WO1988009027A1

WO1988009027A1 - Aircraft collision avoidance

Info

Publication number: WO1988009027A1
Application number: PCT/AU1988/000137
Authority: WO
Inventors: John Harold Dunlavy
Original assignee: John Harold Dunlavy
Priority date: 1987-05-08
Filing date: 1988-05-09
Publication date: 1988-11-17

Abstract

An aircraft collision potential warning system in which each aircraft includes means for transmitting pulse sequences, in pseudo random or random fashion and on a common frequency, which include aircraft identification and altitude information. Each aircraft further includes means to receive and process incoming signals from other aircraft and, on the basis of the altitude information, discard those signals not within a collision potential altitude range. If a signal is within a collision potential altitude range, interrogation is effected specifically to the identified signal source and the closing speed between the two aircraft is determined. If the rate of change of closing speed is below a set figure, a collision potential exists.

Description

AIRCARAFT COLLISION AVOIDANCE:

This invention relates to aircraft potential collision avoidance.

A number of systems are presently under development.

Most existing systems are designed to use existing transponders developed for use with existing secondary surveillance radar (SSR) systems, and aircraft carrying aircraft mounted L-band transponders.

The problems with existing ground based secondary surveillance radar systems is that, as is the case with all radar systems, detection relies upon a reflected signal and this signal can be extremely weak.

In order to overcome the weakness of reflected signal and extraneous reflections, SSR relies upon an aircraft actively replying to a signal received from the ground based radar.

Secondary radar uses a directional narrow beam by which the transponders of an aircraft, when within the beam, can be triggered whereupon the aircraft transponder can reply with appropriate information concerning that aircraft.

So-called mode A transponders are adapted to provide only an identification of the aircraft, whilst mode C can provide altitude information.

It is to be remembered that these types of transponders are adapted to reply only when they receive an active interrogation from the narrow beam of a radar transmitter which is being mechanically swept over the area.

There are at least two major problems associated with such a system these being commonly referred to as "garble" and "fruit". "Garble" occurs when two signals are received at the same time so that the signals overlap on the ground based antenna and often cannot be recovered.

"Fruit" occurs when the coverage of two SSR sites overlap. As a consequence one site may_. receive responses which were in fact replies to an interrogation by the other SSR.

A most recent development using these transponders is known as T.C.A.S. (Threat Alert and Collision Avoidance System).

This system, however, inevftably relies upon a rotating antenna beam both to provide the bearing an any interrogated aircraft and also to reduce garble which must be the frequent result of an omni-directional interrogation.

Rotating antenna beams are inherently complex and expensive.

Although complex signal processing techniques can be used to mitigate to some extent this synchronous garbling problem, it is still a very complex difficulty to indeed separate a multiplicity of time coincident transponder replies and then, of course, selectively decode the information they contain.

In a further attempt to overcome this problem, there is a technique known as "whisper/shout" where the output power of interrogation transmissions are sequentially varied. This can reduce but does not remove the potential for coincident incoming pulse sequences, and further techniques including the directional receiving anteπnaes, have been proposed but once again reduce but do not remove the difficulties.

An object of this invention is to propose an aircraft potential collision avoidance system which will require significantly less complexity in relation to equipment needed for its effective operation and hence shall be less expensive and therefore more accessible for all aircraft operators, and secondarily may provide additional reliability. • According to this invention therefore, it is proposed that each aircraft in the system shall be providing on a randomly separated basis, or pseudo- randomly separated basis, a series of pulse sequences each of which uniquely identifies the transmitting aircraft but which also contains one further piece of information, namely information on the altitude of the aircraft at the time of the transmission by which any receiving aircraft can make a preliminary assessment as to whether any realistic potential for collision exists at all, and if so, the receiving aircraft can then selectively interrogate only that aircraft with which a problem specifically might exist.

The advantages of this arrangement are that this provides for very efficient handling of many signals, and the need then to only deal with those that have some potential for being of concern.

In preference, each transmitted pulse sequence includes other information such as the air speed of the responding aircraft, and the bearing of this aircraft.

This means that when such a pulse sequence is received, it can be assessed on a hierarchial basis so that only if a first criteria indicates a potential for collision, will the further features be looked at and perhaps a selective interrogation occur.

