GB2327002A - Collision avoidance system - Google Patents

Description

Optical collision avoidance system The invention relates to a proximity warning system based upon the automatic detection of pulsed light sources such as strobe lights.

The occurrence of mid air collisions between aircraft has presented an air safety problem which the Civil Aviation Authority (CAA), Ministry of Defence (MOD) and other authorities including the Federal Aviation Administration (FAA) in the USA have been examining for some time.

There have been a number of approaches considered previously for aircraft detection induding radio based systems such as TCAS (Traffic Alert and Collision Avoidance System) for commercial aircraft and optical image processing techniques. Studies sponsored by the FAA have shown that pilot visual acquisition of intercepting aircraft can be low when not alerted by a pilot warning instrument and that the probability of pilot visual acquisition rises by nearly a factor of ten when a warning system, such as TCAS, is employed.

The MOD has primarily been concentrating on fast jet to fast jet collision avoidance for which the technical requirements (due to higher dosing speeds) are more demanding and for which solutions can be of greater cost and complexity than would be appropriate for light aircraft. For instance, the use of on-board radio transponders or sophisticated image processing techniques to detect other aircraft are likely to require high equipment and development costs which could not be justified for fitting to light aircraft.

The system of the present invention is primarily intended to be fitted to light aircraft. It is not intended that the system would ever replace the essential need for any pilot to constantly look outside the cockpit. A detection system may offer enhanced sensitivity compared with the human visual system, but most usefully it allows areas not readily viewed by a pilot to be monitored.

As will be explained in more detail below, a strobe flash has certain characteristics which enable it to be distinguished from any other flashing light, or other interfering sources. A system according to the invention may be sensitive to strobe lights with red and/or white or other coloured lenses.

Many aircraft carry such strobe lights as a warning of their presence and thus, strobe detection is a simple way of indicating to a pilot the proximity of another aircraft.

Such strobe lights are also carried on fixed structures, such as radio masts and tall buildings, whose proximity will also be indicated to a pilot, giving an extra degree of safety. In fact, the system would also indicate the proximity of police cars and ambulances.

Whilst the system has been designed with aircraft in mind, the system of the invention could have other uses, for example in vehicles. Thus, the invention is not limited to airborne applications.

The concept of a proximity warning system of the type described above is not in itself novel. NASA were considering such systems as long ago as 1972 and U.S. patent 3572928 disdoses a proximity warning system based on the detection of xenon flash lamps carried by aircraft. However, none of the earlier proposals seems to have resulted in a commercially viable system.

The present invention provides a method of detecting the proximity of a pulsed light source comprising the steps of: positioning a plurality of optical detectors with different fields of view at a detection station; processing signals from respective light detectors in separate channels; and comparing signals from adjacent channels, whereby a detection is generated only if signals on at least two adjacent channels indicate the proximity of a pulsed light source. The detectors are arranged to have overlapping fields of view. This channel-to-channel comparison results in a dramatic reduction in the false alarm rate due to natural interfering sources (as will be explained in more detail below).

The method according to the invention may comprise the determination of a detection threshold for each channel based on the average signal present on the channel in the absence of a nearby pulsed light source. This average signal will be indicative of the amount of noise present.

Signals on each channel may be filtered so that those not having desired characteristics (such as a typical "strobe" profile") are rejected.

The invention also provides a proximity warning system for detecting a nearby pulsed light source comprising an array of optical detectors having different fields of view and channels for processing signals from the respective detectors whereby a detection signal is generated only if at least two adjacent detectors indicate the proximity of a pulsed light source.

The presently preferred system according to the invention is an optical detection system for detecting a pulsed strobe light induding several optical detectors (arranged at different orientations to collect light over a wide field of view) fitted to the aircraft and connected via suitable electronics to a warning display in the aircraft cockpit. The field of view of an array will be larger than that for a single detector. A system according to the invention can provide information on the relative direction of a strobe source according to the relative signal strengths in the array of detectors.

The system may include an array or arrays of detectors giving a complete view horizontallv around an aircraft. The vertical field of view may be constrained to limit the interfering background light entering the system, particularly from the sun.

In practical airborne situations it is also desirable to provide additional detectors or arrays of detectors with upward and downward views to allow for the detection of other aircraft above and below.

