DE102023118273A1 - Radar-based detection of near-surface objects - Google Patents

Abstract

The invention relates to a device for radar-based location (xo, yo, to) of any objects (2) beneath the surface (3), such as landmines, cavities, metal bodies or rock fragments. For this purpose, the device is based on an antenna arrangement (1) which is moved at a defined height (h) and along a defined, lateral movement trajectory (y) relative to the surface (3). In addition to a first radar antenna (11) which is directed vertically (z) downwards towards the surface (3), the antenna arrangement (1) according to the invention comprises a second radar antenna (12) which is arranged with a right-angled lateral offset (dx) to the first radar antenna (11) in relation to the movement trajectory (y) and is aligned at a defined angle (α) to the first radar antenna (11) in relation to the vertical (z). The lateral offset (dx), the angle (α) and the height (h) are selected so that the main radiation directions of the radar antennas (11, 12) intersect at the surface (3). This arrangement according to the invention increases the transmission/reception dynamics. However, the dimensions of the antenna arrangement (1) remain comparatively compact.

Description

The invention relates to radar-based location of objects that are close to the surface, i.e. directly beneath a surface.

The ability of radar signals to penetrate dielectric materials has become increasingly important in materials testing due to the increasing availability of inexpensive and small microwave radars. One application in this area is the detection of objects or anomalies in optically opaque, dielectric environments.

A special task within this broad application area is the detection of objects that are only slightly below the surface of a medium. The distance resolution of the corresponding radar method is limited by its bandwidth: If the object to be detected is located a short distance below the surface, it is often not possible to separate it from the surface reflection due to the limited distance resolution. In addition, the backscatter intensity of small foreign bodies is typically much smaller than the section of the surface that is illuminated by the main beam of the corresponding radar antenna. The proportion of the received radar signal reflected by the object can therefore be negligible.

This specific problem occurs, for example, with the following types of objects: mines buried just below the ground surface, cavities and undercuts under a road surface, foreign bodies or anomalies just below the surface of concrete layers, screed layers, defects in compacted soils in the construction industry, foreign bodies from machines or processes in food production in surface-passing processes, foreign bodies under film coatings, or protection of surface-processing machines. In this context, the term "just" below the surface is defined as a maximum of 2 free-space wavelengths of the transmitted or received radar signals. Furthermore, the object to be detected is considered "small" within the scope of this invention if its dimensions are at most in the order of magnitude of a free-space wavelength of the radar signal.

Radar-based detection methods are known, particularly in the context of landmine localization: A method based on the principle of “Stripmap-SAR (Synthetic Aperture Radar; in German also known as the synthetic aperture method)” is described in the non-patent literature, for example Heinzel et al.; “Focussing Methods for Ground Penetrating MIMO SAR Imaging within Half-Spaces of Different Permittivity”; EUSAR 2016; Hamburg June 2016 The method described there is based on the use of an antenna arrangement that includes at least two offset radar antennas, which allows the location of buried objects to be mapped three-dimensionally. In order to be able to determine the location precisely, it is necessary to choose an appropriately large offset between the two antennas.

In the publication DE 102017112210 A1 a radar-based method for determining the location of a landmine in a search area is described: In this case, the surface of the corresponding search area is flown over by a drone along a quasi-closed circular trajectory at a defined altitude, whereby the drone comprises a radar-based device whose antenna arrangement is based on a radar antenna aligned laterally at an angle towards the inside of the circle. While flying around, the device regularly records distance values or the corresponding signal propagation times, assigning the corresponding flight position. Based on the flight position-dependent signal propagation times and the flight altitude, the location of the landmine is then determined using the synthetic aperture method. However, flying around the search area in a circle requires a lot of time per area.

The invention is therefore based on the object of improving the search for near-surface objects, such as landmines, in particular from these points of view.