The invention can be said then to reside in an arrangement for effecting a warning of collision potential between aircraft comprising, on a first aircraft, radio frequency transmission means and means to transmit on the said radio frequency transmission means, encoded pulse sequences, each sequence spaced apart in time by a period which is randomly or pseudo-randomly selected so as to be different from subsequent periods between pulse sequences, each pulse sequence including at least means unique to and identifiable of the transmitting aircraft, and the altitude at that time of the transmitting aircraft, and means to receive transmitted pulse sequences from other aircraft and means to assess each received pulse sequence carrying equivalent information from other aircraft, and means to make a first assessment comparing the altitude of the receiving aircraft with the altitude identified in the received signal, and only if this is within a selected range directing the signal to further assessment means.

According to a further form of this invention, each aircraft shall be equipped with a transmitter adapted to transmit within a microwave portion of the electromagnetic spectrum, and adapted to transmit within a selected frequency, a receiver adapted to detect any signal within the said selected frequency, the arrangement being characterized in that there are means to effect from time to time a transmitted signal from the transmitter each signal being uniquely characteristically identifiable, the said signal being transmitted at intervals other than at regularly spaced intervals.

The invention can be said to reside in an arrangement for effecting a warning of collision potential comprising, for an aircraft, transmitting means for transmitting, at a radio frequency, pulse sequences uniquely identifying the sending aircraft, and altitude informatimon of the sending aircraft, means for receiving like pulse sequences from other aircraft, and means to assign a priority to each incoming like signal according to the altitude information deduced from that signal and to direct the signal to further processing means in accord with the assigned priority.

In preference, the signals are transmitted at intervals which are consistently different one from the other such that these can be considered as a random selection of intervals.

In preference, the signals are repeated so that there are a minimum number of signals per given period and a maximum number of signals per given period so that while the actual space interval is randomly selected or pseudo randomly selected, there will be sufficient repetition rate for necessary purposes of consistent interrogation.

In preference, the total pulse sequence length of any signal is a relatively short period as compared to the spaced interval so that the period of transmission of such an arrangement will be relatively small as compared to the period of sensitivity of external signals being receivable. In a further form, the invention could be said to reside in a method of detecting potential collision situations between relatively moving aircraft the method comprising the steps of transmitting from each of the relatively moving aircraft a uniquely identifiable signal on a common frequency wherein each signal is transmitted on a commonly selected frequency, and the signals are spaced apart at intervals which are other than regularly recurring intervals of the same period, and upon such a signal being detected and identified by a second aircraft, effecting communication between the two aircraft.

In preference, the transmission is effected in the microwave portion of the electromagnetics spectrum.

In preference, the uniquely identifying characteristic of any signal is the pulse length of that signal.

In preference, the spaced distance between repetitive signals being transmitted is selected on a basis that would have no chance of being duplicated by any other aircraft transmission.

In preference, such basis can be an essentially randomised selection or a pseudo random selection basis which will be well understood to those familiar with this art.

The concept here of course is that in this way, no one transmission will be transmitting at exactly the same time as the other, other than for the momentary period of one pulse so that hence, there is a best chance of detection of another signal at all times.

In preference, when a first signal from another aircraft is detected there will be transmitted specific information such as the specific identity of the vehicle transmitting the information together with information as to the course direction and altitude of the particular vehicle. Further, such detection can initiate an interrogation whereby the location of the other vehicle such as an aeroplane can be further verified and by having this repeated within a time period, the bearing closing speed, and in fact relative location, can also be determined.

According to a further form, this invention can be said to reside in an aircraft potential collision warning arrangement comprising, with at least two aircraft, on each of the aircraft:-

(a) means for transmitting, at a radio frequency, pulse sequences uniquely identifying the sending aircraft and characteristics of ^• altitude and flight path bearing of the aircraft;

(b) means for receiving like pulse sequences transmitted by the other said aircraft;

(c) means for transmitting at a radio frequency, a signal adapted to effect a dedicated interrogation and inviting a response from the other aircraft; and

(d) means for receiving said interrogation sequence from the said other aircraft and providing an appropriate response including information by which the distance apart of the aircraft can be assessed.

In preference, the initial transmissions and receiving sensitivities are within a limited number of frequencies and transmissions for aircraft, at least within a similar altitude, are at the same frequency.

Confusion is avoided by providing that transmission occurs only during a brief period of a total period.

If the transmission periods were regularly repeating there may still be a chance that two aircraft may be transmitting coincidentally and accordingly there are provided means such that a transmission pulse is effected on an other than regularly repeating cyclic basis. In preference, the transmissions occur on a random basis within, however, a given time space, or at least on pseudo random basis in which an algorithm is used such that there is negligible chance that this could ever be replicated in both real time and a cyclic repetition basis by any other aircraft with the same system.