The detectors may be arranged in one or more sensor units each comprising a generally circular or part-circular array of detectors distributed around the aircraft, with extra detectors oriented so as to look up and down in use.

Of critical importance to the success of the system is that it has a low false alarm rate (ideally less than once per hour) otherwise the system will be unacceptable to the pilot.

To achieve good sensitivity whilst achieving a low false alarm rate, the preferred system exploits four distinct techniques simultaneously: (i) Strobe lights have different spatial coherence properties from interfering sources.

In practical airborne situations the main source of interference for detection using the aircraft strobe lights is sunlight modulated by its passage through the atmosphere. This interference can enter the detector either as direct sunlight or indirectly as a reflection from another object (eg a cloud). The invention discriminates strongly against this type of interference by exploiting its transverse coherence properties, which are different from that of strobe light. This difference originates in the angular extent of the source as viewed from the detector: the sun or objects illuminated by it are spatially extended sources, whereas the strobe light appears as a point source. This results in a transverse coherence length of less than a centimetre for the sunlight, whereas the strobe light coherence is likely to be tens of centimetres. Therefore, by comparing the signal in detectors (which must have overlapping fields of view, and be separated by more than 1cam) the strobe signals are very similar whereas the interfering sunlight signals are not. The invention may therefore strongly reject this interfering signal using the process described above.

(ii) Strobe lights have a distinct flash time profile. The invention may therefore use a signal processing filter which detects the expected strobe pulse shape and rejects non strobe-like signals such as those caused by sunlight noise.

(iii) Strobe lights have a strong spectral peak in the near IR (in the vicinity of 950nm wavelength), whereas the peak in the optical spectrum of daylight is at shorter wavelengths. Placing a filter with a passband in the region 700-1,OOOnm into the system improves the signal-to-noise ratio for the wanted strobe light signal, in conditions where background daylight is the dominant noise source. The invention may therefore use IR filter material placed into the optical path of each detector to increase the signal-to-noise ratio when detecting strobe light against a background of daylight.

(iv) Rejection of non-directional interferers (eg electro magnetic interference (EMI) on all channels). The system may therefore reject any signals which do not exhibit the expected signal strength distribution between separate channels viewing the detected direction.

In addition, false alarms may be generated by an aircraft's own strobe lights (if fitted).

The invention avoids this by disabling the detection for the duration of the aircraft's own strobe flashes. Since the aircraft's own strobe flash duration is ~-O.2ms with a repetition rate of -1Hz, the probability of the system being "blind" to the flash from another aircraft with a similar strobe is only about 1 in 5000. Furthermore, the chances of a second flash also being coincident are very low as the strobe repetition rates are not exactly controlled. The invention provides for distinguishing other strobes from the aircraft's own strobes by the electronic detection of an own strobe flash using an electrical or electromagnetic sensor attached to the aircraft strobe's wiring.

In addition, false alarms may be caused by modulation of light by the propeller on the aircraft, especially for detectors which can view the propeller disk. This invention addresses this problem in two ways: (i) Ideally, the detectors are located on the aircraft so as not to view the propeller disk (eg by fitti:lg to the wingtips and tail as described in the specific design example below).

(ii) For those installations where practical installation constraints do not allow for detectors to be mounted as in (i) above, the invention may use additional signal processing to reject the signal generated by propeller modulation. This processing exploits the fact that: (a) The time profile of the propeller modulation is not the same as a strobe signal and a signal processing filter is used to discriminate against the propeller signal.

(b) The propeller noise is highly repetitive on short timescales (eg is) due to propeller rotation, and signal processing can be used to discriminate against a repetitive signal. This might be an adaptive filter, or a time delay and subtraction process. Propeller rotation rate may be calculated from the detector signal (eg by autocorrelation) or be measured from an electrical sensor on the aircraft.

The invention may utilize a number of novel implementation techniques which dramatically reduce the cost of the system, namely: (i) Low cost large-area solar cells may be used as the detectors. These offer the best possible sensitivity for a given cost, are robust, operate with the required speed and spectral response, and operate over the required temperature range.

(ii) A signal compressor (eg: logarithmic amplifier) in combination with a low-cost analogue to digital converter to produce a detection system with a wide dynamic range.

(iii) A field effect transistor (FET) controlled by a servo loop may be used to back-off the DC current caused by ambient background light falling on the optical detector to further increase the detection system's dynamic range.