• - Eine Antennen-Anordnung, die in einer definierten Höhe und in einer definierten, lateralen Bewegungstrajektorie relativ über der Oberfläche bewegbar ist, mit
  • ◯ einer ersten Radar-Antenne, die in etwa vertikal (also +/- 5°) nach unten gen Oberfläche gerichtet ist,
  • ◯ einer zweiten Radar-Antenne, die in Bezug zur Bewegungstrajektorie mit einem ersten, rechtwinkligen Lateral-Versatz zur ersten Radar-Antenne angeordnet und in Bezug zur Vertikalen in einem definierten Winkel zur ersten Radar-Antenne hin ausgerichtet ist,
  wobei der Lateral-Versatz, der Winkel und die Höhe derart gewählt sind, dass sich Hauptabstrahlrichtungen der Radar-Antennen in etwa an der Oberfläche kreuzen,
• - eine Hochfrequenz-Einheit, mittels der an definierten Positionen entlang der Bewegungstrajektorie jeweils
  • ◯ ein Radar-Signal gemäß eines definierten Radar-Verfahrens in eine der Radar-Antennen einkoppelbar und nach Reflektion am Objekt mittels der anderen Radar-Antenne als Empfangs-Signal empfangbar ist, und
• - eine Auswerte-Einheit, welche ausgelegt ist,
  • ◯ für jede Position entlang der Bewegungstrajektorie gemäß des Radar-Verfahrens anhand der Radar-Signale jeweils eine entsprechende Laufzeit und Intensität einer vom Objekt verursachten Reflektion zu ermitteln, und
  • ◯ mittels der Methode der synthetischen Apertur anhand der Höhe, der ermittelten Laufzeiten und der Intensität, sowie den korrespondierenden Positionen entlang der Bewegungstrajektorie den Ort des Objektes zumindest lateral zu bestimmen.

The invention solves this problem by a device for radar-based location of an object beneath a surface, wherein the device comprises the following components:

• - An antenna arrangement which is movable at a defined height and in a defined lateral movement trajectory relative to the surface, with
  • ◯ a first radar antenna, which is directed approximately vertically (i.e. +/- 5°) downwards towards the surface,
  • ◯ a second radar antenna, which is arranged with a first, right-angled lateral offset to the first radar antenna in relation to the movement trajectory and is aligned at a defined angle to the first radar antenna in relation to the vertical,
  where the lateral offset, the angle and the height are chosen such that the main distance beam directions of the radar antennas intersect approximately at the surface,
• - a high-frequency unit, by means of which at defined positions along the movement trajectory
  • ◯ a radar signal can be coupled into one of the radar antennas according to a defined radar method and, after reflection from the object, can be received as a reception signal by the other radar antenna, and
• - an evaluation unit which is designed
  • ◯ to determine for each position along the movement trajectory according to the radar method, based on the radar signals, a corresponding travel time and intensity of a reflection caused by the object, and
  • ◯ to determine the location of the object, at least laterally, using the synthetic aperture method based on the height, the determined travel times and the intensity, as well as the corresponding positions along the movement trajectory.

If the angle at which the second antenna is aligned is taken into account, the evaluation unit can also determine the depth of the object below the surface if designed accordingly. This means that an object such as a landmine, a cavity, a metal body or a rock fragment can be detected.

The arrangement of the antennas according to the invention results in a synergistic effect in that, on the one hand, the transmission/reception dynamics for a small object to be detected is increased. On the other hand, the dominant, planar surface reflection is reflected away due to Snell's law and is therefore less strongly received by the receiving antenna. In addition, the lateral offset of the antennas relative to one another and the dimensions of the antenna arrangement as a whole can be kept comparatively compact.

To determine the signal propagation times, the FMCW or pulse propagation time method can be implemented in the high-frequency unit or in the evaluation unit as a radar method. The frequency with which the device according to the invention generates or transmits the radar signal should preferably be selected so that it is reflected as well as possible by the object without being reflected by the surface, such as the floor, of the search area. For this purpose, it is advisable according to the invention for the high-frequency unit to generate the radar signal at a frequency of higher than 0.5 GHz and in particular in a frequency band of 1 GHz to 10 GHz. In this context, it is also optimal if the second antenna is aligned at an angle of between 30° and 60° in relation to the vertical.

In order to achieve the relative movement of the antenna arrangement to the surface, the device according to the invention can comprise, for example, a vehicle or an aircraft such as a multicopter, by means of which the antenna arrangement is guided over the surface at the defined height along the defined, lateral movement trajectory.
the unmanned aircraft or the land vehicle can be equipped with a satellite or inertia-based navigation system, or an optically supported navigation system, in order to move along the movement trajectory with as little acceleration as possible. Optionally, the satellite or inertia-based positions can be recorded or any deviations from the planned movement trajectory can be recorded in order to correct the synthetic aperture method accordingly with regard to determining the location of the object.