The system envisages effecting an all directions transmission at a frequency which provides generally efficient non-directional cover and to a range which will provide adequate detection warning considering the speed and general circumstances in which aircraft presently fly. Such a range will be determined by the type of antenna and the radio frequency selected, as well as the location of this on the aircraft, together with the power of the transmission.

It is considered that the transmission will occur for a very brief period over any given time space, but that it will be regularly repeating so that within any larger period, there can be reasonable assurance that a sufficient number of transmitted pulses have been sent for the purpose.

In preference, there is initial information within the first random or pseudo randomly occurring pulses by which information on the altitude of the aircraft, direction of flight, air speed and identity of the aircraft can be assessed.

Once contact has been made, a dedicated interrogation is then effected in respect of the particular signal received and such that the interrogation is directed and received only by that aircraft or equipment to which the interrogation is addressed.

In this way specific information in relation to the two aircraft can then be achieved very specifically, the actual distance apart of the aircraft and an assessment of the closing rate between the two aircraft. In preference, there are means on board each aircraft such that this information, once received, can be rapidly computed so that information on the potential of a collision risk can be made and certain actions initiated.

These can either be a warning to the pilot, or in some cases, execution of change of flying direction so as to ensure avoidance of the collision risk.

According to a further form of this invention, this can reside in an arrangement for assisting avoidance of collision between aircraft wherein at least two of such aircraft include means to effect transmission within a radio frequency of means identifying uniquely the aircraft and such that in respect of a plurality of shared transmission frequencies, transmission from a selected aircraft is significantly less over a period of time than a period allowed for receiving transmissions, the arrangement characterised in that transmission frequencies are selected on a basis of altitude of the aircraft at the time of transmission and/or a rate of change of altitude.

In preference, altitude is divided into a plurality of ranges and a frequency is selected for each of such selected ranges as well as a separate frequency for an aircraft changing altitude above a selected rate of change so that such status is also separately detectable.

In preference, any transmission of this type is transmitted for a very short period of time compared to the period available for receipt of transmissions and in any event such transmission is repetitively effected so that the period between such transmissions is a random or psuedo random period so that there is either negligible or no chance that another aircraft with an equivalent system would duplicate such periodic spacing even if a first transmission coincides.

This is best achieved by a spacing rate which is other than regular and therefore unlikely to be duplicated on any other aircraft with a similar system in either absolute time or periodic spacing ratios. In preference, transmissions will take place at a standard transponder interrogation frequency of 1 ,090 Mhz and a transponder interrogation frequency referred to will occur at a frequency of 1 ,030 Mhz with the arrangement being such that such transmissions are compatible with an existing L-band secondary surveillance radar (SSR system).

The present invention overcomes reliability problems resulting from operational and performance flaws encountered with other airborne collision avoidance systems utilizing prior art. Significantly improved reliability is achieved by the use of radio transmissions, at microwave frequencies, consisting of randomly-recurring, short-duration "bursts", which are emitted by each aircraft at a specific frequency determined by the aircraft's altitude, there being different standard frequencies assigned to different increments of altitude. Reception at the same altitude- determined frequency provides a simple yet highly effective means of eliminating synchronous garbling and interference caused by other aircraft flying at differential altitudes that pose no threat of collision. Another aspect is the encoding of each short-duration burst transmission to identify the transmitting aircraft, its altitude, direction of flight, and airspeed. A further unique combining of technologies is the selective interrogation of an intruding aircraft using transponder techniques to obtain distance and relative velocity information that, along with the altitude, flight direction, and air speed obtained from the reception of the encoded burst transmissions, makes it possible to compute the location of other aircraft without the need of directional antennas or other devices.

Computer circuitry, with memory capacity, forms an integral part of the system. It is used to monitor and control the functioning of all other system components and to continuously process and evaluate the information content of signals received from other aircraft in a manner intended to identify situations posing the threat of a collision.

An embodiment of the system envisioned comprises an omni-directional antenna system, a directional-coupler, a radio transmitter, a radio receiver, a frequency synthesizer, an R.F. circulator, a computer module, an aural/visual display unit and pressure, magnetic-bearing, and airspeed transducers. Optional components include an auto-pilot modem and pilot defeat switch.

