Abstract:
A non-scanning radar system is installed on an aircraft to detect and avoid bird strikes or collisions with other airborne hazards. Target amplitude, range, and Doppler tracking versus time are used to qualify the collision threat. Avoidance is based on a quick minor altitude change by the pilot or autopilot to exit the imminent bird or other airborne hazard altitude window. In one embodiment, a bistatic passive radar receiver antenna is used in conjunction with an existing geostationary satellite signal. Range and Doppler information are obtained via cross correlation processing of the hazard reflection signal with a direct path reference signal from the satellite.

Description:
RELATED INVENTION 
     The present invention claims priority under 35 U.S.C. §119(e) to: “Aircraft Bird Strike Avoidance Method and Apparatus,” Provisional U.S. Patent Application Ser. No. 61/205,697, filed 22 Jan. 2009, which is incorporated by reference herein. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     The present invention relates to the detection and avoidance of imminent aircraft collisions with birds or other airborne hazards such as other aircraft. 
     BACKGROUND OF THE INVENTION 
     Aviation experts say bird-plane collisions happen daily. Nearly 200 people have been killed or injured since 1990 in accidents involving aircraft and wildlife. It is estimated that bird strikes cost military and commercial aviation over $2 billion each year due to damage and other costs. Most birds fly below 5,000 ft. However, bird strikes have occurred at 17,000 ft. and a few sightings have been made above 20,000 ft. In general, birds fly higher at night and during the spring and fall migration periods. They also fly higher in the presence of complete cloud cover. 
     Airports take a variety of measures to reduce bird populations near major airports. In a few cases, ground based radars are used to detect flying birds near aircraft approach and departure paths. However, outside of these few major airports, no bird detection devices are currently available other than the pilot&#39;s see-and-avoid procedures. 
     It is possible to equip an aircraft with a specialized high resolution scanning radar to detect and display the presence of birds. However, such a radar would be expensive and it would be difficult to find antenna installation space even on the largest aircraft. 
     Accordingly, there is a need for small low cost aircraft based detection equipment that would alert the pilot to the possibility of an imminent collision with a bird or other airborne hazard. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, bird strike collision avoidance techniques are presented which will alert the pilot to take evasive action to avoid colliding with birds or other imminent collision hazards such as other aircraft. 
     Briefly, to achieve the desired object of the present invention, two types of radars are presented. Both radars use a non-scanning narrow elevation fan beam to detect the presence of hazards in a short range altitude slice ahead of the aircraft. Target amplitude, range, and Doppler tracking versus time are used to qualify the collision threat. Avoidance is based on a quick minor altitude change by the pilot or autopilot to exit the imminent bird, small aircraft, or other airborne hazard altitude window. 
     To obtain a narrow elevation fan beam antenna pattern, a tall thin antenna is required. A natural place to mount such an antenna is on the forward facing surface of the aircraft vertical tail section. 
     In one embodiment of the invention, a traditional non-scanning radar system is used to illuminate the narrow elevation fan beam volume ahead of the aircraft. 
     In another embodiment, a bistatic passive radar receiver antenna is used in conjunction with an existing (or future) geostationary satellite signal. Radar target range, amplitude, and Doppler information are obtained via cross correlation processing of the airborne hazard reflection signal with a direct path reference signal from the satellite. 
     In another embodiment, an additional non-scanning narrow azimuth fan beam receive antenna is mounted near the nose of the aircraft or on the leading edge of one wing. 
     In another embodiment, an additional non-scanning narrow azimuth fan beam receive antenna is mounted on the leading edges of both wings. 
     Other objects and advantages of the present invention will become obvious as the preferred embodiments are described and discussed below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an elevation fan beam antenna mounted on the vertical tail of an aircraft approaching an airborne hazard. 
         FIG. 2  illustrates the elevation cross section of the elevation fan beam antenna pattern and radar targets on and off antenna boresight. 