A specific embodiment of the invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 shows a typical strobe flash profile; Figure 2 shows a channel response to a strobe flash; Figure 3 represents a block diagram of a possible strobe light detector system according to the present invention; Figure 4 is a plan view of an example of a detector array arrangement on a typical single-engined light aircraft; Figure 5 illustrates a block diagram of the analogue circuitry for processing signals from individual detectors; Figure 6 is a data flow diagram for an example digital signal processing system; Figure 7 shows a diagram of a suitable cockpit display.

A number of optical detectors collecting light from around the required field of view can enable a continuous strobe detection capability to be achieved. A wide angle (nonimaging) optical detection system can be employed to detect the proximity of strobe sources provided the light sensors are fast, have an optical response matched to the peak in the flash tube output and use correlation techniques to limit the effects of natural interference.

The operating conditions for a non-imaging detector to be used in this way are sumznrrised below: - high ambient background light level, hence a large dynamic range is required - operation required for both near and distant targets, also requiring a large dynamic range (for finding the direction of a detection) - audio type bandwidth required (-10kHz) - wide operating temperature range - very high direct exposure to sunlight and/or own strobes hence rapid recovery from saturation and high damage threshold - large area detectors required for maximum sensitivity - fine angular resolution not necessary - low cost - highly robust Silicon photodiodes (solar cells) offer a robust and low cost detector technology with the requisite response speed, spectral sensitivity, dynamic range and operating temperature range. The most common use for large area (several cm2) devices is in solar cell panels, and low-cost devices developed as solar cells provide a good basis for use in a strobe light detector device. An array of these devices, when arranged with optical filters, amplifiers and signal processing offers the most appropriate technology for the basis of the strobe light detector as it can take advantage of the spectral output and flash durations previously described. Such a system is described in some detail below.

The system includes several detector units comprising an array of solar cells. Each unit should be less than 100 mm in diameter, ideally, to simplify mounting and minimise any aerodynamic effects. Aerodynamic effects can also be reduced or eliminated by installing the detector units behind perspex windows from within the aircraft structure.

It is important to provide omnidirectional coverage and preferable to give an indication of the relative position of the other aircraft: this requires a number of similar detection channels, usually one per detector, to cover each of the sectors. The number of channels should be kept low because of the cost and size of the sensors and associated electronics. The example design illustrated here has a total of twelve channels distributed between three detector units each having four detectors: these provide coverage for every sector around the horizon as well as above and below the aircraft.

Three detector units are used, arranged to avoid having detector channels with a direct view of the propeller, whilst maintaining azimuthal coverage. The fields of view of all channels must have significant overlap (the field of view of each detector must be entirely covered by the view of other detectors) to facilitate channel to channel comparison used in noise rejection.

Each channel will see background sky radiation and be subject to other light signals, for example the sun, as well as strobe pulses, and the detector and electronics will introduce additional noise. Digital signal processing is therefore induded to maximize the detection range while maintaining an acceptably low false warning rate.

The system is based upon the detection of the near infra-red content of strobe light pulses of high intensitv and short duration, characteristic of those produced by flash tubes. A typical strobe flash profile is shown in Figure 1 and a typical solar cell response is shown in Figure 2.

A schematic diagram of the system is shown in Figure 3, and an example installation along with the layout of detectors is shown in Figure 4.

Figure 4 shows three detector units, 401, 402 and 403. Units 401 and 402 are mounted on the wing tips and detector 403 is mounted at the tail. Each detector unit 401, 402, 403 has four solar cell detectors indicated by reference numerals 404. Out of each set of four detectors, three cover the horizontal field of view and are arranged at 45" to each other to form an octagonal arrangement. Units 401 and 402 each have a detector 404 oriented forwardly, and a detector with a view below the aircraft denoted by dotted line squares in Figure 4. The tail unit 403 has a detector 404 with an upward field of view.

The solar cell detectors 404 of the tail unit 403 are housed in a protective dome 408 made from infrared filter material. The solar cell detectors 404 of the wing tip units 401 and 402 are mounted behind perspex windows 405 provided in the aircraft wing tip, and have an additional infrared filter 407 provided in their optical paths.

The detector units 401, 402 and 403 communicate with a processing and display unit 406 provided in the aircraft cockpit.