For a better understanding reference will now be made to a preferred embodiment which shall be described with the assistance of drawings in which :-

FIG. 1 is a block diagram of a first embodiment.

FIG. 2 shows the relationship between transmit/receive frequencies and the altitude of the aircraft.

FIG. 3 shows the spectral relationship between the four frequencies typically monitored by the receiver.

FIG. 4 depicts aircraft ascending and descending at a rate exceeding

200 feet per minute and the transmission frequencies emitted by the collision avoidance system.

FIG. 5 illustrates the burst transmissions and their random separation.

FIG. 6 shows a encoding sequence for the burst transmissions.

FIG. 7 shows the sequence of transponder interrogation pulses.

FIG. 8 depicts an alternative protocol for the pulse sequence.

FIG. 9 illustrates the shape of the omni-directional antenna pattern in the horizontal and vertical planes.

FIG. 10 is a block diagram of a typical receiver topology that can be used for receiver 10 of Fig. 1.

FIG. 11 is a schematic layout of functional components of equipment in relation to a second embodiment. FIGS. 12 and 13 illustrate firstly a typical random pulse sequence that could be expected from the second embodiment, and FIG. 13 an encoded message sequence that could be expected from this.

FIGS. 14 and 15 illustrate a technique for assessing collision potential.

Referring to FIG. 1 , antenna 8, which exhibits the preferred radiation pattern shown in FIG. 9, is connected to transmitter 6 and receiver 7 through a directional-coupler 8, which provides a suitable degree of signal isolation to prevent overload of the receiver during periods of transmission.

The computer module 1 is the centre of the entire system. It controls virtually all functions of the system: determines transmitting and receiving frequencies (FIG. 2, FIG. 3 and FIG. 4); defines and generates randomly spaced, short-duration bursts (FIG. 5 and FIG. 6), encoded to identify the aircraft, its altitude, flight direction and airspeed; determines the frequencies for reception of other aircraft by receiver 7; processes the demodulated signals produced by receiver 7; generates the interrogation pulse sequence for the transponder mode (FIG. 7); processes the transponder signals from interrogated aircraft received by receiver 7; determines the relative distance and relative velocity of aircraft intruding the detection range of the system; computes the time and location of potential collisions with other aircraft; computes the most suitable of evasive action, both horizontal and vertical, to avoid a collision; generates the data/information for the aural and visual display unit 9; and optionally provides the aircraft's autopilot system with suitable signals through the autopilot modem 10 to cause the aircraft to execute effective evasive manoeuvres to avoid a collision.

Analog signals from the pressure transducer 2, the magnetic bearing transducer 3, and the airspeed transducer 4, are fed to computer module 1 where they are translated by internal A/D converters into digital form. Using the altitude information supplied by pressure transducer 2, computer module 1 determines the correct transmission frequency, based on values of frequency versus altitude stored in the memory of computer module 1 , similar to the typical values given in FIG. 2. These values are then fed to the frequency synthesizer 5 as a series of digital or analog command signals. If the aircraft is cruising at a fixed altitude, computer module 1 selects a specific, fixed frequency for transmission according to the above protocol. However, if the aircraft is either ascending or descending at a rate typically exceeding about 200 feet per minute, computer 1 instructs the frequency synthesizer 5 to supply the transmitter 6 with a carrier at frequency fo, which is a special alert frequency used by all aircraft that are changing altitude (FIG. 4). In addition computer 1 monitors changes in the aircraft's altitude during ascent or descent manoeuvres and adjusts the normal transmission frequency to coincide with the correct value for the altitude being traversed. When the aircraft reverts to a sustained cruising altitude, transmission at frequency fo ceases and transmission then takes place only at a fixed frequency (f1 through f27), corresponding to the aircraft's altitude.

Receiver 7, depicted in FIG. 10, is designed to simultaneously receive signals on four separate frequency channels, one of fixed frequency, designated as fo, and three which vary in frequency according to the altitude of the aircraft. A typical spectral arrangement of these four receiving channels id depicted in FIG. 3. Frequency channel fo is reserved at all times for the reception of encoded burst transmissions emitted by aircraft in the process of changing altitude, as described in the above paragraph. The frequencies of the other three channels are spectrally adjacent, in a one-two-three sequence, there being a centre channel, designated fo, and upper and lower channels, designated fh and fl, respectively. All three are selected by computer module 1 on the basis of the aircraft's altitude in a manner similar to that described in the preceding paragraph. The frequency of channel fo is chosen to be the same as the frequency of transmission, that is, the frequency matches that allocated to the aircraft's altitude. The adjacent channels, fh and fl are set to receive signals at frequencies transmitted by aircraft flying in the adjacent upper and lower altitude corridors, that is, the altitude segments directly above and below the subject aircraft. However, for the special case of an aircraft flying at either the lowest or the highest of the designated altitude corridors, corresponding to fl and f27 in the example of FIG. 3, reception would be limited to only two channels, f1/f2 and f26/f27, respectively.

