Abstract:
A collision avoidance system mountable on an aircraft for providing to the pilot of that aircraft an early warning of the presence of another nearby threat aircraft within the surrounding air space. The system operates autonomously from that aircraft and does not require the presence of any matched system on board the threat aircraft. The system includes an omni-directional L-band microwave antenna formed by a dielectric sphere cut into eight equal “orange wedge” sectors covering eight distinct beam patterns. Eight L-band microwave signals are transmitted simultaneously from all eight dielectric sectors to provide a sphere of detection around the aircraft. The sectors also act as receivers for detecting a microwave signal reflected back from the threat aircraft, and indicating means provides information to the pilot regarding the direction, closeness and rate of closure of the threat aircraft.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to collision avoidance systems for aircraft and more particularly to a novel, low cost, universal collision, obviation and reduced near miss (UNICORN™) system which is particularly useful and affordable with small private aircraft. 
     In conditions of crowded air traffic zones and/or low visibility it is necessary that the pilot of one aircraft be warned of the presence of a nearby aircraft so that he may maneuver his aircraft to avoid a disastrous collision. Systems known as TCAS (Traffic Alert and Collision Avoidance Systems) employ an interrogator mounted on a commercial jet aircraft and transponders carried on each aircraft it is likely to encounter. In this way, an interrogation is communicated by radar between the aircraft carrying TCAS and smaller, threat aircraft in the vicinity. This is done so that an enhanced radar signal is returned to the jet aircraft to enable the commercial pilot to avoid a collision. The transponder also encodes the returned radar signal with information unique to the threat aircraft on which it is installed. A GPS receiver is also recommended in order to encode information regarding the position of a threat aircraft with respect to the position of the jet aircraft. With TCAS, the burden is on the commercial jet pilot to avoid a collision when an alert signal is received. 
     These systems however are very complicated and very costly and are used primarily on commercial sized aircraft. Because of the high cost, these systems are rarely incorporated on smaller, privately owned aircraft whose pilots are often flying blindly under adverse weather and traffic conditions, a situation which often leads to an unavoidable collision. General aviation pilots are also reluctant to incur the cost of installing a transponder without gaining autonomous control over averting a collision. 
     Thus, there is a need for a low cost, reliable warning and collision avoidance system particularly useful on smaller, privately owned aircraft to enhance flight safety conditions within the air traffic industry. The UNICORN™ system described hereinbelow fulfills that need. 
     SUMMARY OF THE INVENTION 
     Accordingly, the primary object of this invention is to provide a low cost, reliable, universal, omni-directional, sensing system mountable on an aircraft for providing to the pilot of that aircraft an early warning of the presence of another nearby aircraft within the surrounding air space, thus enabling the pilot to take whatever maneuvering action is necessary to avoid a collision with the other nearby aircraft. 
     Another object of this invention is to provide the above-identified system which operates autonomously from a single aircraft and requires no response from or the presence of any matched system on board the other nearby aircraft. 
     Still another object of the invention resides in the provision of the above described sensing system which includes an omni-directional L-band microwave antenna formed by a dielectric sphere cut into a plurality of “orange wedge” sectors, for example eight sectors, covering eight distinct transmitter/receiver direction patterns (up/down, port/starboard, forward/aft). An L-band microwave signal is transmitted simultaneously from all eight dielectric sections to provide an omnidirectional transmission covering a sphere of detection around the aircraft. In addition, the same eight dielectric sectors are employed together with suitable receiver circuitry for receiving microwave signals reflected back from other nearby aircraft. These sectors provide eight video channels by which a pilot of an aircraft receives information regarding the direction, closeness and rate of closure of the nearby aircraft so that a proper collision avoidance maneuver may be effected to avoid a disaster. 
     A further object of the invention resides in the above described system which, although autonomous within itself and requiring no cooperating response mechanism on nearby aircraft, is compatible and may be used together with TCAS-type transponders. 
