufficiently large aperture telescope lens is used. A radius of the imaging beam cross section between 20 mm and 200 mm may for example be used for a focus distance greater than 1000 m and/or for resolving tens of centimeter sized features of larger targets (e.g. meter sized UAVs, military drones, fixed wing drones, helicopters, planes, etc.).

The system is preferably configured to emit the continuous-wave imaging beam at a constant wavelength, in particular without sweeping the wavelength while directing the image laser beam at a position within the image target region to acquire an image pixel. Accordingly, the dwell time for each image pixel may be kept short, thereby achieving high image scan rates.

The optical transceiver module may be configured to scan the imaging beam across a traverse area corresponding to a range of azimuth and elevation angles of between 0.1' x 0.1° and 3° x 3°, such as between 0.5° x 0.5° and 2° x 2°, thus allowing scanning of a traverse area corresponding to a positional resolution of state-of-the art tracking radar systems, which thus may serve as an upstream sensor for a system disclosed herein.

In some embodiments, the optical transceiver module comprises an axially displaceable lens, in particular a lens displaceable along the optical axis of the imaging beam, thereby allowing the imaging beam to be focuses at the detection target region. The system may be configured to control axial displacement of the lens to focus the imaging beam on the detection target region. By changing the axial position of the lens, the system may change the location of the beam focus or beam waist, e.g. so as to axially adjusting the beam focus, e.g. between 100 m to 1000 m, or between 0.4 km to 4 km from said lens. The axially displaceable lens may be a lens of a telescope. The system may further be configured to control axial displacement of the lens so as to switch from the imaging mode to a detection mode, e.g. so as to provide a collimated detection beam.

Embodiments of a system having an imaging mode and a detection mode will be described in more detail below.

In some embodiments, obtaining the position data comprises receiving the position data from an upstream sensor or from operating the continuous-wave coherent lidar unit in a detection mode. Embodiments of a system that may selectively be operated in a detection mode will be discussed in more detail below. Examples of upstream sensors include a radar or a rangefinder. In particular, the position data may include an indication of a target direction or a range of target directions. Alternatively or additionally, the position data may include range information, e.g. as a target range or as an interval of target ranges, so as to allow the system to focus the imaging beam at said target range or at a target range in the obtained interval. The upstream sensor or the detection mode may thus provide a target position where a suspect aerial vehicle, e.g. a suspect UAV, is, including the target range so as to allow the system to focus the imaging beam at the provided range. Proper focusing enhances the signal-to-noise ratio and the image resolution, thereby avoiding image blurring that may otherwise occur if the raster scanned beam is not focused at an appropriate distance. It will be appreciated that the system, at least when operated in imaging mode, may not itself be able to measure a range to the target object. Accordingly, in some embodiments, the lidar unit, at least when operated in imaging mode, is merely used for light detection and imaging rather than for ranging. Nevertheless, the imaging mode is capable of providing high- resolution image information at high frame rates.

In some embodiments of the system, the continuous-wave coherent lidar unit is selectively operable in at least a detection mode and the imaging mode. The system may selectively switch or toggle between operating the continuous-wave coherent lidar unit in either detection mode or in imaging mode. Optionally, the system may be configured to selectively toggle the continuous-wave coherent lidar unit between one or more additional modes, e.g. so as to be able to selectively operate the continuous-wave coherent lidar unit in one of two different detection modes or in the imaging mode, respectively.

In particular, in some embodiments, the continuous-wave coherent lidar unit is configured, when operated in the detection mode: to emit a continuous-wave laser detection beam towards a detection target region, optionally including scanning the detection beam across at least a portion of the detection target region, and to receive a return laser signal from the detection target region by coherent detection; wherein the system is configured: to process the return laser signal, received by the continuous-wave coherent lidar unit when operated in the detection mode, so as to detect one or more signal features indicative of the presence of an aerial vehicle in the detection target region, and to determine one or more attributes of the said aerial vehicle in the detection target region from the detected one or more signal features.

Generally, according to another aspect, disclosed herein are embodiments of a system for detecting aerial vehicles, such as unmanned aerial vehicles, comprising a continuous- wave coherent lidar unit that is selectively operable in at least a detection mode and an imaging mode. In various embodiments of the system, the continuous-wave coherent lidar unit is configured, when operated in the detection mode, to emit a continuous- wave laser detection beam towards a detection target region - and to optionally scan the detection beam across at least a portion of the detection target region - and to receive a return laser signal from the detection target region by coherent detection. The system is configured to process the return laser signal, received by the continuous-wave coherent lidar unit when operated in the detection mode, so as to detect one or more signal features indicative of the presence of an aerial vehicle in the detection target region, and to determine one or more attributes of said aerial vehicle in the detection target region from the detected one or more signal features. In various embodiments of the system, the continuous-wave coherent lidar unit is further configured, when operated in the imaging mode, to scan a focused continuous-wave imaging laser beam across an imaging target region and to receive respective return laser signals from respective positions within the imaging target region by coherent detection. The system is further configured to process the return laser signals, received by the continuous- wave coherent lidar unit when operated in the imaging mode, so as to create at least one two-dimensional image of the imaging target region.

Accordingly, as the continuous-wave coherent lidar unit is selectively operable in a detection mode and an imaging mode, the unit may be used for detection/identification of an aerial vehicle in a target region as well as for creating an image of a detected aerial vehicle, thus providing a cost-efficient detection system that reliably identifies aerial vehicles, such as UAVs, and facilitates threat assessment. The use of continuous-wave coherent lidar allows for a cost efficient, relatively simple implementation of the photonic and electronic components and has the advantage of high signal-to-noise ratio. Moreover, embodiments of the system are capable of detecting and imaging aerial vehicles during daytime and nighttime.

Coherent detection provides detection of reflection intensity and velocities (or speeds) of reflecting objects.

The determination of one or more attributes of the detected aerial vehicles results in a reliable detection, including a reliable distinction between aerial vehicles and e.g. birds, and a reliable distinction between different types of aerial vehicles, such as between different types of UAVs, and/or determination of the presence or type of payload(s), thus improving the threat assessment while reducing the need for a large number of different sensors.

As the continuous-wave coherent lidar unit is also operable to obtain one or more image(s) of a detected aerial vehicle, the aerial vehicle identification and threat assessment is further improved or at least further facilitated.

In some embodiments of the system, the continuous-wave coherent lidar unit is further configured, when operated in the detection mode, to scan the continuous-wave detection laser beam across at least a portion of the detection target region and to receive respective return laser signals from respective positions within the detection target region by coherent detection. Accordingly, a larger detection target region may be covered by the detection beam.

In some embodiments, the system is configured, when operating the continuous-wave coherent lidar unit in a first one of the detection and imaging modes, and responsive to a result of the processing of the return laser signal when operating the continuous-wave coherent lidar unit in said first mode, to initiate operation of the continuous-wave coherent lidar unit in a second one of the detection and imaging modes, different from the first mode, in particular to switch operation of the continuous-wave coherent lidar unit from the first mode to the second mode. Accordingly, efficient use of the continuous-wave coherent lidar unit is made for detecting and identifying an aerial vehicle, utilizing different information obtainable from the operation of the system in different modes and supplementing each other.

In particular, in some embodiments, the system is configured to operate the continuous- wave coherent lidar unit in the detection mode and, responsive to detecting one or more signal features indicative of the presence of an aerial vehicle in the detection target region, to cause the continuous-wave coherent lidar unit to operate in the imaging mode to obtain an image of an imaging target region associated with the detection area that includes the detected aerial vehicle.

Similarly, in some embodiments, the system is additionally or alternatively configured to operate the continuous-wave coherent lidar unit in the imaging mode and, responsive to a failure to identify an aerial vehicle, such as a UAV, in the imaging target region with a desired reliability based on the obtained one or more images, cause the continuous- wave coherent lidar unit to operate in the detection mode. The desired reliability may e.g. be defined by computing a suitable confidence parameter and by comparing the confidence parameter with a predetermined threshold value. For example, the confidence parameter may be indicative of a measure of a resolution of a detected object in the obtained one or more images and/or a measure of a confidence of classification of an identified object as an aerial vehicle, e.g. as an UAV, or as a known type of aerial vehicle, e.g. as a known type of UAV. In some embodiments, the one or more signal features include a Doppler signature of at least one propeller of an aerial vehicle such as a UAV, thus providing a reliable indicator of an identified object being a propeller driven aerial vehicle, such as a UAV or even a particular type of UAV. The signal features may thus be indicative of a frequency of a periodically varying Doppler or beat signal, or another signal feature of the return laser signal. The frequency of the periodically varying Doppler signal may thus serve as a Doppler signature which is indicative of a rotating propeller of an aerial vehicle, e.g. of a UAV propeller. Doppler signatures of one or more propellers of an aerial vehicle are reliably detectable by continuous-wave coherent lidar and allow aerial vehicles to be distinguished from other objects such as birds and different types of aerial vehicles, e.g. different types of UAVs, to be distinguished from each other. Moreover, Doppler signatures of UAV propellers may even be used to assess certain properties of an UAV, such as the presence of a payload.

In some embodiments, the system comprises:

- a laser source,

- a detector, in particular a coherent detector, and

- an optical transceiver system configured to receive laser light from the laser source and to selectively emit the detection beam or the imaging beam.

Preferably, the optical transceiver system is configured to focus the imaging beam so as to illuminate a portion of the imaging target region and to scan the focused imaging beam across the imaging target region so as to successively illuminate respective portions of the imaging target region, e.g. a raster of respective positions within the imaging target region. To this end, the optical transceiver system may comprise a scanner module for scanning at least the imaging beam across the imaging target region, thereby allowing the lidar unit to capture a raster image of the entire imaging target region, or of at least a portion of the imaging target region. The imaging beam may be a focused beam having a beam waist at the imaging target region, e.g. as discussed in connection with the first aspect.

The optical transceiver module may be configured to scan the imaging beam across a traverse area corresponding to a range of azimuth and elevation angles as discussed in connection with the first aspect.