Receiver 7 of FIG. 1 , shown in detail in FIG. 10, is a superhetrodyne type, using double-conversion, with separate Intermediate Frequency amplifiers (I.F. Amp) operating at different centre frequencies. Input from an antenna circulator is fed to an RF filter 13 and RF amplifier 14. There is also a 1 st mixer 16 and a 2-way splitter 20. A single 1 st I.F. Amp 15 is used, tuned to a frequency which is equal to either the sum or difference frequency between the desired receiving frequency and the frequency shown as the 1 st L.O. Signal (local oscillator signal) provided by frequency translator 19 of FIG. 10. Four, separate 2nd I.F. Amps, shown as 21 through 24 in FIG. 10, are used to provide reception of signals at fo, fl , fc, and fh. The I.F. Amp 24 is tuned to a frequency suitable for receiving frequency fo. I.F. Amp 22 is tuned to a frequency suitable to the reception of frequency fc. I.F. Amps 21 and 23 are tuned to separate frequencies needed for reception of fh and fl , respectively. This arrangement of I.F. Amplifiers simplifies the circuitry and number of L.O. frequencies required. For example, the 1 st L.O. becomes a fixed frequency, suitable for tuning the receiver to the centre of the overall frequency band containing all of the sub-frequencies, fo and f1 through f27. Frequency fo is also obtained with a single L.O. frequency, fed to 2nd Mixer #2, block 18 in FIG. 10. Frequencies fo, fl and fh are likewise achieved with a single L.O. frequency fed to 2nd Mixer #1 , block 17 in FIG. 10.

The outputs of all 2nd I.F. Amps are fed to separate detectors of a type suitable for demodulating the type of pulses used by the system. These detectors are shown as blocks 25, 26, 27 and 28 in FIG. 10.

The outputs of these four detectors are fed to Computer Module 1 , as shown in FIG. 1 , where they are initially stored in digital form and then processed by logic circuitry according to a suitable algorithm designed to yield the following information, preferably in the order given: 1. For frequency fo:

(a) Aircraft's identification

(b) Altitude and rate of ascent or descent (c) Direction of flight

(d) Airspeed

2. ^" For frequency fc:

(a) Aircraft's identification (b) Precise altitude of intruding aircraft relative to own aircraft.

(c) Direction of flight

(d) Airspeed

3. For frequency fl and frequency fh: (a) Aircraft's identification

(b) Precise altitude of intruding aircraft relative to own aircraft.

From the above information, stored and continuously up-dated within memory circuitry, the Computer Module 1 is able to establish whether any aircraft is intruding at an altitude that represents less than an acceptable minimum vertical separation for any aircraft received on frequencies fo, fc, fl, or fh. Monitoring of frequencies fl and fh are necessary to preclude the situation where one's own aircraft is flying near the upper or lower boundary separating the increments of altitude used in selecting the frequencies for transmission and reception. If less than an acceptable vertical separation exists, the intruding aircraft is selectively interrogated by means of an encoded pulse sequence which begins with an address code identifying the desired aircraft, as illustrated by the example appearing in FIG. 7. The subject plane received this transmitted sequence and, in the manner of a transponder, re-transmits the short-duration, high-power pulse within a standard delay time at frequency fc. The re-transmitted, transponder pulse is received by the original sending aircraft and demodulated by receiver 7 of FIG. 1. The Computer Module 1 then calculates the distance separating the two aircraft by solving the following simple mathematical expression: Dnm = 0.1618 ft - d)

2 where: Dnm = distance in nautical miles separating aircraft

t = total elapsed time in microseconds between transmissions and reception of transponder pulse

d = delay time with transponder in microseconds

This transponder type of selective interrogation and reply sequence is repeated a number of times sufficient to determine any rate of change in the distance separating the aircraft as a means of computing the relative velocity between them. Once the distance and relative velocity are known, they may be added to the already known flight directions and air¬ speeds of the two aircraft to permit the calculate, by a suitable algorithm, the location of one aircraft relative to the other. It is then a fairly simple problem for the logic circuitry of Computer Module 1 to compute whether the potential for a collision exists and, if it does, the approximate time it is likely to occur, given that all flight parameters remain fixed. A further algorithm is then used to determine the most suitable evasive action for each plane to take to avoid the collision..