     Other objects and advantages will become apparent from reading the following detailed description of the invention which makes reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a general perspective view of the UNICORN™ system of the invention illustrating the novel dielectric antenna mounted on top of an aircraft and the corresponding transparent display unit on or within the cockpit of the aircraft within easy vision of the pilot; 
     FIG. 1A illustrates the spherical antenna in enlarged view; 
     FIG. 1B illustrates the spherical antenna in exploded view; 
     FIG. 1C illustrates the transparent display unit in enlarged view; 
     FIG. 1D illustrates the display unit in exploded view; 
     FIG. 2 is a view taken along line  2 — 2  of FIG. 1B illustrating the coated surfaces on each of the sectors of the antenna; 
     FIG. 3 illustrates an electrical schematic of the sensing and control circuitry incorporated in the UNICORN™ system of the invention. 
    
    
     DETAILED DESCRIPTION OF INVENTION 
     The novel UNICORN™ system  10  of the invention as used primarily with smaller private aircraft  12  includes a hollowed-out spherical antenna  14  (FIG. 1A) constructed from a dielectric material such as polystyrene or Lucite. Antenna  14  is about twelve (12) inches in diameter and is mounted within a protective streamlined, unicorn shaped, transparent radar dome  16  fixed at  18  on the brow of the aircraft above the plane&#39;s cockpit. Dome  16  is constructed of a material which does not interfere with operation of antenna  14 . 
     As shown in the exploded view of FIG. 1B, antenna  14  is formed by dissecting a hollowed-out sphere into a plurality of equal “orange-wedge” sectors, e.g. eight equal octanes  14   a ,  14   b ,  14   c ,  14   d ,  14   e ,  14   f ,  14   g  and  14   h . Each octant has an inner edge  20  scooped out so that when the octants are fastened together as in FIG. 1A, a central cavity is formed which houses a signal transmitting and receiving component assembly  22  within. As will be described in detail with respect to FIG. 3, assembly  22  includes eight transmitting/receiving (T/R) diode switches  24  to which an L-band microwave signal is fed, the T/R switches when in the transmit mode then operating to drive simultaneously in phase all eight sectors or octant horns so as to transmit eight microwave signals omnidirectionally around antenna  14 . 
     Assembly  22  also includes eight low-power, low noise L-band microwave-to-video detection diodes  26 , one associated with each octant, for receiving any return signal reflected back from a nearly aircraft when the T/R switches  24  are in a receiving mode. 
     On each octant, the flat surfaces  30 ,  32  and  34  are coated with a conductive metal coating  36  such as silver or copper applied by a metal sputtering or vapor desposition process. The outer curved surface  38  of each octant is left uncoated and acts as a dielectric electromagnetic radiation and reception face. The coated surfaces  30 ,  32  and  34  on each ocatant guide or funnel the reflected waves received by outer face  38  inwardly to edge or corner  40  located immediately adjacent its associated detection diode  26 . 
     The spherical antenna  14  is assembled by applying cement or glue to the coated surfaces  30 ,  32 , and  34  of each octant and pressing the octants together with the component assembly  22  within the central hollow cavity. The cement should be of a type which may be dissolved by a solvent which does not damage the coatings  36  or the components of assembly  22 , thus repairs may be made to the antenna and a malfunctioning antenna need not be discarded. 
     Antenna  14  may be mounted on the aircraft  12  so that sector  14   d  is facing a port-up/forward direction, sector  14   a  starboard, up/forward, sector  14   c  port/up/aft, sector  14   b  starboard/up/aft, sector  14   h  port/down/forward, sector  14   e  starboard/down/forward, sector  14   g  port/down/aft, and segment  14   f  starboard/down/aft. In this way antenna  14  provides universal, omnidirectional coverage around the aircraft. 
     A coaxial power cable  43  extends externally from antenna  14  and connects assembly  22  via the circuitry of FIG. 3 to a visual indicator display unit  42  mounted within the cockpit directly in front of and slightly above a pilot&#39;s eye level. As such, unit  42  is readily visible to the pilot without obstructing his normal forward view. 