         FIG. 3  illustrates a positional sketch of a geostationary satellite  65 , an aircraft  20 , and an airborne hazard  10 . 
         FIG. 4  illustrates a signal path length sketch between geostationary satellite  65 , aircraft  20 , and airborne hazard  10 . 
         FIG. 5  illustrates the radar signal ambiguity function for various radar signals. 
         FIG. 6  illustrates a block diagram of a traditional transmit/receive radar system. 
         FIG. 7  illustrates a block diagram of a bistatic passive receive-only radar system. 
         FIG. 8  illustrates target triangulation using two receive antennas. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The basic problem and its solution using the present invention is illustrated in  FIG. 1 .  FIG. 1  illustrates a target  10  at range  15  in the flight path of aircraft  20 . Vertical tail mounted narrow elevation fan beam antenna  25  monitors radar reflection signals within the design surveillance volume of fan beam  30  of antenna  25 . Reference antenna  27 , when required, is mounted somewhere along the top surface of aircraft  20 . 
     Azimuth angle  34  and elevation angle  35  of fan beam  30 , along with maximum processing range, are selected based on the required bird strike threat surveillance volume and alert latencies. Signals beyond the maximum processing range are rejected in the radar signal detection processor  270  illustrated in  FIG. 6  and  FIG. 7 . 
     Maximum processing range can be varied based on aircraft speed and altitude. At low altitudes, the processing range can be restricted to avoid antenna  25  sidelobe ground reflections from being confused as possible bird reflections. Lower speeds also allow maximum processing range reductions without reducing alert latency. 
     In a scanning radar, the target parameters of range, elevation, azimuth, amplitude, and Doppler shift are available for target threat assessment. In non-scanning radar, only target range, amplitude, and Doppler shift are available.  FIG. 2  illustrates how the variation of these parameters as the aircraft closes with the target will be used by target processor  275  in  FIG. 6  and  FIG. 7  for threat assessment. 
       FIG. 2  illustrates target  10  along antenna  25  boresight (antenna maximum directivity) and target  12  offset from boresight in either azimuth, elevation, or both. As target  10  closes with aircraft  20 , its Doppler offset frequency and normalized amplitude remains constant. Normalized amplitude is defined here as the target amplitude compensated for range (range sensitivity time control). 
     In contrast, as target  12  closes with aircraft  20 , both its normalized amplitude and Doppler offset frequency decrease with range. The normalized amplitude decreases because the antenna gain decreases as the target&#39;s angular position off boresight increases. The target can actually disappear at close range as it exits the coverage volume of fan beam  30 . The Doppler shift decreases because the target radial velocity, directly towards the antenna, decreases and goes to zero as the target becomes perpendicular to the antenna. 
     This variation of normalized amplitude and Doppler frequency shift, as the aircraft approaches the target, is used to determine the collision threat probability. If the normalized amplitude and Doppler frequency offset are nearly constant as the aircraft approaches the target, the collision threat probability is high. If the normalized amplitude and Doppler offset decrease rapidly, the collision threat probability is low. When the collision threat probability exceeds a predetermined threshold, target processor  275  in  FIG. 6  and  FIG. 7  will alert the pilot to make a small altitude maneuver to avoid a collision. 
     Using a traditional radar, fan beam antenna  25  both transmits the radar signal and receives target reflections. Using a bistatic radar, fan beam antenna  25  is receive only. The bistatic radar transmitter and antenna are located at a distant location. 
       FIG. 3  is an exaggerated illustration of a bistatic radar where the transmitter is placed on geostationary satellite  65  at a height  66  of 22,236 miles above the equator and target  10  is primarily north of aircraft  20 . The geostationary satellite  65  signal is received by aircraft  20  via signal path  70  using top mounted reference antenna  27 . Target  10  is illuminated by geostationary satellite  65  signal via signal path  80 . The target reflected signal is received by aircraft  20  via signal path  75  using tail mounted fan beam antenna  25 . These reference and reflected signals are coherently cross correlated to obtain the aircraft-to-target range, amplitude, and Doppler shift values. 