The optical transceiver system may further be configured to direct the detection beam so as to illuminate at least a portion of the detection target region. To this end, the optical transceiver system may be configured to direct the detection beam towards the detection target region as a collimated beam, as a divergent beam or as a focused beam. The detection target beam may have a beam cross section at the detection target region that is about equal to, or larger than the beam cross section of the imaging beam at the imaging target region. For example, the detection beam may have a beam cross section at the detection target region defined by a radius of the detection beam cross section at the detection target region. In some embodiments, the radius of the detection beam cross section may be between 1 mm and 200 mm, e.g. between 1 mm and 25 mm, or between 10 mm and 50 mm, or between 20 mm and 200 mm. For example, when the detection beam is directed to a detection target region at a distance between 10 m and 1000 m or more, a radius of the detection beam cross section between 1 mm and 50 mm, such as between 1 mm and 20 mm may allow probing of millimeter to centimeter sized features of the target, at least when a sufficiently large aperture telescope lens is used. A radius of the detection beam cross section between 20 mm and 200 mm may for example be used when directing the detection beam at a detection target region at a distance greater than 1000 m and/or for probing tens of centimeter sized features of larger targets (e.g. meter sized UAVs, military drones, fixed wing drones, helicopters, planes, etc.). The optical transceiver module may be configured to scan the detection beam across the detection target region so as to successively illuminate respective portions of the detection target region, e.g. a raster of respective positions within the detection target region. The transverse area scanned by the detection beam at the detection target region may be smaller than, equal to, or larger than the transverse area scanned by the imaging beam at the imaging target region. For example, the optical transceiver module may be configured to scan the detection beam across a transverse area corresponding to a range of azimuth and elevation angles of between 0.1° x 0.1° and 3° x 3°, such as between 0.5° x 0.5° and 2° x 2°. In some embodiments, the optical transceiver module is configured to scan the detection beam across the detection target region so as to obtain a sparse sampling of the detection target region, i.e. to successively illuminate respective positions within the detection target region such that there are gaps between the successively illuminated positions. Accordingly, detection of aerial vehicles in a detection target region may be achieved with no or only limited scanning of the beam, while a relatively high resolution of an image of the imaging target region may be achieved with beam scanning. Moreover a detection within a relatively large detection target region and an imaging of an imaging target region may be obtained with a single continuous-wave coherent lidar unit, thus reducing the cost of an efficient detection and identification system. The choice of a divergent or a collimated or a focused detection beam may depend on the available laser power of the laser source and the desired size of the detection target region. When the laser source provides sufficiently high laser power, the use of a divergent beam may be particularly useful to cover a larger detection area per scan position. In some embodiments, the system is configured to scan the detection beam across the detection target region, while, in other embodiments, the continuous-wave coherent lidar unit is configured to direct the detection beam along a fixed direction, fixed relative to the continuous-wave coherent lidar unit, i.e. the detection beam may be a non-scanning beam. It will be appreciated that the detection beam may nevertheless be directed towards different detection target regions, in particular by performing pan and/or tilt movements of the entire continuous-wave coherent lidar unit.

When operated in detection mode, the system may be configured to obtain a return laser signal over a detection dwell time, while directing the detection beam to a certain detection region during said detection dwell time. When the detection target beam is scanned across respective positions within the detection target region, the system may be configured to obtain respective return laser signals over a detection dwell time from each of said positions. In some embodiments, the detection dwell time is chosen large enough to cover at least two cycles of the periodic micro-Doppler/propeller signature of the types of aerial vehicles to be detected. In some embodiments, the detection dwell time is between 10 ms and 100 ms, such as between 15 ms and 100 ms.

When operated in imaging mode, the system may be configured to obtain a return laser signal, while directing the imaging beam to a particular position within the imaging target region, over an imaging dwell time, which may be shorter than the detection dwell time. The imaging dwell time may be determined by the scanning speed of the scanner module for scanning the imaging beam. Accordingly, the system provides fast and relatively high-resolution imaging in the imaging mode, while being able to detect signal features in the detection mode that extend over relatively long periods of time, e.g. so as to detect Doppler signatures of propellers of an aerial vehicle with high accuracy.

Generally, the imaging target region and the detection target region may be the same target region or they may be different, optionally partially coinciding, target regions. For example, the imaging target region may be a sub-region of the detection target region or the detection target region and the imaging target regions may otherwise overlap. The detection target region may be defined by a set, e.g. a range, of respective radial directions from the continuous-wave coherent lidar unit. Each radial direction may be defined by a pair of azimuth and elevation angles. Similarly, the imaging target region may be defined by a set, e.g. a range, of respective radial directions from the continuous-wave coherent lidar unit.

In some embodiments, the optical transceiver system may include separate optical transceivers for the imaging and detection modes, respectively, and the continuous- wave coherent lidar unit may be configured to selectively use one of the separate transceivers. In other embodiments, the optical transceiver system may comprise a combined optical transceiver module that is selectively operable in detection mode and imaging mode, i.e. the imaging mode and the detection mode may share some or even all of the optical components of the combined optical transceiver module. In some embodiments, the optical transceiver module comprises a scanner module selectively operable in imaging mode, e.g. at an imaging scanning rate, and in a detection mode, e.g. at a detection scanning rate, lower than the imaging scanning rate. Generally, in some embodiments, the optical transceiver module includes a telescope and a beam scanner. The beam scanner may be a MEMS beam scanner. The beam scanner may be positioned upstream from the telescope in the transmission path. In some embodiments, the optical transceiver module may include a lens or other optical component that is movably arranged between at least a first position corresponding to operation in detection mode and a second position for operation in imaging mode. Movement of the optical component may be controllable by the control unit. For example, a lens or other optical component may selectively be movable between an active position on the optical path and a stand-by position removed from the optical path. Alternatively or additionally, a lens or other optical component may be movable along the optical path. For example, the telescope may include a first lens and a second lens. At least the first lens may be axially displaceable such that the axial position of the first lens is controllable by the control unit, thereby allowing the beam geometry to be modified, e.g. so as to switch between a focused imaging beam and a collimated or even divergent detection beam and/or in order to focus the imaging beam at a different target range.

Accordingly, the optical transceiver module may be controlled to selectively operate in the detection and imaging modes by moving one or more optical components and/or by changing an operating parameter, e.g. a scanning rate and/or scanning amplitude, of the beam scanner.

In some embodiments, the system is operable to adjust the axial position of the first lens to change the location of the beam focus or beam waist, e.g. so as to adjust the beam focus within an interval of target ranges, e.g. between 100 m to 1000 m, or between 0.4 km to 4 km. It will be appreciated that the size of the telescope aperture may be selected dependent on the desired interval of target ranges, where large target ranges may require large telescope apertures.

The coherent detector is configured to receive the return laser signal and a local reference laser signal, also referred to as local oscillator (LO) signal. The local reference signal may be a portion of the laser light from the laser source, which may be passed through an optical reference path. Accordingly, in some embodiments, the optical transceiver system is configured to receive a first portion of the laser light from the laser source and to emit the first portion as imaging beam or detection beam, wherein the continuous-wave coherent lidar unit is configured to direct the return laser signal and a second portion of the laser light from the laser source to the coherent detector.

In some embodiments of the various aspects disclosed herein, the coherent detector may be a single detector, e.g. comprising one or more photo diodes, receiving both the return laser signal and the local reference signal. To this end, the continuous-wave coherent lidar unit may comprise an optical coupling unit configured to optically couple the return laser signal with the local reference laser signal and to direct the optically coupled signal to the coherent detector. Alternatively, in some embodiments, the coherent detector may be a balanced detector comprising separate optical detectors, e.g. separate photodiodes, for detecting the return laser signal and the local reference laser signal, respectively.

In some embodiments, the system is configured to:

- obtain position data indicative of a detection target region in which presence of an aerial vehicle is suspected,

- operate the continuous-wave coherent lidar unit to direct the detection beam or the imaging beam towards said target region.

In particular, in some embodiments, the system is configured to:

- operate the continuous-wave coherent lidar unit in the detection mode and to direct the detection beam towards said target region,

- responsive to detecting one or more signal features indicative of the presence of an aerial vehicle in said detection target region, to cause the continuous-wave coherent lidar unit to operate in the imaging mode to obtain an image of an imaging target region, the imaging target region comprising at least a portion of the detection target region.

Alternatively or additionally, the system may be configured to:

- operate the continuous-wave coherent lidar unit in the imaging mode and to direct the imaging beam towards said target region to obtain one or more images of said target region, - process the obtained one or more images to identify an aerial vehicle, in particular a UAV, in said target region,

- responsive to a failure to identify an aerial vehicle in said target region based on the obtained one or more images, cause the continuous-wave coherent lidar unit to operate in the detection mode and to direct the detection beam towards said target region.

Accordingly, the system may receive position data indicative of a target region where presence of an aerial vehicle is suspected, and the system selectively directs the detection beam and/or the imaging beam towards the target region indicated by the received position data. Accordingly, the continuous-wave coherent lidar unit does not need to scan a large field of view in order to identify potential aerial vehicles, but may selectively investigate specific target regions. The received position data may be received from an upstream sensor, such as a tracking radar, one or more radiofrequency (RF) sensor(s), a laser rangefinder, an electro-optical/infra-red (EO/IR) camera, a short-wave infrared (SWIR) camera, or the like, or a combination thereof. In some embodiments, the upstream sensor may have a detection range larger than a detection range of the continuous-wave coherent lidar unit and/or the upstream sensor may have scanning speed larger than a scanning speed of the continuous-wave coherent lidar unit. For example, the upstream sensor may be capable of identifying potential aerial vehicles at large distances, thus allowing an early detection of approaching objects, which the present system may then identify/classify as aerial vehicles or other objects, and/or as UAVs or as non-UAV objects.

Preferably, the system includes a controllable pan-and-tilt mount or is otherwise configured for mounting on a controllable pan-and-tilt mount, thereby allowing the system to selectively direct the imaging beam and/or the detection beam in different directions. A pan-and-tilt mount allows the system to selectively direct a viewing direction of the continuous-wave coherent lidar unit towards different target regions. In particular the pan-and-tilt mount allows the system to direct the viewing direction in two degrees of freedom. To this end, the pan-and-tilt mount is controllable to perform a pan movement, e.g. along a horizontal path such as a horizontal arc, and a tilt movement, e.g. along a vertical path such as a vertical arc.