     Display unit  42  is a transparent hollow sphere cut into eight “orange-wedge” sectors  42   a - 42   h  corresponding to sectors  14   a - 14   h  respectively, of antenna  14 . A cluster of eight different colored indication lights  44   a - 44   h  is mounted within the sphere, with each light  44   a - 44   h  positioned adjacent its respective corresponding sector  42   a - 42   h . Sectors  42   a - 42   h  are removably fastened together, e.g. by transparent Velcro connectors  46  mounted on the flat surfaces of the sectors, so that the lights may be quickly and easily changed when necessary. Display unit  42  is mounted in the cockpit in such a way that each of sectors  42   a - 42   h  is positioned and aligned with its corresponding sector  14   a - 14   h  so that when a pilot observes a lighted sector  42   a - 42   h , he immediately knows the location of a nearby aircraft. For example, sector  42   a  would indicate a starboard/up/forward location or sector  42   e , indicates a port/down/aft location. The pilot may then quickly take necessary evasive action. 
     The UNICORN™ system  10  of the invention mounted on aircraft  12 , is controlled by a digital clock, counter/synchronizer  50  which is the central element in a “sing-around” feedback loop  52 . In its quiescent mode, clock  50  feeds timing pulses to pulse modulator  54  at a minimum pulse repetition range (PRF) consistent with a desired radii of a “sphere of safety” around aircraft  12 . Pulses from modulator  54  are then fed to a power amplifier/oscillator  56  which is tuned to one of certain L-band microwave frequencies permitted by the FCC. The power amplifier/oscillator  56  may be of a Gunn-diode type or an Impatt diode type. When a Gunn-diode type is used, the radius of the “sphere of safety” around the aircraft is approximately 2 nautical miles. Using a more expensive, higher end Impatt diode power amplifier oscillator, the radius of the sphere of safety is extended to about 5 nautical miles, which obviously improves the early warning time to the pilot or aircraft  12  against collisions with other threat aircraft such as aircraft TA within that sphere of safety. 
     The power amplifier/oscillator feeds a cluster of eight transmit/receiver (T/R) switches  24  each of which is coupled to one of the eight sectors  14   a - 14   h  of dielectric spherical antenna  14 . When in the transmit mode, switches  24  operate to drive simultaneously in phase all eight sectors  14   a - 14   h  which thereby transmit eight microwave radar beams omnidirectionally around antenna  14  and aircraft  12 . 
     When switches  24  are in a receive mode, any wave reflected off a threat aircraft TA, a forward terrain  60 , or ground G and returning to and through the dielectric surface  30  of a corresponding one of the sectors  14   a - 14   h  will be detected by the one of a cluster of eight L-band microwave diode detectors  26  which is associated with that sector. As mentioned above, the conductive coatings  36  on flat surfaces  30 ,  32  and  34  guide or funnel the reflected returned waves to detectors  26  and thereby provide an aperture gain that enhances the detection capabilities of the system. 
     The returning L-band radar echo will provide return energy that will arrive at one of the receiver sectors  14   a - 14   h  close to the Maximum Respone Axis (MRA) of the receiver beam pattern of that segment. Around L-band frequency, e.g. approximately 1 gigahertz, the microwave emissions have a wavelength of about 3×10 8  m/s/10 9  hertz=0.3 meters=30 cm=1 foot, approximately. The bulk electromagnetic (em) wave in the dieletric waveguide antenna  14  travels at a phase wave-speed reduced below 3×10 8  m/s (the free space speed of light) by a ratio of the reciprocal of the square root of the permitivity of the dielectric chosen to that of free space. Thus, dieletric sphere  14  may have a diameter slightly less than one (1) foot. 
     Each of the detector diodes  26  provides a unidirectional pulsed rectified signal  62  to an aircraft collision peak selector and channel indicator  64 . Diodes  26  will include a passive video matched filter, or a transistor video amplifier with a resistor/capacitor feedback circuit to effect an active form of video matched filtering, to produce signals  62 . 
     The eight video amplified/video filtered signals  62  from each sector  14   a - 14   h  and its associated detector  26  are then subjected to an eight diode channel peak selection process by selector  64 . The selector selects the largest of the eight signals  62  and only passes that signal as an output signal  66  to a channel indicator gate  68 . The sector whose receive-beam pattern MRA is closer to the echo-arrival direction will be the one selected through channel indication logic within selector  64 . Output signal  66  will indicate which one of sectors  14   a - 14   h  receives the peak signal, and thus indicates the direction from which that peak signal was received corresponding to the location of the threat aircraft TA or other obstacle. As a high end option, selector  64  may include a logrithmic amplifier using, e.g. a back-biased Zener solid state diode to facilitate receive-beam interpolation by pairwise differencing between adjacent port-starboard and up-down receiver-beam channels, respectively, to enhance azimuth and elevation angle accuracy. 