In contrast to the variant in which the antenna arrangement is moved over the fixed surface, it is also conceivable within the scope of the invention that the antenna arrangement is arranged statically at the defined height and the surface to be examined with the object to be detected is moved against the movement trajectory under the antenna arrangement, such as on a conveyor belt.

Which of the two radar antennas is used for sending or receiving is in principle irrelevant due to the reciprocal radar propagation properties. The high-frequency unit can either transmit the radar signal towards the surface via the second, obliquely arranged radar antenna and receive the corresponding reception signal after reflection via the first, vertically downward-oriented radar antenna, or vice versa. To increase the probability of detection on natural surfaces with unknown, slight inclines, the antenna arrangement can optionally also include a third radar antenna, which is directed approximately vertically downwards in accordance with the first radar antenna, but has a horizontal offset from the first radar antenna and, like the first radar antenna, is used for receiving. For this purpose, appropriate interferometric or phase-based methods can be used in the evaluation unit, which relate to the signals received by the two radar antennas.

Since the height of the antenna arrangement above the surface is required for object detection according to the invention, che must be known or correctly set, the high-frequency unit can be designed so that, for example, in a specially designed mode of the device, it sends out the radar signal via the first antenna and, after reflection on the surface, receives it again via the first antenna in order to determine the actual, current altitude based on the corresponding signal propagation time. In this case, the device can adjust the altitude accordingly if it deviates from its target value.

• - Relative Bewegung der Antennen-Anordnung in einer definierten Höhe über der Oberfläche und in der definierten, lateralen Bewegungstrajektorie, wobei in definierten Positionen entlang der Bewegungstrajektorie gemäß des Radar-Verfahrens anhand der jeweiligen Radar-Signale jeweils entsprechende Laufzeiten ermittelt werden, und
• - Bestimmung des Objektes am Ort anhand der Höhe, der ermittelten Laufzeiten sowie der korrespondierenden Positionen entlang der Bewegungstrajektorie.

The method for radar-based detection of the location of the object on the surface corresponding to the device according to the invention comprises the following method steps:

• - Relative movement of the antenna arrangement at a defined height above the surface and in the defined lateral movement trajectory, whereby in defined positions along the movement trajectory according to the radar method, corresponding travel times are determined based on the respective radar signals, and
• - Determination of the object at the location based on the height, the determined travel times and the corresponding positions along the movement trajectory.

It is advantageous to move the antenna arrangement along the movement trajectory at such a speed and/or to determine the respective signal runtime at such a clock rate that the individual positions at which the runtime is determined in relation to the movement trajectory are spaced apart by a maximum of half a wavelength of the radar signal. This ensures sufficient spatial resolution of the object to be detected.

A filtered back projection algorithm can be used to create a two-dimensional visualization of the object's location along the motion trajectory. This can then be displayed on a mobile device such as a tablet PC.

• 1: Eine erfindungsgemäße Antennen-Anordnung zur SAR-basierten Suche von Objekten unter einer Oberfläche.

The invention is explained in more detail using the following figure. It shows:

• 1 : An inventive antenna arrangement for SAR-based search of objects beneath a surface.

Imaging methods that are based on radar-based time-of-flight measurement and are generally used to locate objects are already technically mature and are in commercial use. They are based on the principle of the so-called “synthetic aperture radar” (SAR) and are already used, among other things, to record terrain topographies using satellite-based radar. For this purpose, the surface 3 to be examined is flown over with a corresponding device along a defined movement trajectory y and at a known height h, with an antenna arrangement 1 of the device sending a radar signal S _HF towards the surface 3 at defined positions y _i along the movement trajectory y and receiving it as a received signal R _HF after reflection there. The transmission and reception take place on the basis of a defined time-of-flight method, such as the FMCW or pulse time-of-flight method. To generate and process the radar signals S _HF , R _{HF ,} the device comprises, depending on the implemented time-of-flight method, a suitable high-frequency unit which, in the case of the FMCW method, is based on a PLL (“Phase Locked Loop”) on the transmitting side, for example, and on a mixer and subsequent Fourier transformation on the receiving side.