Various embodiments of the system are capable of providing image data indicative of different types of information, thus facilitating an improved detection of aerial vehicles and/or of attributes of the detected aerial vehicles. In particular, in some embodiments, the system is configured, when operated in the imaging mode, to reconstruct a signalstrength image from the return laser signal, the signal-strength image representing respective spatially resolved signal magnitudes of the respective return laser signals, which are indicative of a reflectivity of the detected object. Alternatively or additionally, in some embodiments, the system is configured, when operated in the imaging mode, to reconstruct a speed image from the return laser signal, the speed image representing respective spatially resolved Doppler shift frequencies of the respective return laser signals.

In some embodiments, the system is configured, when operated in the imaging mode, to reconstruct a combined image from the return laser signal, the combined image being based on respective spatially resolved signal magnitudes of the respective return laser signals and on respective spatially resolved Doppler shift frequencies of the respective return laser signals or spatially resolved radial velocities (speeds) of the target. In particular, the system may combine or process the signal-strength image and the speed image to enhance the image contrast. For example, when the aerial vehicle flies in a field of view with other objects in the background (e.g. trees, buildings, etc.), presence of these objects may degrade the contrast of the signal-strength image of the aerial vehicle. However, if the objects in the background are stationary, they appear as zerovalued pixels in the speed image, resulting in high contrast speed image of the aerial vehicle when the magnitude of the radial velocity of the aerial vehicle is significantly higher than zero. On the other hand, when the aerial vehicle is in a field of view with low-lying moving clouds in the background, the contrast of the speed image may be degraded if the radial velocity magnitudes of the cloud and the aerial vehicle are nearly identical. But in the corresponding signal-strength image, the cloud may appear as group of low-valued pixels and the aerial vehicle as a group of high-valued pixels especially if the beam focus is closer to the axial position of the aerial vehicle and farther from the axial position of the clouds.

In some embodiments, the system is further configured to perform image processing of the at least one two-dimensional image to identify one or more properties of the detected aerial vehicle, in particular one or more of the following properties: a type of aerial vehicle, a speed of the aerial vehicle, an estimated time of arrival of the unmanned vehicle, a make and/or model of aerial vehicle, a configuration of the aerial vehicle, an orientation of the aerial vehicle, a presence, size and/or profile of payload carried by the aerial vehicle. Accordingly, the system is capable of providing information useful for threat assessment and/or effective interdiction deployment. For example, transverse position (e.g. more precise than what the upstream sensor can provide), speed and/or orientation information may be used for path prediction and, hence, for threat assessment and effective interdiction deployment. Moreover, the speed image and the signal-strength image may be combined, e.g. so as to suppress a non-moving background portion of the signal-strength image or otherwise. The transverse position refers to a position that may be defined by an azimuth angle and an elevation angle.

Alternatively or additionally, the attributes of the aerial vehicle detectable from the signal feature of the return laser signal, which is received responsive to emitting the detection beam, may include one or more of the following attributes: a type of aerial vehicle, a speed of the aerial vehicle, a rotation speed of a propeller of the aerial vehicle, if present, say in rotations per minute (RPM), a make and/or model of aerial vehicle, a presence and/or weight of payload carried by the aerial vehicle. In some embodiments, the system is further configured to output identification data indicative of a property of one or more identified aerial vehicles. Examples of such identification data include a speed, a size, a shape, an orientation and/or a threat level associated with the one or more identified aerial vehicles. The system may be configured to communicate the identification data to an interdiction system, to a downstream sensor and/or to a controllable pan-and-tilt mount controller. The interdiction system, e.g. a ballistic, or laser weapon or the like, may use the identification data for threat assessment and for controlling possible interdiction measures. A downstream sensor may use the identification data to control the downstream sensor. For example, a camera or other downstream sensor associated with an interdiction system may use the identification data. The interdiction system and/or downstream sensor may e.g. use the identification data for controlling a pan- and-tilt mount of the interdiction system or of the downstream sensor, or otherwise. Similarly, the pan-and-tilt mount controller may use the identification data so as to facilitate tracking of the detected aerial vehicle.

In some embodiments, the continuous-wave coherent lidar unit is selectively operable in one or more additional mode, e.g. in a frequency-modulated continuous-wave (FMCW) mode, in particular a FMCW detection mode. Alternatively or additionally, when operated in detection mode, the continuous-wave coherent lidar unit may be configured to operate in an FMCW mode. Operation in FMCW mode allows the continuous-wave coherent lidar unit to not only obtain speed but also range information. Nevertheless, when operated in imaging mode, it is generally preferred to operate the continuous- wave coherent lidar unit at a constant wavelength so as to obtain high image frame rates at high image resolutions, as discussed above.

Various embodiments of the system disclosed herein may be configured to detect different types of aerial vehicles, in particular manned or unmanned aerial vehicles, civil or military aerial vehicles, propeller-driven aerial vehicles or aerial vehicles having a different propulsion system, and/or the like. While some embodiments of the system may be configured to specifically detect aerial vehicles of certain types, e.g. aerial vehicles of certain sizes, travelling at certain speeds, etc. other embodiments may be configured to detect a wider range of aerial vehicles, e.g. micro-drones as well as larger payload-carrying drones, etc. For the purpose of the present disclosure the term aerial vehicle is intended to include manned and unmanned aerial vehicles. Examples of manned aerial vehicles include helicopters, airplanes, manned dual-mode vehicles that can fly and drive on the ground/road (sometimes referred to as "flying cars" or "readable aircrafts"). The term aerial vehicle generally refers to airborne vehicles.

The present disclosure relates to different aspects, including the system described above and in the following, further methods, systems, devices and product means, each yielding one or more of the benefits and advantages described in connection with one or more of the other aspects, and each having one or more embodiments corresponding to the embodiments described in connection with one or more of the other aspects described herein and/or as disclosed in the appended claims.

In particular, according to one aspect, disclosed herein are embodiments of a method for detecting aerial vehicles, such as unmanned aerial vehicles, the method comprising: emitting a continuous-wave laser detection beam towards a detection target region, receiving a return laser signal from the detection target region by coherent detection, processing the return laser signal to detect one or more signal features indicative of the presence of an aerial vehicle in the detection target region, determining one or more attributes of the said aerial vehicle in the detection target region from the detected one or more signal features; scanning a focused continuous-wave imaging laser beam across an imaging target region, receiving respective return laser signals from respective positions within the imaging target region by coherent detection, processing the return laser signals to create at least one two-dimensional image of the imaging target region.

The above method steps may be performed under the control of a suitable control unit for controlling operation of a continuous-wave coherent lidar unit, e.g. the continuous- wave coherent lidar unit described above and in the following. Embodiments of the method steps described herein as being carried out or controlled by a control unit can be implemented by means of hardware comprising several distinct elements, and/or at least in part by means of a suitably programmed microprocessor or other type of control unit. Generally, the control unit may include a suitably programmed microprocessor or any other circuit and/or device suitably adapted to perform the data- and/or signalprocessing functions and/or control functions described herein. In particular, the processing unit may comprise a general- or special-purpose programmable microprocessor, such as a central processing unit (CPU), a digital signal processing unit (DSP), an application specific integrated circuit (ASIC), a programmable logic array (PLA), a field programmable gate array (FPGA), a Graphical Processing Unit (GPU), a special purpose electronic circuit, etc., or a combination thereof. To this end, the control unit may be suitably programmed by software and/or firmware configured to be executed by the control unit. The software and/or firmware may be stored on a suitable memory of the control unit. In some embodiments, the control unit may be implemented as a single unit or by multiple units that may be communicatively coupled to each other, e.g. an onboard control unit and a remote signal/data processing unit.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects will be apparent and elucidated from the embodiments described in the following with reference to the drawing in which: FIG. 1 schematically illustrates an example of a system for detecting unmanned aerial vehicles.

FIG. 2 schematically illustrates an example of how a system disclosed herein may be operable as a part of a UAV detection and interdiction system.

FIG. 3 schematically illustrates an embodiment of a continuous-wave coherent lidar unit of a system for detecting unmanned aerial vehicles.

FIG. 4 schematically illustrates another embodiment of a continuous-wave coherent lidar unit of a system for detecting unmanned aerial vehicles.

FIG. 5 schematically illustrates yet another embodiment of a continuous-wave coherent lidar unit of a system for detecting unmanned aerial vehicles.

FIG. 6 schematically illustrates a more detailed view of an embodiment of a continuous- wave coherent lidar unit of a system for detecting unmanned aerial vehicles.

FIGs. 7A-C schematically illustrate examples of an optical transceiver module of a system for detecting unmanned aerial vehicles.

FIG. 8 illustrates an example of a method for detecting unmanned aerial vehicles.

FIG. 9 schematically illustrates an example of a control unit for controlling operation of a continuous-wave coherent lidar unit of a system for detecting unmanned aerial vehicles. FIG. 10 illustrates Doppler signatures of an UAV or one of its propellers detected by an embodiment of a system for detecting unmanned aerial vehicles operated in detection mode.

FIG. 11 illustrates lidar images of UAVs captured by an embodiment of a system for detecting unmanned aerial vehicles operated in imaging mode.

DETAILED DESCRIPTION

FIG. 1 schematically illustrates an example of a system for detecting UAVs. The system comprises a continuous-wave (CW) coherent lidar unit 1 mounted on a pan-and-tilt mount 30. The pan-and-tilt mount is operable to perform pan and tilt movements, respectively, of the CW coherent lidar unit 1 so as to direct a viewing direction of the CW coherent lidar unit 1 towards different target regions. The system is operable to detect one or more UAVs 40 within a detection range of the system. In particular, the system is capable of distinguishing a UAV from other flying objects, such as birds. To this end, as will be described in greater detail below, the CW coherent lidar unit 1 is selectively operable in a detection mode and an imaging mode.

When operated in the detection mode, the CW coherent lidar unit is operable to emit a CW laser detection beam towards a detection target region and to receive a return laser signal from the detection target region by coherent detection. The system is configured to process the return laser signal so as to detect one or more signal features indicative of the presence of a UAV 40 in the detection target region, and to determine one or more attributes of said UAV 40 in the detection target region from the detected one or more signal features. For example, the system may be operable to detect a Doppler signature of at least one of the propellers 41 of the UAV 40.