     The channel indicator gate  60  passes a signal  70  to an audible alarm and light flicker/color light generator  72  which sends an audio alarm signal  74  to loudspeaker horn  76  and a light generating signal  78  to the transparent display sphere  42 , both of which are mounted in the cockpit. The signal  78  carries information concerning the location or direction of a threat aircraft TA, the range of the threat aircraft, and the rate of closure of that aircraft. Consequently, the one of lights  44   a - 44   h  and its associated transparent sector  42   a - 44   h  which correspond to the one receiver section  14   a - 14   h  and its detector  26  which detected the peak return signal will light up and flicker and provide that information to the pilot. Similarly, by virtue of the “sing-around” action, the beeping and rate of beeping of horn  76  provides information regarding the distance or range of the threat aircraft TA and the rate of closure. With this information providing sufficient early warning time, the pilot of aircraft  12  can maneuver his plane to aovid a collision with aircraft TA. 
     A peak signal  80  is also passed from selector/indicator  64  to an aircraft collision threshold device  82  to establish an acceptable false alarm/contact rate in accordance with a span of contact to sphere-of-safety range dependent on SIR determined probabilities of detection. 
     The false contact rate (e.g. from clutter or other echoes) is further reduced by use of a split range gate  84  that indicates when a video signal, that has exceeded its respective threshold, exactly straddles between an early and a late range gate. This is indicated by differencing the area of the portion of the video pulse—where area is obtained through short-term integration and that falls in the early versus the late range gate. When the difference indication passes through zero, the center of the video pulse is located. Logic  86  is provided to ensure that the first contact is normally selected. 
     The selector and channel indicator  64  can be upgraded by providing a plug-in module  90  to facilitate a ground poximity warning and a plug-in module  92  to facilitate a forward terrain collision and angular directional avoidance indicator. Module  90  receives information from lower sectors  14   e  (starboard/down/forward),  14   f  (starboard, down/aft),  14   g  (port/down/aft), and  14   h  (port.down/forward) to derive as peak selected signal. Module  92  receives information from sectors  14   e  (starboard/down/foward) and  14   h  (port/down/forward) to derive a peak selected signal. As with the peak output signal  80  from unit  64 , the peak ouput signal  94  from module  90  is subjected to thresholding circuitry  96  and split range gating circuitry  98  and then is passed into the logic circuitry  86 . Similarly, the peak output signal  100  from module  92  is subjected to thresholding circuitry  102  and split range gating circuitry  104  and then passed into logic circuitry  86 . 
     When one of the sectors  14   a - 14   h  detects a threat aircraft TA and selector  64  ultimately provides signal  80  which is processed through threshold device  82  and range gate  84  and then passed onto logic circuitry  86 , that first threat contact is selected by that circuitry and a corresponding priority output signal  106  is captured by the sing-around feedback loop  52 . Signal  106  is passed to sing-around rate counter threshold circuitry  108  which ensures that a ground proximity alarm will not be sounded or indicated during a normal landing glide-slope-descent rate situation. A signal  110  is passed from circuitry  108  to clock  50  to acitvate the next sing-around feedback loop cycle. 
     Each of the signals  94  and  100  from modules  90  and  92 , respectively, may override the first threat contact of signal  80  by way of override determination circuitry in logic  86  so that the output signal  106  is representative of the highest priority threat. For example, if a ground echo would arrive in one of the four channels of sectors  14   e - 14   h , the peak signal selected by logic  86  would be derived from output signal  100 . 
     Another ouput signal  112  from logic  86  is fed to both channel indicator gate  68  and a delayed Schmidt trigger  114 . Signal  112  as fed to gate  68  not only carries information regarding the channel selected, but also the channel in a contact-option group selected, e.g. information pertaining to a forward terrain collision alert as sensed by module  92  or a ground proximity alert as sensed by module  90 . This information conditions the channel indicator gate  68  to activate the generator  72  to selectively produce differing identifying audible alarms at horn  76  and light flickering/color light signals on display sphere  42  from which the pilot of aircraft  12  may quickly take maneuvering action to avoid a collision. 