During the above communications between one's own aircraft and the intruding aircraft, what is referred to as a "handshake" takes place between the computers of the two aircraft. Such a handshake has the advantage of permitting the two computers to compare their independent findings as a means of insuring maximum reliability by reducing the probability of error. Another benefit of such a handshake is for the two computers to independently evaluate and agree upon the most suitable evasive action to avoid a collision and to co-ordinate that action with the respective pilots of both aircraft through to completion and satisfactory resolution of the threat. The special case of an aircraft changing altitude at a rate resulting in it transmitting on frequency fo requires the use of an algorithm different from that usable for aircraft received frequency fc. In addition to considering the flight parameters mentioned above, it is necessary for the fo algorithm to calculate the time at which the two aircraft will be at the same altitude and then to determine if this will occur at the same instant they are at the same lateral co-ordinates. Although this represents a more complex calculation than that required for two aircraft flying at the same altitude, the geometry involved is relatively simple and straightforward in terms of mathematics.

An alternative protocol to the transmitting sequences shown in FIG. 6 and FIG. 7 would be that illustrated in FIG. 8, wherein only aircraft identification and altitude are transmitted during the short-duration burst transmissions depicted in FIG. 5. Airspeed and flight-direction are then reserved for transmission during the transponder sequence. This has the advantage of increasing the probability of detecting an intruding aircraft within the shortest time by increasing the total time available for reception between the burst transmissions. However, it has the disadvantage of complicating the transponder sequence and requiring additional time for assessing the parameters of the threat. It appears that either protocol could be used with about the same overall performance and results.

It is firstly noted in respect of FIG. 11 that there should be the following major components available to operate the system of the second embodiment. These can be :-

31. A transmitter operating within the microwave portion of the electromagnetic spectrum, having a power input sufficient to accomplish the purpose of the system over limited distances of perhaps 10 kilometres.

32. A sensitive microwave receiver tuned to the same frequency as the aforementioned transmitter. 33. A microwave antenna which exhibits an omni-directional pattern azimuthally and a very narrow beam width vertically (a flattened, donut- shaped pattern) peaking on the horizon when the aircraft is in level flight.

34. A transmit/receive switch connected between the transmitter, receiver, and the antenna, capable of disabling or blanking the receiver during transmission of individual pulses or encoded message sequences.

35. A dedicated computer with suitable A/D and D/A interfaces and a visual display unit and/or other suitable means for displaying information and producing an alarm signal when a threat is identified, said computer being connected to both the transmitter and the receiver.

36. A visual display unit.

The interaction of the components comprising the above system is illustrated in block diagram form in FIG. 11. The operation of the system is best understood by also making reference to FIGS. 12 and 13.

To begin, the computer 35 generates a sequence, e.g. 37 of short- duration electrical pulses, each having a fixed width of less than 10 microseconds, with an average of about 10 pulses occurring per second. The repetition rate is set to vary continuously in a random manner within certain limits, that is, the time separating one pulse from the next varies randomly from about 100 microseconds to about 100 milliseconds (see FIG. 12).

The pulses generated by the computer 35 are fed to the transmitter 31 through a digital-to-analog interface. Within the transmitter 31 , the pulses are converted to microwave energy which is sent to the antenna 33 through the transmit/receive switch 34. The computer 35 supplies a suitable command to the transmit/receive switch 34 to connect the antenna 33 to the transmitter 31 and disconnect it from the receiver 32 during the transmission of each pulse and simultaneously commands the receiver sensitivity to be muted during the same period. During the interval between pulses, the computer 35 causes, by similar means, the antenna 33 to be disconnected from the transmitter 31 and reconnected to the receiver 32, with sensitivity returning from muted to normal.

Because the system transmits during less than 1/10,000 of the total time, over 999.9 out of each 1 ,000 milliseconds is available, on a statistically random basis, for the reception of pulses transmitted by nearby aircraft at a similar altitude. If a pulse transmitted by another plane is heard by the receiver, a suitable signal is sent to the computer 35 through a high¬ speed analog-to-digital interface. The computer 35 is programmed to identify the received pulses on the basis of whether it possesses the correct, fixed pulse-width and aircraft identification, and then assesses altitude information. If the bona fides of the received pulse are established, the computer ceases its generation of the random pulse train and effects an interrogation sequence.