When operated in the imaging mode, the CW coherent lidar unit 1 is further configured to scan a focused CW imaging laser beam across an imaging target region and to receive respective return laser signals from respective positions within the imaging target region by coherent detection. The system is further configured to process the return laser signals so as to create at least one two-dimensional image of the imaging target region and, in particular of a detected UAV 40 in the imaging target region. Generally, the image of the imaging target region may have an image resolution high enough to distinguish the imaged UAV from other objects, such as birds. More preferably, the image of the imaging target region may have an image resolution high enough to distinguish different types of UAVs, e.g. different makes and/or models of UAVs.

Alternatively or additionally, the image of the imaging target region may have an image resolution high enough to distinguish different hardware configurations of an image UAV, e.g. whether or not the UAV is carrying a payload 42 and/or high enough to distinguish different types of payload. 1

Generally, when operated in detection mode, the system may be operable to detect UAVs within a maximum detection range, which may, in some embodiments, be between 500m and 2 km, such as between 700 m and 1 km. The system may be operable to detect propeller Doppler signatures and/or other attributes of UAVs that are within the maximum detection range. When operated in imaging mode, the system may be operable to obtain images of UAVs within a maximum imaging range, which may, in some embodiments, be between 300 m and 1 km, such as between 500 m and 700 m.

Various embodiments of a system disclosed herein may be operable as a stand-alone system or they may be integrated with one or more other detection sensors in an UAV detection and classification system. Alternatively or additionally, embodiments of the system disclosed herein may be integrated with one or more UAV interdiction systems in a UAV detection and interdiction system.

FIG. 2 schematically illustrates an example of how a system disclosed herein may be operable as a part of a UAV detection and interdiction system. In particular the UAV detection and interdiction system of FIG. 2 includes a system 1 for detecting UAVs as disclosed herein and further a tracking radar 2 and a UAV interdiction system 3.

The tracking radar 2 may serve as an upstream sensor for the CW coherent lidar unit 1. For example, the tracking radar 2 may be operable to detect flying objects within an upstream detection range Rl, and forward position information about detected objects to the CW coherent lidar unit 1 and/or the controller of its pan-and-tilt mount 30. When a detected object enters a detection range R2 of the detection mode of the CW coherent lidar unit 1, the CW coherent lidar unit 1 may operate in the detection mode to determine whether the detected object is a UAV and, optionally, what type of UAV. To this end, the CW coherent lidar unit 1 detects one or more attributes, such as a Doppler signature of one or more propellers, of the detected object. Alternatively or additionally, when a detected object enters a detection range R3 of the imaging mode of the CW coherent lidar unit 1, the CW coherent lidar unit 1 may operate in the imaging mode to capture one or more images of the detected object. Based on the captured images, an identification and/or classification of the detected object as a UAV can be made and, optionally, additional features of the UAV may be determined.

The CW coherent lidar unit 1 may forward identification data to the interdiction system 3 so as to facilitate threat assessment and possible launch of UAV countermeasures operable to disable or even destroy the identified UAV. Various embodiments of the CW coherent lidar unit provide a cost-efficient yet reliable system for UAV identification and classification. The system may provide identification data facilitating one or more of the following: distinguishing UAVs from birds with high precision, detection of payloads for threat assessment, detection of UAV distance and speed, calculation of estimated time of arrival to target, etc.

The detection mode and the imaging mode may have different detection ranges. In some embodiments, the detection mode may have a detection range larger than the imaging mode. For example, the detection range of the detection mode may be between 500 m and 2 km, such as between 700 m and 1 km while the detection range of the imaging mode may be between 300 m and 1 km, such as between 500 m and 700 m. The upstream sensor may have a detection range R1 larger than the detection ranges R2 and R3 of the respective operational modes of the CW coherent lidar unit 1.

It will be appreciated that other systems may include alternative or additional upstream sensors, such as one or more radio-frequency (RF) sensor(s), a laser rangefinder, an electro-optical/infra-red (EO/IR) camera, a short-wave infrared (SWIR) camera, or the like, or a combination thereof. Similarly, other systems may include additional interdiction systems. Yet further, some systems may not include any interdiction system but merely be operable for UAV detection and classification. Yet further, some systems may only include a CW coherent lidar unit as disclosed herein and a UAV interdiction system, i.e. the upstream sensor may be omitted.

FIG. 3 schematically illustrates an embodiment of a CW coherent lidar unit 1 of a system for detecting unmanned aerial vehicles. The CW coherent lidar unit 1 of FIG. 3 comprises a detection lidar unit 10, an imaging lidar unit 20, a control unit 15 for controlling the detection lidar unit 10 and the imaging lidar unit 20, and a signal processing unit 16 for processing the return laser signals received from the detection lidar unit 10 and from the imaging lidar unit 20. The detection lidar unit 10 is configured to emit a collimated detection beam 11 towards a detection target region. To this end, the detection lidar unit 10 comprises a laser source 112 and an optical transceiver 131 for directing an output from the laser source 112 as a collimated detection beam 11 towards a target detection region. Alternatively, the detection lidar unit may be configured to emit a divergent or focused detection beam. Furthermore, the detection lidar unit may be configured to perform a 2D scanning of the detection beam across the target detection region. The detection lidar unit is further configured to receive, as a return laser signal, light that has been reflected by an object in the detection target region responsive to being illuminated by the detection beam 11. To this end the detection lidar unit 10 comprises a coherent detector 114, and the optical transceiver 131 is configured to direct the return laser signal towards the coherent detector 114. The coherent detector 114 is also configured to receive a local reference laser signal. The imaging lidar unit 20 is configured to emit a focused imaging beam 21 towards an imaging target region and to scan the focused imaging beam 21 across the imaging target region. To this end, the imaging lidar unit 20 comprises a laser source 212 and an optical transceiver 132 for directing an output from the laser source 212 as a focused imaging beam 21 towards a target imaging region and to perform a 2D scanning of the imaging beam across the target imaging region. The imaging lidar unit is further configured to receive, as a return laser signal, light that has been reflected by an object in the imaging target region responsive to being illuminated by the imaging beam 21. To this end, the imaging lidar unit 20 comprises a coherent detector 214, and the optical transceiver 132 is configured to direct the return laser signals from respective positions in the target imaging region towards the coherent detector 214. Alternatively to providing separate lidar units for detection and imaging, respectively, the detection lidar unit 10 and the imaging lidar unit 20 may be partly or completely be integrated into a single lidar unit and share one or more components, e.g. as described in connection with FIG. 4.

The control unit 15 is configured to selective operate the detection lidar unit 10 or the imaging lidar unit 20, e.g. responsive to control commands from the signal processing unit 16 or otherwise. The signal processing unit 16 receives the return laser signals received by the detection lidar unit 10 or the imaging lidar unit 20, either directly from the respective lidar units or via the control unit 15, which optionally may perform a preprocessing of the detected return laser signals prior to forwarding them to the signal processing unit 16. The control unit 15 and the signal processing unit 16 may be implemented as a single unit or as separate units that may be communicatively coupled with each other. In some embodiments, the signal processing unit 16, or at least a processing unit implementing some of its functionality, may be implemented by a data processing system external to the CW coherent lidar unit 1, e.g. in a separate control box, as part of an overall UAV detection control system, etc. The signal processing unit 16 processes the return laser signals received from the detection lidar unit 10 and the imaging lidar unit 20 so as to determine attributes of a detected UAV and/or provide one or more images of a detected UAV.

FIG. 4 schematically illustrates another embodiment of a CW coherent lidar unit 1 of a system for detecting unmanned aerial vehicles. The CW coherent lidar unit 1 of FIG. 4 is similar to the CW coherent lidar unit of FIG. 3 in that it is configured to selectively emit a collimated, divergent or focused detection beam 11 and a focused imaging beam 21, which the CW coherent lidar unit directs or scans across a detection target region or an imaging target region, all as described above. To this end, the CW coherent lidar unit comprises a control unit 15, also as described above. In the example of FIG. 4 the system comprises a signal processing unit 16 external to the CW coherent lidar unit and communicatively coupled to the control unit 15. As described above, the signal processing unit 16 processes the return laser signals received from the CW coherent lidar unit 1 so as to determine attributes of a detected UAV and/or provide one or more images of a detected UAV.

The CW coherent lidar unit 1 of FIG. 4 differs from the CW coherent lidar unit of FIG. 3 in that the CW coherent lidar unit 1 of FIG. 4 includes a single lidar unit for both detection and imaging. The CW coherent lidar unit 1 comprises a laser source 12, an optical transceiver module 13, a coherent detector 14, and the control unit 15. The coherent detector 14 is configured to receive a local reference laser signal. The optical transceiver module 13 may comprise a first transceiver module 131 for directing and/or scanning an output from the laser source 12 as a collimated, divergent or focused detection beam 11 towards a detection target region and for directing a corresponding return laser signal received from the detection target region towards the coherent detector 14. The optical transceiver module 13 may further comprise a second transceiver module 132 for scanning an output from the laser source 12 as a focused imaging beam 21 across an imaging target region and for directing corresponding return laser signals received from respective positions in the imaging target region towards the coherent detector 14. The optical transceiver module may include one or more optical elements, e.g. one or more movable mirrors, for selectively directing the output from the laser source to the first or second transceiver module, and for selectively directing return laser signals from the first or second transceiver modules to the coherent detectors. It will be appreciated that the first and second transceiver modules may partly or completely be integrated as a single transceiver module, e.g. using suitably variable telescopic optics. Alternatively, the optical transceiver module may be a combined transceiver module that is selectively operable in detection mode and imaging mode, e.g. as described in connection with FIG. 7C. FIG. 5 schematically illustrates yet another embodiment of a CW coherent lidar unit 1 of a system for detecting unmanned aerial vehicles. The CW coherent lidar unit 1 of FIG. 5 is similar to the embodiment of FIG. 4 in that it comprises a laser source 12, a coherent detector 14, an optical transceiver module 13 and a control unit 15, and in that it is communicatively coupled to a signal processing unit 16, all as described in connection with FIG. 4.