     Using aircraft GPS information, aircraft-to-target range and Doppler shift information can be accurately determined from the known positions of geostationary satellite  65  and aircraft  20 . However, to a first approximation, a simple lookup table can be used to compensate cross correlated aircraft-to-target range and Doppler measurements for the geostationary satellite  65  signal path length differences between aircraft  20  and target  10  using the flat earth geometry illustrated in  FIG. 4 . 
     The elevation angle  67  to geostationary satellite  65  is equal to the latitude position of aircraft  20  which varies from around 30 degrees at the southern US border to 49 degrees at the northern US border. When target  10  is directly north of aircraft  20 , the cross correlated measured aircraft-to-target signal path  75  increases by the amount that signal path  80  is longer  79  than signal path  70 . Likewise, when target  12  is directly south of aircraft  20 , the cross correlated measured aircraft-to-target signal path  77  decreases by the amount that signal path  82  is shorter  78  than signal path  70 . The lookup table range parameter compensation is a simple cosine function of heading (north equal to 0 degrees) whose peak amplitude is based on the latitude of aircraft  20 . 
     No compensation is required when target  10  is directly east or west of aircraft  20  since path lengths  70 ,  80 , and  82  are equal. That is, the cross correlated measured distances  75  or  77  are equal to the actual physical aircraft-to-target distances. 
     Only the Doppler variation with range is important for threat assessment. Except for aircraft or targets in a tight turn, the geostationary satellite Doppler shift should not change during the threat observation time. Therefore, cross correlation Doppler shift variation is still primarily a function of the aircraft-to-target closure geometry as was illustrated in  FIG. 2   
     Radar detection is a very mature field with many choices available for frequency, transmit power, and waveform design. Both pulsed and CW waveforms can be used as is well known by anyone skilled in the art. 
     The present invention radar should be looking at empty space, free of targets, until a collision hazard appears. Therefore, its primary task is target detection. Any short range reflected energy is a potential cause for pilot concern. Target position detection is of secondary importance, but is required to determine the collision threat probability because statistically most small targets will harmlessly pass by the aircraft without actually colliding with the aircraft. 
     Radar detection is a function of transmit energy, irrespective of the transmit signal waveform design. However, range resolution and Doppler resolution are determined by the actual waveform design as illustrated in  FIG. 5 . 
       FIG. 5  illustrates the ambiguity function of common radar waveforms. Range and Doppler resolution are defined by the radar signal ambiguity function which is simply the cross correlation between the transmit pulse and its range and Doppler shifted versions as is well known by those skilled in the art. 
     Long pulse  200  is characterized by low range resolution  205  and high Doppler resolution  206 . Short pulse  210  is characterized by high range resolution  207  and low Doppler resolution  208 . Pseudo random noise (PN) coded pulse compression waveform  225  has both high range and Doppler resolution as illustrated by thumbtack response  209 . 
     Although pulse type radars can be used in the present invention, a more appropriate radar for this application is a CW radar using a PN coded pulse compression waveform  225 . A CW radar does not have the close range reception dead zone caused by transmit pulse blanking required in pulse type radars. A block diagram of a CW radar&#39;s preferred implementation is illustrated in  FIG. 6  for a traditional radar and in  FIG. 7  for a bistatic radar. 
     Transmitter/receiver section  240  couples transmitter  248  and receiver  247  to common antenna  25  using coupler  255 . These elements are placed close to antenna  25  to minimize RF signal losses. 
     Signal detection processor  270  digitizes radar reflection signals using analog to digital converter (ADC)  260 , cross correlates the transmit and received signals using Doppler bank of matched filters  265 , and quantizes their amplitude, range, and Doppler shift using signal detector  271 . Target processor  275  applies the threat algorithm to the varying range, amplitude, and Doppler shift values of the detected signals to determine collision threat probability. When a high probability collision threat is detected, an alert is issued to the pilot or a command is issued to the autopilot to commence an evasive maneuver. Slight altitude changes should be sufficient to exit the target collision threat window. 