The interrogation sequence includes an encoded message consisting of a brief sequence of digital bits conveying information regarding the transmitting plane's altitude, airspeed, and direction of flight. The beginning and end of the message includes a short identification section which identifies the sending plane by registration number and also serves to deactivate the transmission of random-pulses by the other plane by virtue of the program in its computer.

The transmission of this encoded message is followed by the radiation of a radar pulse, intended to be reflected by the metallic surfaces of the other plane. This radar pulse, travelling at the precise speed of light, returns to the sending plane within a total time of about 6.7 microseconds for each kilometre of separation. The computer uses this time interval to determine the distance which separates the two planes and, by comparing the rate of any change in distance, also determines the rate of closure between them.

In an alternate fashion, the other plane duplicates the first plane's transmission of the encoded message sequence and radar pulse, such that each plane thereafter is aware of the other plane's presence, its altitude, airspeed, direction of flight, and rate of closure. By simple calculation, each plane's computer is then able to establish the relative position of the other plane and whether or not a collision course exists. From this information, the computer can then determine what, if any evasive action might be required (most likely a mutual change in altitude, which the two computers co-ordinate with each other, by means of transmitted message sequence, before advising the pilots and/or instructing the auto pilot systems of each plane).

Accordingly there is provided simultaneous transmission by several planes, within a relatively short distance of each other, or pseudo randomly or randomly spaced, short duration pulses at a microwave frequency in a manner which permits each plane to reliably hear and

• identify the presence of other planes. This is made possible by the randomly occurring "holes" between the transmitted pulses, allowing the reception, within a statistically brief period, of another randomly occurring pulse radiated by a nearby aircraft.

It is anticipated that many variations of the basis system configuration described above are possible and obvious to those versed in the art. For example, with a system designed for use within countries which typically experience a relative high density of air traffic, it may prove necessary to allocate separate microwave frequencies for each 1 ,000 feet of elevation (with overlap), such that the transmissions of planes flying at elevations separated by more than about 1 ,000 feet do not interfere with each other. It may also prove advisable to establish a universally monitored, priority frequency for the transmissions of planes that are in the process of changing altitude or are engaged in take-off and landing phases of flight, since these modes represent a much higher risk factor than level flight.

The following description demonstrates a preferred technique for an aircraft to determine the relative position of another aircraft on a collision course, using only the flight parameters of the two aircraft and the distance between the aircraft at two different times. The following description is in terms of two aircraft in level flight at the same altitude although it can be simply modified to take into account climbing and falling aircraft. Furthermore, in the following description aircraft A determines the relative position of aircraft B, although the roles can easily be reversed. In FIG. 14, aircraft will obtain from the pseudo-random transmissions of aircraft B both its flight parameters and the closing speed. From this it deduces that aircraft B is, in fact, on a collision course. Aircraft A then issues, at a time To, an interrogation to aircraft B to determine the distance DTo between the two aircraft. Since the altitude of B is known, aircraft A can deduce that aircraft B lies on a circle of radius DTo centred around the current location of aircraft A. Because aircraft A also knows the velocity of aircraft B it can deduce that aircraft B will lie on the dashed circle shown in FIG. 14 after at a time T1 , where T1 To.

FIG. 15 shows the situation at the time T1. Aircraft A already knows that aircraft B must lie on the dashed circle. By taking a second distance measurement at time T1 , aircraft A can deduce that aircraft B must also lie on the solid circle of radius DT1 shown in FIG. 15. Although these circles would usually intersect at two points, when aircraft B is on a collision course there is only one point of intersection, as shown in FIG. 15. As a consequence it is possible for aircraft A to uniquely determine the position of aircraft B relative to the current position of aircraft A at time T1.

While there is a very small chance in the arrangements described that overlaps will occur between received signals from different aircraft, nonetheless if this does occur it could create difficulties.

Accordingly there is provided a separation of the code providing the unique identification for the aircraft sending the signal.

If a part of the identification code is at the front of the pulse sequence and the remainder at the end of the pulse sequence, if any overlap occurs, the received signal will no longer have an identifiable uniquely characteristic signal which can be assigned to a known aircraft, in any event, if by chance the garble signal did identify an existing aircraft, it would be statistically impossible that such an aircraft was within interrogating range of the first aircraft. Accordingly, there can be provided additional safeguard as to the avoidance of the garble problem.