The laser source 12 may be any suitable type of laser, such as a diode laser, fiber laser, or solid-state laser, etc. In some embodiments, a relatively low output power of the laser source is sufficient, e.g. a laser source having an output power of 10 W or less, such as of 5 Watt or less, or such as of 20 mW or less. The laser source may provide a laser output of a suitable wavelength, e.g. between 1000 nm and 2000nm, such as between 1000 nm and 1800 nm, such as between 1400 nm and 1600 nm. The line-width of the laser output is preferably between 0.1 kHz and 10 MHz, such as between 0.1 kHz and 5 MHz, such as between 0.1 kHz and 1 MHz. Optionally, the laser source may include a laser power amplifier.

The CW coherent lidar unit 1 comprises a beam splitter 121 for dividing the laser power output from the laser source 12 into two portions, where a first portion is used as a local oscillator (LO) signal and fed through a reference path, while a second portion is forwarded to the optical transceiver module 13. Prior to being fed to the optical transceiver 13, the second portion may be sent to one or more optical components, which may include a suitable optical amplifier 182 and a circulator 183. The optical transceiver 13 selectively transmits either a detection beam or an imaging beam to a target region where presence of a UAV is suspected, and receives a corresponding return laser signal that is then coupled to the first input arm of a receiving beam splitter/combiner 122. The LO beam is coupled to the second input arm of the receiving beam splitter 122 and the receiving beam splitter 122 mixes or otherwise combines the LO beam with the return laser signal, and the mixed or otherwise combined signal is fed to the coherent detector 14. The control unit 15 may include suitable digital-to-analog and analog-to-digital conversion circuitry and a suitably programmed processor, e.g. an FPGA, for controlling the optical transceiver module 13 and for acquiring and preprocessing the detector signals from the coherent detector 14. The control unit 15 is communicatively coupled, e.g. by a wired or wireless communication interface, to a signal processing unit 16, which may be external to the CW coherent lidar unit 1, as illustrated in FIG. 6, or which may at least partially be integrated into the CW coherent lidar unit 1.

The optical transceiver module 13 may comprise a first transceiver module 131 for directing and/or scanning the output from the circulator 183 as a collimated, divergent or focused detection beam towards a detection target region and for directing a corresponding return laser signal received from the detection target region towards the circulator 183. An example of a first transceiver module will be described in more detail in connection with FIGs. 7A and 7C. The optical transceiver module 13 may further comprise a second transceiver module 132 for scanning an output from the circulator as a focused imaging beam across an imaging target region and for directing corresponding return laser signals received from respective positions in the imaging target region towards the circulator 183. Respective examples of a second transceiver module will be described in more detail in connection with FIGs. 7B and 7C. The optical transceiver module 13 may include one or more optical elements, e.g. one or more movable mirrors, for selectively directing the output from the circulator 183 to the first or second transceiver module, and for selectively directing return laser signals from the first or second transceiver modules back to the circulator 183. Examples of a suitable movable mirror include a MEMS mirror and a galvanometer scanning mirror. In some embodiments, the optical transceiver module may be implemented by a single module, e.g. as illustrated in FIG. 7C, that can selectively be operated in detection mode and imaging mode. As discussed above, embodiments of the CW coherent lidar unit, or at least of components thereof, may be mounted on a pan-and-tilt mount. It will be appreciated that the entire CW coherent lidar unit 1 may be accommodated in a housing mounted on a pan-and-tilt mount. In other embodiments, only some components of the CW coherent lidar unit 1 may be mounted on a pan-and-tilt mount, e.g. only the optical transceiver module 13 and, optionally further components of the optical path, e.g. the circulator 183, thereby reducing the size and weight of the unit that needs to perform pan and tilt movements. To this end, the optical transceiver module, and optionally further components of the optical path, may be accommodated in a first housing, which is mounted on a pan-and-tilt mount, while the remaining components are accommodated in one or more additional housings, which may be operationally connected to the first housing via one or more optical fibers or otherwise.

FIG. 6 schematically illustrates a more detailed view of an embodiment of a CW coherent lidar unit 1 of a system for detecting unmanned aerial vehicles.

The CW coherent lidar unit 1 of FIG. 6 is similar to the embodiment of FIG. 5 in that it comprises a laser source 12, a coherent detector 14, an optical transceiver module 13, a beam splitter 121, an optical amplifier 182, a circulator 183, a beam combiner 122 and a control unit 15, and in that it is communicatively coupled to a signal processing unit 16, all as described in connection with FIG. 5.

The CW coherent lidar unit 1 of FIG. 6 employs an external cavity laser as a laser source 12, e.g. a 1550 nm planar external cavity seed laser (e.g. PLAN EX™, available from Redfern Integrated Optics) with a CW output power (ex-fiber) of 17 mW, or another suitable laser source. In the present example, the beam splitter 121 is a 90/10 fiberoptic beam splitter (FOBS), even though other types of beam splitters may be used. The reference path may comprise a variable fiber-optic attenuator (FOA) 171 and fiber delay line of a suitable length, e.g. 22 m or another suitable length. The output of the delay line provides an LO signal at a suitable power, e.g. 0.5 mW power. Prior to being fed to the optical transceiver 13, the second part is initially fed through another variable FOA 181, which may generate a suitable input signal for the optical amplifier 182, e.g. a 0.25 mW input signal. The optical amplifier 182 may be an Erbium-doped fiber amplifier (EDFA) (e.g. a CEFA-C-PB-HP-37 available from Keopsys). In one example, the output from the optical amplifier may provide an adjustable optical power for the optical transceiver via the circulator 183. The circulator 183 may include a fiber-coupled collimating lens (CL1) emitting a Gaussian beam (e.g. having a 1/e² radius w = 1 mm) and providing the adjustable optical power (e.g. of up to 5 W) for the optical transceiver 13. In the example of FIG. 6, the circulator 183 is a free-space optical circulator and comprises a polarizing beam splitter (PBS), a quarter-wave plate (QWP) and a second collimating/coupling lens (CL2). The optical transceiver 13 selectively transmits a detection beam or an imaging beam to a target region where presence of a UAV is suspected, and receives the return laser signal that is then coupled by CL2 to the first input arm of receiving beam splitter 122, e.g. a 50/50 FOBS. The LO beam is coupled to the second input arm of the receiving beam splitter 122 and the receiving beam splitter 122 mixes the LO beam with the return laser signal. The two output arms of the receiving beam splitter 122 are connected to a balanced detector (e.g. a PDB430C-AC available from Thorlabs). In one example, all optical fibers of the CW coherent lidar unit 1 are polarization-maintaining single-mode fibers. It will be appreciated that various modifications may be made to the optical configuration of the CW coherent lidar unit. For example, alternative examples may include a different type of coherent detector and/or a different type of laser source.

The optical transceiver module 13 may comprise a first transceiver module 131, a second transceiver module 132, and a movable mirror also as described in connection with FIG. 5. Alternatively, the optical transceiver module may be implemented by a single module, also as described in connection with FIG. 5. As discussed above, some embodiments of the CW coherent lidar unit, or at least components of some embodiments of the CW coherent lidar unit, may be mounted on a pan-and-tilt mount.

FIG. 7A schematically illustrates an example of an optical transceiver 131 suitable for transmitting a divergent, collimated or focused detection beam and for receiving corresponding return laser signals. In particular, the optical transceiver 131 is suitable for detection of the Doppler signature of drone propellers or other useful attributes of a detected UAV. The optical transceiver 131 comprises a movable lens LI and a large- aperture second lens L2. For example, the lens LI may be an aspheric lens LI, e.g. having a focal length of 11 mm. The lens LI may be mounted on a linear stage. The second lens L2 may be a doublet lens having a suitably large aperture, such as a 6-inch aperture, and a suitable focal length, e.g. a focal length of 560 mm. When the optical transceiver module is configured to provide a detection beam having a suitably large beam diameter, e.g. larger than 100 mm, such as 102 mm, the detection beam may cover a large transverse area, without the need for a beam scanner or at least with a limited number of raster-scan positions, thus facilitating long dwell/exposure times for reliably detecting Doppler signatures of UAV propellers. When the detection beam is divergent an even larger transverse area can be covered without the need for any scanning operation or with only a sparse or limited scanning. The detection beam may be made divergent, collimated or focused by suitable axial translation of the lens LI or otherwise. In other embodiments, one or both lenses may be axially fixed. The detection beam may be scanned by the pan-and-tilt mount where the lidar unit is mounted.

FIG. 7B schematically illustrates an example of an optical transceiver 132 suitable for transmitting and scanning a focused imaging beam and for receiving corresponding return laser signals. In particular, the optical transceiver 132 is suitable for raster-scan imaging of flying UAVs. The optical transceiver 132 comprises a pair of telescope lenses LI and L2, respectively, and a beam scanner 133. In one specific example, the lens LI is an aspheric lens LI, e.g. having a focal length of 6.38 mm, and the lens L2 is a 1-inch aperture doublet lens L2, e.g. having a focal length of 60 mm. Accordingly, the optical transceiver 132 for the imaging mode may be smaller than the optical transceiver for the detection mode and configured to transmit a focused imaging beam. The effective focus of the optical transceiver may be between 40 m and 200 m, such as between 40 m and 100 m such as between 40 m and 60 m. The relatively small optical transceiver facilitates use of a beam scanner with a limited effective aperture size, e.g. having a diameter of 25 mm. In one example, the beam scanner 133 may be a combination of three separate plane mirrors actuated by two galvo scanners (e.g. a 6260H galvanometer available from Cambridge Technology) and one fast resonant scanner (e.g. a resonant optical scanner SC-25 available from Electro-Optical Products Corporation). It will be appreciated that other examples of the optical transceiver may include different components and/or otherwise have different specifications, e.g. a larger transceiver aperture to allow beam focus at large distances, e.g. at 1 km or more. In some embodiments, the transceiver aperture may be larger than 4 inch.