     For the bistatic radar, reference antenna  27 , receiver  249 , and ADC  261  are added as illustrated in  FIG. 7 . The signals from antennas  25  and  27  are cross correlated in signal detection processor  270 . Target processor  275  alert outputs will be passed to existing aircraft aural, display, or autopilot resources using any appropriate interface method. For very light aircraft, any appropriate means of alerting the pilot can be implemented as is well known to those skilled in the art. 
     The use of a bank of Doppler matched filters is based on the assumption that the cross correlated radar signal will approximate a thumbtack ambiguity function, which provides high Doppler resolution, and therefore requires the cross correlated signals to be matched in Doppler offset. 
     The selection of transmit power, waveform design, and all radar hardware implementation tradeoffs are well known by practicing radar engineers for both traditional and bistatic radars. The radar cross section for a large variety of birds at different frequencies and aspect angles are available in the literature. The nominal radar cross section of a pigeon is 0.01 square meters. 
     Ideally, the FAA or other government entity could provide an optimally designed geostationary satellite signal to be used for the bistatic radar implementation of the present invention. The bistatic radar implementation lowers the aircraft equipment cost and complexity for all users within the geostationary satellite service coverage, with respect to a traditional radar implementation, since no aircraft transmitter is required. However, an existing geostationary satellite signal can also be used. 
     One of the most powerful geostationary satellites available is used for XM satellite radio. XM broadcasts a 4 MHz wide signal with a 98 dBm EIRP at a downlink center frequency of 2334.5 MHz. The processing gain of cross correlating a 4 MHz bandwidth signal using a 1 Hz and 10 Hz integration bandwidth is 66 dB and 56 dB respectively. Reflected target energy will increase or decrease by 6 dB for each dividing or doubling respectively of aircraft-to-target range. In contrast, it increases or decreases by 12 dB for a traditional radar. For both a traditional and bistatic radar, reflected target energy will increase by about 10 dB if the target is a flock of birds and by about 20 dB if the target is a small aircraft. 
     At an aircraft speed of 200 knots, this bistatic radar can provide approximately 10 seconds warning of an imminent collision with a flock of birds. Thus, the XM satellite signal power is adequate for use as a bistatic radar transmitter in the present invention. 
     It is important to note that the XM 4 MHz bandwidth signal does not have to be demodulated. Instead, this signal functions as a noise radar signal. In noise radar, a random noise source is used as the transmit signal. It has a thumbtack ambiguity function similar to a PN coded pulse compression waveform. Since the XM signal is composed of many random data subcarriers (music channels), the composite XM signal is essentially random noise before demodulation. 
     There are many other geostationary satellites available to choose from whose frequencies range from L-Band to Ka-Band. Most communication satellites contain 24 C-band and 24 to 36 Ku-band transponder channels of which half are vertically polarized and half are horizontally polarized. The bandwidth of each C-band Fixed Satellite Services (FSS) transponder channel is 36 MHz. The bandwidth of each Ku-band Broadcasting Satellite Services (BSS) transponder channel is 27 MHz. 
     In general, these other choices have lower power densities and wider bandwidths which makes signal cross correlation more difficult than using the XM signal. However, higher frequencies allow smaller and higher gain antennas to be used on the aircraft. Cross correlation can be performed for each individual transponder channel or for the entire satellite bandwidth. To obtain an even higher bistatic radar transmit power, the signal from multiple adjacent satellites could also be used. 
     Frequency reuse via geostationary satellite spacing typically requires that reference antenna  27  be steered towards the satellite being used. Such antennas are currently available for direct broadcast TV, Satcom, and other communication applications on various aircraft. XM has the advantage that it is a protected frequency designed to be received using a small low gain antenna with nearly hemispherical coverage. One option might be to use an existing or modified aircraft communication antenna, such as for Mode S, or other currently installed antenna for XM signal reference antenna  27 . 