Each detector 404 comprises a large area solar cell positioned behind an IR filter 407 or 408 that only passes light in the near infrared band. The back surface of the IR filter is coated with a thin film of transparent conductor which is then grounded to the metallic shell of the detector unit housing, to provide EMC screening. The field of view of detectors looking horizontally is limited by baffles (not shown) to f20" in elevation, to limit the effects of ambient light sources. The horizontal view of detectors is not limited, to allow maximum overlapping field of view between detectors, which is required by the signal processing (detailed further below).

The components of the system are illustrated schematically in Figure 3. Each solar cell detector 404 is electrically connected to a detector analogue channel 500 to be described in more detail below with reference to Figure 5. The signals from the channels are passed to digitizer and telemetry circuitry 301 provided for each detector unit 401, 402 or 403 from where they are passed via hard-wired databuses 302 to receiving and processing unit 406 provided in the aircraft cockpit.

Telemetry receivers 303 decode received signals for supply to a processing unit 304 which provides signals to a visual cockpit display 700 (see Figure 7). The display includes user controls (08) which are an audio alert volume control knob (incorporating an on/off switch), an audio mute switch, and a self-test button. An audible warning device 305 may also be provided to alert the pilot's attention to the display. Cockpit unit 406 is connected to the aircraft's own strobe unit 306 to receive signals for disabling the system for the duration of the aircraft's own strobe.

Referring now to Figure 5, the analogue circuitry within each channel consists of the following elements: - Sensitive front end low-noise amplifier. Low-noise amplification of the detector output is important for achieving a system with an adequate detection range. The noise introduced should ideally be less than the current shot noise generated by the detector in daylight, as then the system approaches its theoretical limit of sensitivity: a system with a noise of lnV/wHz measured relative to the input will approach this limit.

- DC current back-oif circuitry 504. DC backoff circuitry is used to ensure that the detector operates at all times with zero or close to zero emf across it, as this maximises the detector's sensitivity. Circuitry 304 backs-off the DC current generated within the detector by background ambient light. The invention described might use a low-noise JFET 503 controlled via a DC servo loop.

- AC coupling and filtering circuit 505. AC coupling is used so that further stages of amplification do not saturate, and filtering is required to limit the bandwidth of the signal to match that expected from a strobe light flash time profile.

- Logarithmic amplifier 506. This is used to increase the dynamic range of the system cost-effectively, ie without using an expensive digitiser. The invention described might use a logarithmic amplifier with 50dB dynamic range to allow the system to indicate direction for both very distant and very near strobe lights.

- Anti-alias filter. This is used prior to the signal being digitised for noise reduction purposes, by an analogue to digital converter, not shown.

Figure 2 shows the response of a detector channel as described above to a typical strobe light source.

The signal from each detector channel is then input to a multiplexer and Fbit digitiser (not shown) at 20kHz for each channel. The digitisation is best done close to the detector channels to minimise the possibility that electromagnetic interference might add noise to the wanted signal.

The sensor units will be relativeiv exposed to the slip stream and a wide temperature range. Ideally electronics within the sensors should work from approximately 400C to 70 C.

Figure 6 shows a data rlow diagram for the digital signal processing system. The key processes are as follows: - Detection is based n an adaptive threshold for each channel, to maximise sensitivitv whilst maintaining an acceptable false alarm rate for all operating conditions likely to be experienced by the system. The adaptive threshold is calculated in the following steps: (i) After sampiing at step 600 signals are mitered at step 601 and then, for each channel, a running average for the magnitude of the amplitude is calculated at step 602. This 'average ampiitude" 603 is a good measure of the noise level in each channei. The average is calculated over timescales of order a second, to ensure that the calculated value is both accurate and the system can adapt to fast changes in lighting conditions.

n An adaptive threshold 605 for each channel is calculated at step 604 by tnuitislving the average amplitude bv a ratio ("Mapping"). The mapping can be adjusted according to (ie a function of) the lighting conditions or the number of false aiarms being generated. This allows the system to adapt to cnanging operationai conditions without generating excessive false warning or losing sensitivity.

- The filter at step 601 is designed to reject any signal that does not have the characteristic strobe-like response (eg as depicted in Figure 2). This might be achieved by a linear interpolation and a comparison with the expected shape or by correlating the signal with the expected strobe pulse response. A basic detection 606 passes this process if the signal peak exceeds the adaptive threshold for that channel.