FIG. 7C schematically illustrates an example of an optical transceiver module 13 suitable for transmitting and scanning a focused, a collimated or a divergent detection beam and/or a focused imaging beam and for receiving corresponding return laser signals. In particular, the optical transceiver module 13 is suitable for raster-scan detection or raster-scan imaging of flying UAVs, i.e. the optical transceiver module 13 of FIG. 7C may serve as an optical transceiver for the imaging mode, as an optical transceiver for the detection mode, or as a combined optical transceiver module for both the imaging and the detection mode. The optical transceiver module 13 comprises a pair of telescope lenses LI and L2, respectively, and a beam scanner 133. In the example of FIG. 7C, the beam scanner is implemented as a dual-axis MEMS mirror and placed between the circulator 183 and the telescope lenses LI and L2. Employing a MEMS-based beam scanner provides a compact, inexpensive implementation. In particular, due to the small angular field-of-view (FoV) requirement for the lidar-based raster-scan imaging system disclosed herein, the use of the bulky galvo-resonant beam scanner can be avoided. The MEMS-mirror beam scanner 133 may be placed at the input side of the transmission path of the optical transceiver, i.e. at the input side of lenses LI and lens L2. It may be worthwhile noting that the beam deflection away from the optical axis by the MEMS- mirror results in translation of the focused beam in the "Point Object Plane" 134. This lateral translation has a corresponding magnified translation in the "Point Image Plane" where the lidar probe beam is focused at a distance from lens L2. For example, the "Point Image Plane" may be located between 500 m and 2 km, such as about 1 km from lens L2. A summary of design parameters of a specific example of an optical transceiver module as shown in FIG. 7C is summarized in table 1 below. However, it will be appreciated that other embodiments may employ other parameters.

Table 1

A typical MEMS mirror (e.g., 02.4 mm, Mirrorcle Technologies Inc.) has a resonant frequency around 860 Hz. This allows the optical transceiver shown in FIG. 7C to create raster-scan images with 400 x 400 pixels at 4.3 frames/s or 100 x 100 pixels at 17.2 frames/s. The reduction of density of scan points or pixels per area at higher frame rates may be compensated for by decreasing the angular FoV or the maximum optical deflection angle of the output beam out. With its cm-level spatial resolution (even at 1 km distance to a UAV) and improved frame rate, the system disclosed herein can further refine the UAV position data (azimuth and elevation) and speed data. The increased precision in position and speed data can enhance tracking and flight behavior prediction, thereby benefiting cUAS interdiction systems.

In the example of table 1, the optical transceiver provides scanning of an 8.72 m by 8.72 m transverse area of the imaging beam at the imaging target region at 1 km distance. The focused imaging beam has a radius of 13 mm at the imaging target region. When an image with 400 by 400 pixels is obtained, adjacent pixels will thus correspond to (8720/400) = 21.8 mm transverse dimension, which is slightly less than the focused beam diameter (26 mm). This means that the raster-scan image will not be a sparse point-cloud image, but a dense point cloud that provides a high resolution image. When the optical transceiver of the above example is operated in detection mode and uses a collimated beam, the beam radius is 40 mm at a detection target region at 1 km distance. Accordingly, to densely scan an area 8.72 m by 8.72 m with a collimated beam diameter of 0.08 m in detection mode, around 109x109 scan positions may be required. For scanning micro-Doppler detection mode, using a dwell time of 15 ms per position, scanning of 109x109 scan positions will take about 3 minutes. When the optical transceiver emits a divergent beam that produces a beam radius of e.g. 80 mm at 1 km, a complete and dense scanning only requires 54x54 scan positions; hence, the scan time is reduced to 44 s. If the transverse area to be scanned in detection mode is reduced and/or if the number of scan positions is otherwise reduced (e.g. by only performing a sparse scanning), the detection scan time can be further reduced. A transverse area of 8.72m x 8.72m at 1 km distance is sufficient to cover a transverse area corresponding to a typical cUAS radar sensor, which may provide tracking accuracies of 0.5° azimuth x 0.5° elevation. Accordingly scanning a transverse area of this size also in detection mode may provide a reliable identification of a possible UAV detected by an upstream tracking radar.

The optical transceiver module 13 may selectively be operated as a transceiver for the detection and beam and as a transceiver for the imaging beam, i.e. it allows for switching between detection and imaging mode, with an advantage of utilizing the same optical transceiver for both modes. When the optical transceiver module 13 of FIG. 7C is used for micro-Doppler detection, the scan rate of the MEMS mirror may thus be reduced compared to typical imaging scan rates of the MEMS mirror when operated in imaging mode. The reduced scan rate may be selected so as to ensure that, at each scan position, the detection beam probes said position with sufficient dwell time, e.g. for at least 15 ms.

Moreover, by translating the lens LI along the optical axis, the beam geometry may be modified between a focused beam and a collimated or even divergent beam, thereby facilitating selective operation in either imaging or detection mode.

In the example of FIG. 7C, the circulator is shown as in integral part of the optical transceiver module. However, it will be appreciated that the circulator may be external to the optical transceiver module as shown in FIG. 6. FIG. 8 illustrates an example of a method for detecting unmanned aerial vehicles. The method may be performed by any of the embodiments described herein, or otherwise.

At initial step SI, the process receives position data, e.g. including an azimuth and an elevation angle, defining a direction towards a target region. The process may receive the position data from a UAV detection system, from an upstream sensor or otherwise. For example, the position data may indicate a direction at which an upstream sensor has detected a suspected UAV. Alternatively or additionally, the UAV detection system may control the process to successively direct the CW coherent lidar unit towards different directions, e.g. so as to scan at least a portion of the total accessible FoV that can be covered by the system. The position data may optionally include additional information, e.g. distance information indicative of a distance of the target region including a suspected UAV, velocity information indicative of a speed and/or direction of movement of the suspected UAV, and/or the like.

At step S2, the process directs the viewing direction of the CW coherent lidar unit towards the target region. To this end, the process may actuate a pan-and-tilt mount on which the CW coherent lidar unit is mounted, or the process may direct the CW coherent lidar unit in a different manner.

At step S3, the process selects operation of the CW coherent lidar unit in either the detection mode or the imaging mode. The process may select a default mode, e.g. it may always initially select either the detection mode or the imaging mode. In other embodiments, the choice of mode may depend on the upstream sensor data, e.g. on the distance of the suspected UAV from the CW coherent lidar unit and/or on the estimated velocity of the suspected UAV. Depending on the selected mode, the process either proceeds at step S4 or at step S6. At step S4, when operated in the detection mode, the CW coherent lidar unit emits a CW laser detection beam towards the target region, and receives a return laser signal from the detection target region by coherent detection as described herein or otherwise. The process processes the return laser signal so as to detect one or more signal features indicative of the presence of a UAV in the detection target region, and to determine one or more attributes of the said unmanned aerial vehicle in the detection target region from the detected one or more signal features. In one embodiment, the process obtains a Doppler signature, e.g. as a representation of respective distributions of detected Doppler shifts over time. Examples of obtained Doppler signatures are shown in FIG 10. The process may then detect a propeller speed of the UAV, e.g. from a detected frequency of a periodic variation of the distribution of detected Doppler shifts.

At subsequent step S5, based on the detected one or more attributes, the process may confirm whether the suspected UAV indeed is a UAV. Moreover, the process may determine the type of UAV detected in the target region, e.g. by comparing the obtained Doppler signatures with previously obtained reference Doppler signatures of known types of UAVs. It will be appreciated that such classification may be performed by a trained machine-learning model, trained on previously obtained reference Doppler signatures of known types of UAVs. The process may then return to step SI to obtain updated position data, e.g. so as to analyse another target region for a suspected UAV or to track an already detected UAV.

At step S6, when operated in the imaging mode, the CW coherent lidar unit may scan a focused CW imaging laser beam across the target region, and receive respective return laser signals from respective positions within the imaging target region by coherent detection. At subsequent step S7, the process may then process the return laser signals to create at least one two-dimensional image of the target region. The process may display the created image or forward the created image to an UAV detection system for display and/or further analysis. Alternatively, the process may perform image processing of the created image, in particular so as to identify a UAV in the image and/or to classify the identified UAV as a particular type of UAV, e.g. a particular make or model, and/or to identify a payload carried by the UAV or another type of hardware configuration of the detected UAV. The image processing may be based on image processing techniques known as such in the art. The classification of UAVs and/or the detection of payloads may likewise be based on image processing techniques known in the art. Optionally the classification may be performed by a trained machine-learning model, e.g. a neural network, such as a convolutional neural network. Examples of images obtained by an embodiment of the CW coherent lidar unit disclosed are illustrated in FIG. 11.

The process may then return to step SI to obtain updated position data, e.g. so as to analyse another target region for a suspected UAV or to track an already detected UAV. It will be appreciated that, during operation of the system in detection mode or imaging mode, the process may concurrently operate the pan-and-tilt mount so as to track a moving UAV. The tracking of the UAV may be based on position data from a UAV detection system or an upstream sensor and/or the tracking may be based on the information extracted by the present process from the information obtained by operation of the CW coherent lidar unit in detection or imaging mode. Such information may include velocity information and/or orientation information which may be extracted from the obtained Doppler signals. The process may then again proceed to step S2 to adjust the viewing direction of the CW coherent lidar unit based on the updated position data. The process may then again proceed to step S3 to determine whether or not to switch operation of the CW coherent lidar unit to the other mode. Accordingly, if the CW coherent lidar unit is initially operated in the detection mode, the process may continue operation in the detection mode or switch operation from the detection mode to the imaging mode. If the CW coherent lidar unit is initially operated in the imaging mode, the process may continue operation in the imaging mode or switch operation from the imaging mode to the detection mode. In some embodiments, the process may repeatedly switch back and forth between the two modes, e.g. for tracking an approaching UAV and/or for successively obtaining more accurate and/or higher resolution information. The switching may be conditioned on the information obtained in the initial mode of operation. For example, if the process is initially operated in the imaging mode, and the obtained image does not allow an identification/classification of a UAV with sufficient confidence, the process may switch to the detection mode so as to obtain a Doppler signature. Alternatively, if the system is initially operated in the detection mode, the process may switch to the imaging mode responsive to the detection of a Doppler signature indicative of a UAV, e.g. so as to determine a type of UAV or whether the UAV is carrying a payload.