     The delayed Schmidt trigger  114  turns the threshold video signal  112  into a digital triggering ouput  116  fed to counter  108 . The delay created by trigger  114  allows circuitry settling time after a returned threat signal is received and before the next sing-around feedback loop cycle is activated. 
     The sing-around rate control/threshold  108  already has been described. It is noted that apart from radial range information being implicit in the time between sing-around feedback loop cycles, the changes in the PRF of those cycles convey information on relative radial-range closure rate. This latter quantity is an important measure in gauging the imminence of a collision. However, under certain low closure rate circumstances (e.g., the descent rate in approaching ground proximity during a normal glide-slope landing), an audible alarm or a visual warning indication might be distractive. Of course, a manual override could be applied but requires some fore-thought on the part of the pilot. The purpose of counter  108  is to apply thresholding to the radial range closure rate information in order to preview the sing-around feedback loop  52  from being prematurely triggered during benign circumstances. Then, triggering is only effected when logic  86  dictates it is reasonable to consider the event as possibly threatening; otherwise, the controller returns the sing-around feedback loop to its quiescent state. 
     The channel indicator gate  58  already has been described. In the case of the basic UNICORN™ system without modules  90  and  92 , its only function is to take the channel indication of signal  80 , as validated through circuitry  82  and  84  with first contact selection performed by logic  86  as a means to instruct the audible alarm and light flickering colored lights how to perform in conveying radial range, radial range closure rate and octant sector contact information in a rapid but distinguishable and unambiguous way to the pilot. 
     The audible alarm and light flickering/colored light generation is used to generate the signals which drive the loudspeaker horn  76  as an audible alarm and the octant heads-up display  42  so as to provide the pilot with rapid, unambiguous and clear indications of impending collision situations along with concise information that would enable an immediate autonomous collision avoidance maneuver or sufficient early warning to not only obviate a collision but, also, to facilitate reducing a near miss. 
     The loudspeaker  76  would be used to reproduce various audible alarm tone sequences whose sound content would identify the type of impending collision or near miss while also conveying radial range and radial range closure rate through the changing PRF of these beep sequences—namely beep - - - beep - - - beep - - - beep - - - beep - - - beep—beep conveying some sense of urgency regarding the need to rapidly react. 
     In the transparent display sphere  42 , different color-coded lights would identify the eight (8) threat corridors—namely port/up/forward, port/up/aft, port/down/forward, port/down/aft, starboard/up/forward, starboard/up/aft, starboard/down/forward, and starboard/down/aft. Furthermore, different color coding would be used to indicate ground-proximity warnings and forward terrain collision warning and avoidance indication. Light-flickering of the colored lights (similar to the audible alarm) will be used to convey degree of urgency in responding to an impending collision or near miss. 
     In very small cockpits in which overhead space is limited, it may be desirable to replace spherical display unit  42  with a pair of forward and aft flat screen displays mounted on the instrument panel. 
     As mentioned, there is a desire to make UNICORN™ compatible with TCAS requirements for smaller aircraft lacking a strong radar cross section (RCS) to respond to a TCAS installed aircraft radiated interrogation with a transponded signal encoded with aircraft type, GPS position (if available) and other information useful in rapidly assessing the likelihood of a collision. The decoding and encoding required for this transponder mode may be easily accommodated as an upgraded option along with the installation of a suitable GPS-receiver capability, respectively, accommodated in elements  64 ,  50  and  54 . In addressing a concern about mutual interference, which would be much less prevalent with the lower microwave power levels associated with a UNICORN system (particularly under the Gunn diode option) relative to TCAS, a “whisper and shout” mode might be employed as a further upgraded option. This would involve pulsing the PA/OSC module  56  to radiate lower power during the quiescent mode than would be employed at full power once an alert cycle was being initiated. 