     Detection sensitivity (peak of the ambiguity function) of cross correlated geostationary satellite signals can be increased by increasing the cross correlation integration time. 
     The shape of the ambiguity function is determined by the geostationary satellite&#39;s signal characteristics but should have a sharp peak and low sidelobes similar to the ambiguity function of PN coded signals due to their randomness. 
     Mounting additional antennas on the aircraft may be desirable in some implementations. For example, a non-scanning long narrow horizontal mounted receive antenna  31  illustrated in  FIG. 1 , which will have a narrow azimuth and wide elevation beamwidth, can be mounted near the nose of the aircraft or on the leading edge of a wing. This also requires the addition of another receiver/ADC section  242  as was illustrated in  FIG. 7  for reference antenna  27 . The purpose of this new implementation is to create two receive beams whose overlapping surveillance volume only includes the collision threat area. Target processor  275  would issue an alert whenever a target at similar ranges was simultaneously detected within the overlapping surveillance volume of both antennas. This implementation reduces the radar processing complexity since it alerts on any targets detected at similar range within the overlapping antenna beamwidths, but this implementation also requires an additional antenna and receiver/ADC section. 
     As another example, non-scanning narrow horizontal mounted receive antennas  32  and  33  can be mounted on the leading surface near the wingtips of both wings as illustrated in  FIG. 1 . Two additional receiver/ADC sections  242  are also required. In this implementation, the range and azimuth positions of target  310  are determined by triangulation of range ring  300  of antenna  32  and range ring  305  of antenna  33  as illustrated in  FIG. 8 . Using this implementation, the pilot can be given the threat direction via voice alerts, aircraft panel display, or heads-up windscreen indicator. 
     XM currently has two geostationary satellites parked at 85 and 115 degrees west longitude for spatial/frequency/time diversity. Both satellites can be used simultaneously to form a composite radar signal since their transmission frequencies are unique. Multiple geostationary satellites can also be used simultaneously. Likewise, multiple transmitters can be mounted on the aircraft and their signals transmitted through a single or multiple antennas. 
     The primary purpose of each of these implementations is to alert the pilot of imminent collision hazards after leaving the airport. During takeoff and landing, most aircraft cannot execute abrupt maneuvers to avoid bird strikes. Instead, the airport must take steps to minimize the presence of birds near the runways or delay takeoffs and landings until birds have moved a safe distance from the runways. 
     Although the preferred embodiments of the invention have been illustrated and described in detail, it will be readily apparent to those skilled in the art that various modifications may be made therein without departing from the spirit of the invention. 
     For example, separate co-located antennas (monostatic radar) can be used in  FIG. 6  in place of antenna  25  and coupler  255  to increase the transmit-to-receive isolation of a CW radar. Alternatively, antenna  25  could be used for transmit and antenna  31  for receiving. Or, both receiving antennas  32  and  33  could be used to provide azimuth triangulation. 
     Antenna locations other than those illustrated may be more appropriate. By using a very high transmit frequency, it may be possible to find adequate space to mount a large enough single circular or conformal patch antenna to simultaneously obtain both the desired elevation and azimuth beamwidths. 
     It is also possible to use the bistatic radar technique with existing terrestrial transmitters such as TV stations or with specially constructed transmitters, but service would be very local. However, a high frequency beacon directed along airport approach and departure routes might make sense with the present invention. 
     The concept of using a geostationary satellite signal as a bistatic radar transmit signal is not limited to aircraft collision hazard avoidance applications. This invention can also be used for open area terrestrial intrusion detection applications. With both the geostationary satellite transmitter and the ground receiving antenna stationary, Doppler filtering can be used to detect any moving objects. Two or more receiving antennas can be used to indicate the position, direction, and track of the movement. 
     Many other simple modifications are also possible without departing from the spirit of the invention.