FIG. 9 schematically illustrates and example of a control unit 15 for controlling operation of a CW coherent lidar unit of a system for detecting unmanned aerial vehicles, in particular for operation of the CW coherent lidar unit in the imaging mode. Operation of the CW coherent lidar unit in detection mode may be performed by a simplified control process, e.g. as described below.

The control unit 15 shown in FIG. 9 may be implemented in various ways. In one example, the control unit may be based on a PXIe-1083 chassis equipped with a PXIe- 5763 four-channel PXI FlexRIO Digitizer and a Thunderbolt interface (all from National Instruments) for interfacing with an external signal processing unit 16. In addition, the chassis may hold a dual-channel digital-to-analog converter (DAC) module 801. Each DAC channel may have a 16-bit depth and a suitable sample rate of e.g. up to 500 kS/s - one to control the scanner 802 of the azimuth (galvo) scanning mirror M and another for the scanner 805 of the slow (galvo) scanning elevation mirror M. The digitizer PXI2-5763 has four analog inputs each with a bandwidth of 225 MHz, 500 MS/s and 16-bit analog- to-digital converter (ADC). In one example, only one channel is used.

The control unit 15 includes an FPGA (KU060, Xilinx) 150, where all the fast signal processing tasks are performed up to the image pixel creation. The control of the scanning mirrors M may also be implemented here so as to synchronize the pixel generation and the actual mirror position, a task which is well suited for an FPGA implementation. The scanning control includes an azimuth X control block 161, an elevation y control block 162 and a Y-scan reconstruct block 163.

In one example, the elevation scanning may be the fastest scanning axis, which may be performed by a resonant scanner 807. The resonant scanner may have a mechanical resonance with a high Q. factor. Therefore, the frequency of scanning can, in some implementations, not be controlled in a dynamic manner - it acts as a fixed frequency (248 Hz) mechanical oscillator. A digital phase-locked loop (DPLL) in the FPGA interpolates the position of the mirror based on a timing signal from the resonant scanner. The rest of the scanning system in azimuth derives its timing from the movement of the resonant scanner.

The digital signal processing pipeline starts in at an analog-to-digital conversion block 151, e.g. a 500 MS/s ADC. The relatively high sample rate of 500 MS/s may be multiplexed into four streams with 125 MS/s each. This data rate may be reduced in a decimating filter 152, e.g. by a factor of 8 to 62.5 MS/s. At this rate, it can be handled by a single stream at 125 MHz clock rate. This stream of real-valued samples may transformed into a complex-valued spectrum using a fast-Fourier transform (FFT) algorithm (256 points) implemented by block 153. The FFT algorithm may be a flow- through architecture having the samples in natural order at the input and the output, with the reordering performed inside a FFT library package. The imaginary part of the complex-valued input may be set to zero resulting in a symmetric spectrum. At magnitude block 154, the spectrum is converted to magnitude and its symmetry is used in block 155 to decimate the data rate by a factor of 2 by only using one side, e.g. the positive side, of the spectrum. The data rate at this stage is 500 MS/s / 8 / 2 = 31.25 MS/s. Each sample, i.e., the magnitude of the spectral component, is at this stage tagged with the corresponding frequency bin number from 0 to 127. Within this stream of data, the bin with the maximum magnitude is detected and extracted using a peak-finding algorithm implemented by block 158 - the range of search for peak in both magnitude and frequency may be a static parameter, e.g. under control from the external signal processing unit 16. After peak finding, the data rate is 31.25 MS/s / 128 = 244141 S/s which is subsequently sent via a first-in-first-out (FIFO) over a suitable interface (IF), e.g. a Thunderbolt connection to the external signal processing unit 16 with the current position X-Y (or azimuth and elevation angles) of the scanning mirrors.

This data stream may be represented as a stream of position-tagged pixels as the imaging beam scans the target field-of-view. In the signal processing unit 16, the stream of position-tagged pixels is received and used to reconstruct 2D images - e.g. one image based on the magnitude of the return signal and one image on the Doppler shift frequency (which is proportional to the radial speed of the target). The stream of data in the FPGA 150 may be clocked at a frequency of 125 MHz and paced by a data-valid signal in parallel to this data stream (as not all clock cycles necessarily hold valid data).

The signal processing unit 16 may be a suitably programmed computer or other suitable data processing system.

The signal processing for the detection mode may be simplified, when no synchronization with scanning mirrors is needed and when lower data rates are used. For example, the FPGA based processor's rate of Doppler spectra may be reduced, e.g. from 244 kSpectra/s to 763 Spectra/s. This may be done by averaging over successive spectra, in one example over 320 successive spectra. At this lower rate, the averaged spectra may be transferred to the signal processing unit 16 allowing the signal processing unit to display and/or process the received spectra. For example, the signal processing unit may display a plot of Doppler spectrum versus time. Alternatively, a speed spectrum versus time plot may be obtained by converting the frequency axis (in Hz) of the Doppler spectrum to m/s by multiplying the factor A/2 (where, in one example, the wavelength A is A = 1.55 pm). In one example, based on the Nyquist criterion, the maximum modulation frequency detectable in a speed profile versus time plot (e.g., frequency of the periodic blade motion) is around 380 Hz. As the propeller partly blocks the incident detection beam twice per full rotation, observing 380 Hz modulation corresponds to 11400 rotations per minute (RPM). Hence, the 763 Spectra/s is sufficient to detect the maximum rotational speeds of propellers of a typical UAV (rpm< 9000). If higher rotational speeds are to be detected, the processing rate of the spectra may be increased.

FIG. 10 illustrates Doppler signatures of UAVs detected by an embodiment of a system for detecting unmanned aerial vehicles operated in detection mode. The data displayed in FIG. 10 was obtained by the CW coherent lidar unit described in connection with FIG. 6, FIG. 7A and FIG. 9 above. The results demonstrate the performance of an embodiment of the system for detecting the periodic signatures associated with the propellers of a UAV, in this example a DJI Phantom 3 Professional. The data was obtained with the UAV located at a distance of around 500 m from the CW coherent lidar unit. The upper left graph in FIG. 10 shows an 8-second periodogram (speed distribution versus time plot) obtained by the CW coherent lidar unit from the return laser signal that it receives from the rotating blade for the case when the detection beam is collimated. Zooming in to a portion shown in the inset, a periodic pattern with a frequency of 50 Hz can easily be noticed. As the unit detects two adjacent cycles of the pattern for every full rotation of the propeller, 50 Hz corresponds to 1500 blade rotations per minute (RPM). In the periodogram shown in the upper right of FIG. 10, the RPM of the target propeller was increased using the UAV's remote control. Based on the data section shown in the inset, the propeller blade's rotational speed increased to 2800 RPM. The lower left periodogram in FIG. 10 shows the case when the detection beam emitted by the optical transceiver of the CW coherent lidar unit was focused at the 500 m UAV position by slightly increasing the axial distance of lens LI from lens L2 (see FIG. 7 A). Here, the inset shows a repeat of the 1500 RPM as the blade speed was set to this minimum when the remote control throttle were at neutral positions. In front of the target propeller, the collimated beam has larger cross section but lower intensity while the focused beam has smaller cross section but higher intensity. It is worthwhile noting that, with a higher intensity detection beam, the signal-to-noise ratio (SNR) of the return laser signal in the present example is higher by about 18 dB than for the case of a collimated detection beam. When the blade rotation is increased, the focused detection beam is also able to detect the increased blade RPM (around 4300) as shown in the lower right periodogram in FIG. 10. An advantage of the focused detection beam geometry is the high SNR in the lidar echoes but an advantage of the collimated (or even a semi-divergent) detection beam is a more relaxed requirement on beam pointing stability. I will be appreciated that, in all the 8-second periodograms in FIG. 10, dark bands are present (i.e., for both focused and collimated cases) as the detection beam line-of-sight wandered due to wind gusts during the experiment, which affected the lidar pointing stability during the outdoor field measurement - deflecting the beam away from the rotating propeller intermittently. These data dropouts can be reduced by further improvements to the mechanical stability of the lidar setup housing and/or mount.

FIG. 11 illustrates lidar images of UAVs captured by an embodiment of a system for detecting unmanned aerial vehicles operated in imaging mode. In particular FIG. 11 shows the lidar raster-scan images of three micro-UAVs of different sizes and shapes. The images are either based on the line-of-sight speed pixel information or the magnitude of the FFT peak (normalized by the maximum in each video file). Each image corresponds to an angular FoV of 2.5° x 2.5°. The inventors have demonstrated that an embodiment of the CW coherent lidar unit is successful at capturing high-contrast video images of the three target quadcopters (2.5 frames/s for an image with 200 x 200 pixels to 5 frames/s for one with 100 x 100 pixels) as the UAVs fly from a short distance of 20 m to as far as 70 m. The quality of the images (and of corresponding videos based on these images) is clearly sufficient for drone classification or identification, and likely for discriminating against birds. A careful examination of the image frames shown in FIG. 11 also reveals the ability of the anti-drone lidar to image a small camera carried by each drone, which means that larger payloads will be much easier to detect and therefore increase the potential to assess threat levels and improve decision-making for countermeasures. The maximum range of the system's imaging functionality of the embodiment used for obtaining the images of FIG. 11 is presently limited by the use of a lens L2 with a smaller aperture diameter (1 inch). This limitation is due to the use of a 25 mm diameter resonant scanning mirror for the beam scanner and can be avoided by employing a different types of beam scanner with a larger effective aperture that suits the 6-inch diameter lens L2 used in FIG. 7C, which has the ability to focus the lidar beam at 500 m or beyond.