According to the principle of "ground penetrating radar", it is known that the antenna arrangement 1 is not aligned vertically downwards towards the surface 3, but rather at a defined angle α between 30° and 60° in relation to the vertical z. As a result, the intensity of the received signal R _HF is not dominated by the reflection of the transmitted radar signal S _HF on the surface 3, but the intensity also results to a sufficient extent from the reflection on any object 2 to be detected below the surface 3. This is also supported by the fact that the high-frequency unit is set to radar frequencies above 0.5 GHz, preferably in a frequency band between 1 GHz and 10 GHz.

1 shows a corresponding embodiment of the antenna arrangement 1 according to the invention. According to the invention, however, a first radar antenna 11 of the antenna arrangement 1 is aligned vertically downwards, while a second antenna 12 is arranged in contrast at an angle α of 45° in relation to the vertical, with alignment x towards the first antenna 11. The term "alignment" in relation to the radar antennas 11, 12 refers to their main radiation directions.

bemessen. Hieraus folgt, dass sich die Hauptabstrahlrichtungen der zwei Radar-Antennen 11, 12 an der Oberfläche 3 kreuzen, sofern sich die Radar-Antennen 11, 12 genau in der vorgesehenen Höhe h befinden, wie in 1 illustriert ist. Hieraus wird zudem ersichtlich, dass die laterale Achse x, die durch den lateralen Versatz dx zwischen den zwei Radar-Antennen 11, 12 ausgebildet wird, möglichst senkrecht zur Bewegungstrajektorie y verlaufen muss. Diese Vorgabe ist bei der Positionierung der Antennen-Anordnung 1 an demjenigen (Luft-) Fahrzeug der erfindungsgemäßen Vorrichtung, welches die Antennen-Anordnung 1 während der Ortung trägt, konstruktiv bzw. steuerungstechnisch entsprechend zu berücksichtigen. Bei der Darstellung in 1 ist die Bewegungstrajektorie y geradlinig. Prinzipiell ist es jedoch auch denkbar, dass die Bewegungstrajektorie y einen lateral zickzackförmigen, elliptischen, kreisförmigen oder kreissegmentförmigen Verlauf aufweist.At the same time, the second radar antenna 12 is arranged with a defined lateral offset dx to the first radar antenna 11. The lateral offset dx is determined according to the relationship $$dx=h\ast \text{tan}\left(\alpha \right)$$

It follows that the main radiation directions of the two radar antennas 11, 12 cross at the surface 3, provided that the radar antennas 11, 12 are located exactly at the intended height h, as in 1 This also shows that the lateral axis x, which is formed by the lateral offset dx between the two radar antennas 11, 12, must run as perpendicularly as possible to the movement trajectory y. This requirement must be taken into account in terms of design and control technology when positioning the antenna arrangement 1 on the (air) vehicle of the device according to the invention which carries the antenna arrangement 1 during the location. In the representation in 1 the movement trajectory y is straight. In principle, however, it is also conceivable that the movement trajectory y has a laterally zigzag, elliptical, circular or circular segment-shaped course.

Advantageous in 1 The advantage of the antenna arrangement 1 according to the invention shown is that the influence of the surface 3 as a disturbing reflection source is minimized by the orientation of the second radar antenna 12 tilted by 45°, but the vertical orientation of the first antenna 11 enables a high transmission/reception dynamic.

According to the SAR principle, each transmitted or received radar signal S _HF , R _HF is calculated from a 1 A corresponding runtime t _i or signal intensity S _i is determined by the device's evaluation unit (not explicitly shown). This is then assigned to the position y _i of the device on the movement trajectory y at which the underlying radar signal S _HF , R _HF is transmitted and received. If designed accordingly, the evaluation unit can record the signal intensity S _i in a complex value or taking the phase into account. This has an advantageous effect on the SAR method in that in this case any phase corrections do not have to be replaced by corresponding runtime corrections.

• - die - immer gleiche - Höhe h,
• - die jeweilige Position y_i auf der Bewegungstrajektorie y, und
• - die entsprechende Laufzeit t_i bzw. Signalintensität S_i umfasst. Die Anzahl an Datenpunkten innerhalb des Datensatzes ergibt sich wiederum aus
• - der Länge der Bewegungstrajektorie y
• - der relativen Geschwindigkeit der Vorrichtung entlang der Bewegungstrajektorie y, und
• - der Messrate, in der die Radar-Signale S_HF, R_HF ausgesendet/empfangen werden.