     It is worthy of mention that UNICORN™ does not face the problem addressed by TCAS. The TCAS was designed with the problem of large commercial airlines colliding with or descending onto smaller aircraft. These smaller aircraft have much smaller RCSs and, generally, do not have the air-speed performance of the airliner but, nevertheless, are better able to take violent evasive action. The smaller RCSs require radar echo return enhancement as provided by a pulse transponder. This, of course, enables other useful information, such as GPS receiver and other data to be added to the encoded transponder return by the smaller aircraft. Conversely, UNICORN™ 0  has smaller aircraft applications in mind, with the larger RCS airliners as one of the potential threat aircraft. Other lower RCS small aircraft (generally because of their lower airspeed) do not close as rapidly; thereby making the shorter range associated with a smaller RCS more acceptable due to the corresponding less rapid closure vis-a-vis the reduced early warning time. 
     With passenger and cabin crew safety in mind, commercial airliners are less apt to take a violent evasive maneuver to avoid a collision with a smaller aircraft until it became patently obvious that disaster was imminent. Conversely, a smaller aircraft, general aviation or corporate aviation, pilot provided by a UNICORN system may take autononomous control in sufficient time to avoid a catastrophe. 
     As described above, the eight sectors  14   a - 14   h  are glued together to form a spherical antenna  14  mounted on top of aircraft  12 . On some aircraft, e.g. a high winged small aircraft such as a Piper cub, EM “shadowing” by the structure of the aircraft may be a consideration. In such a case, antenna  40  may be formed as separate upper and lower hemispheres each enclosed within its own protective dome  16 . The upper hemisphere would be mounted in its normal position above and slightly behind the cockpit, while the lower hemisphere would be mounted diametrically opposite the upper hemisphere but below the plane as shown, e.g., in broken line at  15  in FIG.  1 . Both hemispheres would be suitably connected via coaxial cables to display unit  42 . Further, in very small aircraft in which the full spherical antenna may create aerodynamic drag, the antenna may be separated into more than two and as much as eight solid-angle sectors which would enable those sectors to be embedded into the aircraft skin in uniform distribution around the cross-section of the aircraft. This would minimize any aerodynamic drag. 
     UNICORN™ also could fulfull a role as an inexpensive marine collision warning device for use by recreational boaters avoiding collisions with large marine vessels. In this application only the upper hemisphere of the dielectric antenna would be needed. Furthermore, the omni-directional transmission would be polarized so as to reinforce the L-band microwave transmitter hemispherical coverage by virtue of a reinforcement by an ocean-surface reflected EM wave constructively interferring with the direct EM wave. The U.S. Coast Guard is conducting a large scale test of automatic identification system (AIS) technology. AIS bears great resemblance to TCAS; while also requiring GPS information as part of the transponded response. Importantly, UNICORN™ could not only be made compatible with AIS in addition to having its autonomous mode of operation, but, because of a four (4) quadrant angular capability (in the upper hemisphere), it would significantly improve the ability to enable Kalman-filter recursive/predictive processing to more accurately determine if a collision course were imminent. Again, “whisper and shout” operation could be employed to minimize mutual L-band EM interference with other numerous recreational craft and large vessels. 
     As with the airborne collision avoidance UNICORN™ system, various upgrade options could be offered recreational boaters in accord with the amount they already had invested in their craft. For those recreational craft operators that have invested in a GPS receiver, the U.S. Coast Guard hope that they also will invest in an AIS transponder possibly might may be fulfilled. However, the generic UNICORN™ system could prove to be a better investment for the owners of relatively inexpensive recreational boats. Moreover, its compatability with AIS might encourage such an owner to upgrade by adding a GPS receiver and the appropriate UNICORN™ system AIS upgrade module at a later date. 
     Another Department of Transportation (DOT) application of UNICORN™ could involve something akin to the forward terrain collision warning capability adapted to receive two or three adjacent beams to interrogate directly ahead and, when traveling on a curved part of a railroad track, left an right. This would act to obviate rear end collisions of a follower train plowing into a leader train. Conversely, a similar UNICORN™ adaptation mounted on the rear of a train could instruct the train engineer to speed up so as to avoid the opposite situation. Again, an AIS capability could be integrated into such a UNICORN™ system adaptation. 
     The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein. In the claims, for example, the use of the term “generally spherical” is intended to cover both the spherical antenna  14  and the hemispherical or other solid-angle sector adaptations described above.