Generally, various embodiments of the system disclosed herein, which has two operating modes, have various potential cUAS applications, particularly for target classification and identification. In some embodiments the two modes are employed for the sensing of micro-Doppler signatures from drone propellers and for the raster-scan imaging of the entire drone profile and payload, respectively. In the case of a focused beam geometry of both operating modes, a nominal lidar beam focus of around 1-2 cm appears to be sufficient. The ability of a CW coherent lidar unit to focus the lidar probe beam is ultimately limited by diffraction. When the focused probe beam should be able to reach a distance of 1 km, the inventors have found that a 4.5-inch telescope aperture is sufficient and can result in a more compact optical transceiver than the 6-inch telescope used in the above embodiments. As was described above, various embodiments of the CW coherent lidar unit may be mounted on a controllable highspeed pan-and-tilt mount and be integrated with an upstream sensor that can efficiently track a drone with high angular resolution and accuracy, such as obtainable by commercial cUAS radar systems, such as a system available from Echodyne. Echodyne's cUAS radar sensor provides tracking accuracies of 0.5° azimuth x 0.5° elevation over a large FoV (130° azimuth x 90° elevation). The tracking radar can aid the pan-and-tilt mount for continuously adjustable pointing, which enables the CW coherent lidar unit to scan a narrow FoV in search of an airborne target. The CW coherent lidar unit may thus complement the radar with its ability to detect micro-Doppler signatures and obtain high resolution images via raster-scanning. By limiting the raster-scan to an angular FoV of 0.5° x 0.5°, an area covering 8.72 m x 8.72 m can be scanned at 1 km. If a raster-scan image with 400 x 400 pixels is obtained, a single pixel corresponds to 2.18 cm, which is well matched with the diameter of the focused lidar beam that can be produced with a 4.5-inch aperture optical transceiver.

Accordingly, disclosed herein are various embodiments of a CW coherent lidar unit which provides micro-Doppler detection and raster-scan imaging of drones. These two operating modes suggest the prospect of embodiments of the system disclosed herein as a cUAS sensor that can distinguish drones from birds. Embodiments of the CW coherent lidar unit disclosed herein allow lidar based sensing of micro-Doppler signals from drone propellers up to a remote distance of at least 0.5 km. In addition, embodiments of the CW coherent lidar unit disclosed herein are capable of producing videos of flying drones up to at least 70 m distance and/or with frame rates of at least 2.5 (or 5) frames/s at 200 x 200 (or 100 x 100) pixels.

In the apparatus claims enumerating several means, several of these means can be embodied by one and the same element, component or item of hardware. The mere fact that certain measures are recited in mutually different dependent claims or described in different embodiments does not indicate that a combination of these measures cannot be used to advantage.

It should be emphasized that the term "comprises/comprising" when used in this specification is taken to specify the presence of stated features, elements, steps or components but does not preclude the presence or addition of one or more other features, elements, steps, components or groups thereof.

Claims

1. A system for detecting aerial vehicles, wherein the system comprises a continuous- wave coherent lidar unit and is configured to obtain position data indicative of a detection target region in which presence of an aerial vehicle is suspected and, responsive to the obtained position data, to operate the continuous-wave coherent lidar unit in an imaging mode; wherein the continuous-wave coherent lidar unit is configured, when operated in the imaging mode: to focus a continuous-wave imaging laser beam at the detection target region and to scan the focused continuous-wave imaging laser beam across an imaging target region, the image target region including at least a portion of the detection target region, wherein the continuous-wave imaging laser beam is a narrow line-width continuous-wave imaging laser beam; and to receive respective return laser signals from respective positions within the imaging target region by coherent detection, and wherein the system is configured to process the return laser signals, received from respective positions by the continuous-wave coherent lidar unit, to create at least one two-dimensional image of the imaging target region.

2. The system according to claim 1, comprising:

- a laser source,

- a coherent detector, and

3. The system according to claim 2, wherein the optical transceiver system is configured to receive a first portion of the laser light from the laser source, wherein the continuous- wave coherent lidar unit comprises an optical coupling unit configured to direct the return laser signal and a second portion of the laser light from the laser source to the coherent detector.

4. The system according to claim 2 or 3, wherein the optical transceiver module comprises an axially displaceable lens and wherein the system is configured to control axial displacement of the lens to focus the imaging beam on the detection target region.

5. The system according to any one of the preceding claims, wherein obtaining the position data comprises receiving the position data from an upstream sensor or from operating the continuous-wave coherent lidar unit in a detection mode.

6. The system according to any one of the preceding claims, wherein the continuous- wave coherent lidar unit is selectively operable in at least a detection mode and the imaging mode, wherein the continuous-wave coherent lidar unit is configured, when operated in the detection mode: to emit a continuous-wave laser detection beam towards a detection target region, optionally including scanning the detection beam across at least a portion of the detection target region, and to receive a return laser signal from the detection target region by coherent detection; wherein the system is configured: to process the return laser signal, received by the continuous-wave coherent lidar unit when operated in the detection mode, so as to detect one or more signal features indicative of the presence of an aerial vehicle in the detection target region, and to determine one or more attributes of the said aerial vehicle in the detection target region from the detected one or more signal features.

7. A system for detecting aerial vehicles, the system comprising a continuous-wave coherent lidar unit selectively operable in at least a detection mode and an imaging mode, wherein the continuous-wave coherent lidar unit is configured, when operated in the detection mode: to emit a continuous-wave laser detection beam towards a detection target region, optionally including scanning the detection beam across at least a portion of the detection target region, and to receive a return laser signal from the detection target region by coherent detection; wherein the system is configured: to process the return laser signal, received by the continuous-wave coherent lidar unit when operated in the detection mode, so as to detect one or more signal features indicative of the presence of an aerial vehicle in the detection target region, and to determine one or more attributes of the said aerial vehicle in the detection target region from the detected one or more signal features; wherein the continuous-wave coherent lidar unit is further configured, when operated in the imaging mode: to scan a focused continuous-wave imaging laser beam across an imaging target region, and to receive respective return laser signals from respective positions within the imaging target region by coherent detection, and wherein the system is configured to process the return laser signals, received from respective positions by the continuous-wave coherent lidar unit when operated in the imaging mode, to create at least one two-dimensional image of the imaging target region.

8. The system according to claim 6 or 7 , configured to operate the continuous-wave coherent lidar unit in the detection mode and, responsive to detecting one or more signal features indicative of the presence of an aerial vehicle in the detection target region, to cause the continuous-wave coherent lidar unit to operate in the imaging mode to obtain an image of an imaging target region associated with the detection area that includes the detected aerial vehicle.

9. The system according to any one of claims 6 through 8, wherein the one or more signal features include a Doppler signature of at least one propeller of an aerial vehicle.

10. The system according to any one of claims 6 through 9, comprising:

- a laser source,

- a coherent detector, and

- an optical transceiver system configured to receive laser light from the laser source and to selectively emit and/or scan the detection beam or the imaging beam, the imaging beam being focused towards the imaging target region, and wherein the optical transceiver system comprises a scanner module for scanning the detection beam and/or the imaging beam across the detection or imaging target region.

11. The system according to claim 10, wherein the optical transceiver system is configured to receive a first portion of the laser light from the laser source, wherein the continuous-wave coherent lidar unit comprises an optical coupling unit configured to direct the return laser signal and a second portion of the laser light from the laser source to the coherent detector.

12. The system according to claim 10 or 11, wherein the optical transceiver module comprises an axially displaceable lens and wherein the system is configured to control axial displacement of the lens to focus the imaging beam on the detection target region.

13. The system according to any one of claims 6 through 12, wherein the detection beam is a collimated beam, a divergent beam or a focused beam.

14. The system according to any one of claims 6 through 13, configured to:

- operate the continuous-wave coherent lidar unit in the detection mode and to direct the detection beam towards said target region, optionally including scanning the detection beam across at least a portion of said target region,

15. The system according to any one of claims 6 through 13, configured to:

- operate the continuous-wave coherent lidar unit in the imaging mode and to scan the imaging beam across said target region to obtain one or more images of said target region,

- process the obtained one or more images to identify an aerial vehicle in said target region, - responsive to a failure to identify an aerial vehicle in said target region based on the obtained one or more images, cause the continuous-wave coherent lidar unit to operate in the detection mode and to direct the detection beam towards said target region.

16. The system according to claim 14 or 15, wherein obtaining the position data comprises receiving the position data from an upstream sensor.

17. The system according to any one of claims 6 through 16, wherein the at least one attribute of the aerial vehicle includes one or more of the following attributes: a type of aerial vehicle, a speed of the aerial vehicle, a rotational speed of a propeller of an aerial vehicle, a make and/or model of aerial vehicle, a transverse position of the aerial vehicle, a presence and/or size of payload carried by the aerial vehicle.

18. The system according to any one of the preceding claims, wherein the continuous- wave imaging laser beam has a wavelength between 1 pm and 2 pm and a linewidth of between 0.1 kHz and 10 MHz, such as between 0.1 kHz and 5 MHz, such as between 0.1 kHz and 1 Mhz.

19. The system according to any one of the preceding claims, wherein the continuous- wave imaging laser beam has a constant wavelength.

20. The system according to any one of the preceding claims, configured for mounting on a controllable pan-and-tilt mount.

21. The system according to any one of the preceding claims, configured to, when operated in the imaging mode, to reconstruct a signal-strength image from the return laser signal, the signal-strength image representing respective spatially resolved signal magnitudes of the respective return laser signals.

22. The system according to any one of the preceding claims, configured to, when operated in the imaging mode, to reconstruct a speed image from the return laser signal, the speed image representing respective spatially resolved Doppler shift frequencies of the respective return laser signals or spatially resolved radial velocities of the target.

23. The system according to any one of the preceding claims, configured, when operated in the imaging mode, to reconstruct a combined image from the return laser signal, the combined image being based on respective spatially resolved signal magnitudes of the respective return laser signals and on respective spatially resolved Doppler shift frequencies of the respective return laser signals or spatially resolved radial velocities of the target.

24. The system according to any one of the preceding claims, further configured to perform image processing of the at least one two-dimensional image to identify one or more properties of the detected aerial vehicle, in particular one or more of the following properties: a type of aerial vehicle, a speed of the aerial vehicle, a make and/or model of aerial vehicle, a configuration of the aerial vehicle, a transverse position of the aerial vehicle, an orientation of the aerial vehicle, a presence, size and/or profile of payload carried by the aerial vehicle.

25. The system according to any one of the preceding claims, further configured to output identification data indicative of a property of one or more identified aerial vehicles, in particular a transverse position, a speed, a size, a shape, an orientation and/or a threat level associated with the one or more identified aerial vehicles, to a computer, to a control system, to an interdiction system, to an upstream sensor, to a downstream sensor, and/or to a controllable pan-and-tilt mount controller.