Overall, based on the SAR method, a data set of data points is recorded over the motion trajectory y, with each data point

• - the - always the same - height h,
• - the respective position y _i on the motion trajectory y, and
• - the corresponding runtime t _i or signal intensity S _i . The number of data points within the data set is again determined by
• - the length of the motion trajectory y
• - the relative speed of the device along the motion trajectory y, and
• - the measurement rate at which the radar signals S _HF , R _HF are transmitted/received.

The synthetic aperture method can be applied to the resulting data set. This allows an appropriately implemented algorithm in the evaluation unit of the device according to the invention to determine the location x _o , y _o of the object 2 at least laterally based on the recorded data set. By mathematically including the angle α and the at least approximately known dielectric constant in the vicinity of the object 2 below the surface 3, the depth t ₀ of any object 2 below the surface 3 can also be determined.

This means that the density of data points along the movement trajectory y is higher the slower the speed or the higher the measurement rate. The contrast or the intensity of the object reflection increases with the density of data points. The resolution in the lateral direction x and in the depth z or t ₀ increases with the length of the movement trajectory y and the bandwidth of the radar signal.

In order to achieve a sufficient unambiguous resolution in the direction of the motion trajectory y, these parameters must be chosen such that the data points or the positions y _i each have a distance of at most half the wavelength that corresponds to the mean frequency of the radar signal S _HF , R _HF .

The reference system to which the determined location coordinates x _o , y _o , t ₀ of the object 2 refer is not fixed per se within the scope of the invention. A local reference point can be defined. In this case, optical triangulation methods (for example laser-based with corresponding reflectors) can be used to determine the position y _i of the device in relation to the local reference point. The geographical coordinate system can also be used as a reference system. For this purpose, the device according to the invention can, for example, be equipped with a GPS receiver (“Global Positioning System”) so that it can determine its position y _i and thus the location x _o , y _o , t ₀ of any object 2 in geographical coordinates. The advantage of this is that the device can determine any deviation from the movement trajectory y or from the selected height h during the location determination on the basis of its GPS position y _i . This can be used to counteract the (air) vehicle and/or to subsequently calculate Compensation for this deviation can be used within the synthetic aperture algorithm.

list of reference symbols

1

antenna arrangement

2

object

3

surface

11

First radar antenna

12

Second radar antenna

dx

lateral offset

h

height of the antenna array above the surface

to

Depth of the object in relation to the surface

RHF

Reflected radar signal

SHF

radar signal

y

movement trajectory

yi

positions along the motion trajectory

z

vertical

α

Angle of the second antenna relative to the vertical

QUOTES INCLUDED IN THE DESCRIPTION

This list of documents listed by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions.

Cited patent literature

• DE 102017112210 A1 [0006]

Cited non-patent literature

• Heinzel et al.; “Focussing Methods for Ground Penetrating MIMO SAR Imaging within Half-Spaces of Different Permittivity”; EUSAR 2016; Hamburg June 2016 [0005]

Claims (16)