By including a test in relation to the closing speed between the respective aircraft when the aircraft is assessed as a potential threat, it has been found that if the closing speed remains substantially constant, then this can indicate a potential collision situation.

Accordingly there are proposed means where by the closing speed between a first aircraft and a second aircraft is assessed, and in the event that the assessed closing speed remains within a selected range of values between successive readings, then an appropriate signal is activated.

Claims

C AlMS"

1. An arrangement for effecting a warning of collision potential comprising, for an aircraft, transmitting means for transmitting, at a radio frequency, pulse sequences uniquely identifying the sending aircraft, and altitude information of the sending aircraft, means for receiving like pulse sequences from other aircraft, and means to assign a priority to each incoming like signal according to the altitude information deduced from that signal and to direct the signal to further processing means in accord with the assigned priority.

2. An arrangement for effecting a warning of collision potential as in the last preceding claim further characterised in that the further processing means include means to interrogate uniquely the aircraft from which the signal was sent.

3. An arrangement for effecting a warning of collision potential as in the last two preceding claims further characterised in that the means for transmitting the pulse sequences are adapted to transmit such sequences at intervals which are pseudo-randomly selected in length of time as between each successive sequence.

4. An arrangement for effecting a warning of collision potential between aircraft as in any one of the preceding claims further characterised in that the transmission means are adapted to transmit the said pulse sequences on a common carrier frequency irrespective of the altitude of the aircraft at the time of transmission.

5. An arrangement for effecting a warning of collision potential between aircraft as in any one of the preceding claims 1 , 2 or 3 further characterised in that the transmission means are adapted to transmit the said pulse sequences on carrier frequencies which are selected in accord with the range of altitude of the aircraft at the time of the transmission.

6. An arrangement for effecting a warning of collision potential between aircraft as in any one of the preceidng claims further characterised in that the pulse sequence includes interpretable information relating to the bearing and the air speed of the aircraft.

7. An arrangement for effecting a warning of collision potential between aircraft as in any one of the preceding claims further characterised in that the arrangement includes at least two aircraft a first of which is adapted to send a first interrogation pulse sequence adapted to be uniquely responded to only by the second aircraft and the second aircraft being adapted to uniquely respond with information whereby the closing speed between the two aircraft can be determined.

8. An arrangement for effecting a warning of collision potential between aircraft as in the last preceding claim wherein the closing speed is determinable by means to send a signal comprising a burst of a selected frequency and the said second aircraft on receipt adapted to apply to the signal a multiplying by a fixed ratio and retransmitting the thus changed burst at the new frequency.

9. An arrangement for effecting a warning of collision potential between aircraft as in any one of preceding claims 1 , 2, 3, 4, 5, 6, 7 and 8 further characterised in that the further processing includes assessing the rate of change of closing speed and if the said rate is less than a selected value effecting a warning of potential collision signal.

10. A method for detecting a collision potential between aircraft which comprises the steps of effecting from a first aircraft and on a repeating basis a transmitted signal on a radio frequency comprising a pulse sequence including information including an encoding uniquely identifying the said first aircraft and as well the altitude or a range of the altitude of the said first aircraft at the time of the transmission.

11. A method for detecting a collision potential between aircraft as in the last preceding claim further characterised in that the transmitted signal is repeated with intervals which are selected as between successive pulse sequences on a random or pseudo-random basis.

12. A method for detecting a collision potential between aircraft as in either of the last two preceding claims further characterised in that there is included the further step in that each aircraft upon detection the said pulse sequence from another aircraft assesses the signal on the basis of the altitude information contained therein and effects a priority further assessment only if the altitude detected is within a preselected range of altitudes.

13. A method for detecting a collision potential between aircraft as in any one of the preceding method claims further characterised in that if the altitude information is detected as being within a selected range indicating an initial collision potential there Is effected the next step of uniquely interrogating the aircraft originating the signal.

14. A method for detecting a collision potential between aircraft as in the last preceding claim further characterised in that the interrogation includes effecting a measurement of the closing speed between the respective aircraft.

15. A method for detecting a collision potential between aircraft as in the last but one preceding claim further characterised in that the interrogation includes effecting a measurement of the change of closing speed between the respective aircraft and if this is detected as being less than a selected value effecting a potential collision warning signal.

16. A method of assessing collision potential avoidance substantially as described in the specification with reference to and as illustrated by the accompanying illustrations.