- The signals from adjacent channels are then compared with each other at step 607. A warning is only indicated if the same strobe-like signal is present on adjacent channels (ie channels with an overlapping field of view) in the correct proportion. This process can also be used to provide a good estimate of the direction of the strobe detection, and to check that the signal appears to originate from a physical source (rather than, for example, electro-magnetic interference).

This process distinguishes strongly against false alanns caused by the principle source of natural interference, ie sunlight, because this interferer does not produce a signal that correlates well between adjacent channels.

- Rejected events 608, ie: detections with unacceptable face to face correlation, are used at step 609 to adjust the mapping, or ratio, 610 if necessary.

- Warnings are also suppressed at step 611 if the calculated average amplitude changes too rapidly on a given detection. This suppresses unwanted false warnings that might occur due to sudden lighting condition changes, eg if the aircraft flies out from doud into sunlight - Accepted events 612 are used to operate the cockpit user interface 700, at step 613, eg by warnings 614 under the control 615 of the user.

The signal processing system 304, Figure 3, also operates the display and audio.

The pilot display should preferably indicate to the pilot both the presence of another aircraft and its relative position. An audible indication is desirable to attract the pilot's attention to the display.

For maximum visibility, the display could be light emitting with a brightness automatically compensating for ambient cockpit light levels or controlled via existing cockpit lighting controls. The audio warning can be from a built-in loudspeaker and/or through existing radio headphones.

Figure 7 shows an example of how a simple display 700 could look based on twelve light emitting diodes 701 (LEDs) arranged around a facia and with three LEDs 702 to one side to indicate a hazard above, level with or below the aircraft. The display size could be made to match standard instrument mountings. The cockpit unit 700 could be miniaturised enough to allow its installation behind the display within the instrument dash.

Claims (19)

1. A method of detecting the proximity of a pulsed light source comprising the steps or; positioning a plurality of optical detectors with different fields of view at a detection station; processing signals from respective light detectors in separate channels; and comparing signals from adjacent channels, whereby a detection signal is generated only if signals on at least two adjacent channels indicate the proximity of a pulsed light source.

2. A method as daimed in claim 1 comprising determining a detection threshold for each channel based on the average signal present on the channel in the absence of a nearby pulsed light source.

3. A method as claimed in claim 2 in which detection signals are suppressed if the calculated average amplitude changes particular rapidly at the time of detection.

4. A method as claimed in claim 2 or 3 in which the detection threshold is adjusted according to the frequency of false detections.

5. A method as claimed in claim 2, 3 or 4 in which the detection threshold is adjusted according to the frequency of signals which exceed the detection threshold but do not appear on adjacent channels.

6. A method as claimed in any preceding claim in which signals on each channel are filtered to reject signals that do not have desired characteristics.

A . A method as claimed in claim 6 in which the signals on each channel are filtered to match the time profile of a strobe light flash.

8. A method as claimed in any preceding claim in which the detectors are arranged to have overlapping fields of view.

9. A proximity warning system for detecting a nearby pulsed light source comprising an array of optical detectors having different fields of view and channels for processing signals from the respective detectors whereby a detection signal is generated only if at least two adjacent detectors indicate the proximity of a pulsed light source.

10. A system as claimed in claim 9 in which the detectors comprise solar cells.

11. A system as claimed in claim 9 or 10 including means for filtering signals from each of the detectors to reject signals that do not have certain time characteristics.

12. A system as claimed in claim 9, 10 or 11 in which signals from each detector are filtered to match the time profile of a strobe light flash.

13. A system as claimed in any of claims 9 to 12 in which the detectors have overlapping fields of view.

14. A system as claimed in any of claims 9 to 13 comprising means for determining a detection threshold for each channel whereby a detection signal is generated only if the signal on the channel exceeds the threshold.

15. A system as claimed in any of claims 9 to 14 in which each detector is shielded by an infrared filter.

16. A system as claimed In any of claims 9 to 13 including d.c. blocking means on each channel.

17 A system as claimed in claim 16 in which the d.c. blocking means comprise a transistor controlled by a d.c. loop.

18. A method of detecting the proximity of a pulsed light source substantially as hereinbefore described with reference to the accompanying drawings.

19. A proximity warning system uostannally as hereinbefore described with reference to the accompanying drawings.