Device for radar-based detection of an object (2) at a location (x _o , y _o , t _o ) under a surface (3), comprising: - an antenna arrangement (1) which is movable at a defined height (h) and along a defined, lateral movement trajectory (y) relative to the surface (3), with ◯ a first radar antenna (11) which is directed approximately vertically (z) downwards towards the surface (3), ◯ a second radar antenna (12) which is arranged with a first, right-angled lateral offset (dx) to the first radar antenna (11) in relation to the movement trajectory (y) and is aligned with the first radar antenna (11) at a defined angle (α) in relation to the vertical (z), wherein the lateral offset (dx), the angle (α) and the height (h) are selected such that that the main radiation directions of the radar antennas (11, 12) intersect approximately at the surface (3), - a high-frequency unit, by means of which at defined positions (y _i ) along the movement trajectory (y) ◯ a radar signal (S _HF ) can be coupled into one of the radar antennas (11, 12) in accordance with a defined radar method and, after reflection at the object (2), can be received as a received signal (R _HF ) by the other radar antenna (11, 12), and - an evaluation unit which is designed ◯ for each position (y _i ) along the movement trajectory (y) in accordance with the radar method based on the respective radar signal (S _HF , R _HF ) to determine a corresponding runtime (t _i ) and intensity (S _i ) of a reflection caused by the object (2), and ◯ to use the method of synthetic Aperture to determine the location (x _o , y _o ) of the object (2) at least laterally based on the height (h), the determined travel times (t _i ) intensity, as well as the corresponding positions (y _i ) along the movement trajectory (y). device according to claim 1 , wherein the evaluation unit is designed to additionally determine a depth (t ₀ ) of the object (2) in relation to the vertical (z) based on the angle (α). device according to claim 1 or 2 , whereby the FMCW or pulse transit time method is implemented as radar method in the high frequency unit and in the evaluation unit for determining the signal transit times (t _i ). Device according to one of the preceding claims, wherein the high-frequency unit is designed to generate or process the radar signals (S _HF , R _HF ) with a frequency of higher than 0.5 GHz, in particular in a frequency band from 1 GHz to 10 GHz. Device according to one of the preceding claims, comprising: - A vehicle by means of which the antenna arrangement (1) can be moved at the defined height (h) along the defined lateral movement trajectory (y) over the surface (3). device according to claim 1 until 4 , comprising: - An in particular unmanned aircraft such as a multicopter, by means of which the antenna arrangement (1) can be guided at the defined height (h) along the defined, lateral movement trajectory (y), in particular vertically over the surface (3). device according to claim 5 or 6 , wherein the unmanned aircraft or the vehicle is designed to move along the movement trajectory (y) on the basis of a satellite or inertia-based navigation method and/or an optically supported navigation method, in particular without acceleration. device according to claim 7 , which is designed to record the satellite or inertia-based positions (y _i ) and to detect any deviations from the movement trajectory (y) in order to correct the synthetic aperture method accordingly. Device according to claim 1 until 4 , wherein the antenna arrangement (1) is statically arranged at the defined height (h), and wherein the surface to be examined (3) is movable counter to the movement trajectory (y) beneath the antenna arrangement (1). Device according to one of the preceding claims, wherein the high-frequency unit is designed to transmit the radar signal (S _HF ) via the second radar antenna (12) towards the ground (3) and to receive the received signal (R _HF ) after reflection via the first radar antenna (11) aligned vertically downwards. Device according to one of the preceding claims, wherein the second antenna (12) is aligned with respect to the vertical (z) at an angle (α) between 30° and 60°, in particular 45°. Device according to one of the preceding claims, wherein the antenna arrangement (1) comprises a third radar antenna which is directed approximately vertically (z) downwards and has a horizontal offset to the first radar antenna (11). Device according to one of the preceding claims, wherein the high frequency unit is designed to transmit the radar signal (S _HF , R _HF ) via the first antenna (11) and, after reflection, to received, and wherein the evaluation unit is designed to determine the height (h) based on the corresponding signal propagation time (t _i ). Method for radar-based detection of the location (x _o , y _o , t _o ) of the object (2) on the surface (3) by means of the device according to one of the preceding claims, comprising the following method steps: - relative movement of the antenna arrangement (1) at a defined height (h) above the surface (3) and in the defined, lateral movement trajectory (y), wherein in defined positions (y _i ) along the movement trajectory (y) according to the radar method corresponding travel times (t _i ) are determined on the basis of the respective radar signals (S _HF , R _HF ), and - determination of the object (2) at the location (x _o , y _o , t _o ) on the basis of the height (h), the determined travel times (t _i ) and the corresponding positions (y _i ) along the movement trajectory (y). procedure according to claim 13 , wherein the antenna arrangement (1) is moved relatively along the movement trajectory (y) at such a speed and/or the respective signal propagation time (t _i ) is determined at such a clock rate, so that the individual positions (y _i ) at which the propagation time (t _i ) is determined in relation to the movement trajectory (y) are spaced apart by a maximum of half a wavelength of the radar signal S _HF , R _HF . Method according to at least one of the Claims 13 until 14 , wherein a filtered back projection algorithm is applied for a two-dimensional visualization of the location (x _o , y _o , t _o ) of the object (2) along the motion trajectory (y). procedure according to claim 13 until 15 , wherein the object (2) detected is a landmine, a cavity, a metal body or a rock fragment.

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