WO1990001202A1

WO1990001202A1 - Improvements to aircraft collision avoidance

Info

Publication number: WO1990001202A1
Application number: PCT/AU1989/000321
Authority: WO
Inventors: John Harold Dunlavy; Richard Lane
Original assignee: John Harold Dunlavy
Priority date: 1988-07-28
Filing date: 1989-07-28
Publication date: 1990-02-08

Abstract

A collision avoidance arrangement where aircraft transmits and receives radio signals modulated by a digital pulse sequence (34), if a potential collision is detected (35) the arrangement is adapted to transmit and receive interrogation radio signals and from these signals each aircraft estimates the closing speed of the aircraft (36) and then determines the range between the aircraft (37), and the arrangement also being adapted such that each aircraft transmits to the other the estimate of the closing speed calculated aboard the aircraft and then averages the estimates of the closing speed from each aircraft to obtain a more accurate value for the closing speed of the aircraft.

Description

IMPROVEMENTS TO AIRCRAFT COLLISION AVOIDANCE:

This invention relates to aircraft collision avoidance arrangements and has particular application to an arrangement described in Patent Application

P.C.T. /AU88/00137 and entitled 'AIRCRAFT COLLISION AVOIDANCE' in th name of John Harold DUNLAVY.

Quite obviously it is a tragedy when two aircraft collide in mid-air especially s when passenger aircraft are involved. With the growing demand and use of aircraft the risk of mid-air collision is increasing. Studies have shown in general that as the density of aircraft in a given space is doubled the risk of collision is quadrupled. Around the major airports of the world the number of aircraft is increasing placing greater burden on aircraft controllers and equipment. A consideration of Heathrow or O'Hara airports provide ample illustration of this greater use of aircraft. Compounding this problem is that th aircraft themselves are becoming faster so lessening the time in which to act given a potential collision scenario.

Whilst busy airports spring to mind when considering mid-air collisions, these areas are normally well serviced by ground radar station and air traffic controllers.

But large areas between airports especially between region of high population interspaced by a region of low population may not be well service by ground radar.

Consequently, whilst the number of aircraft within a given space may be low, the information available to the pilot may also be low and also the long periods of inactivity associated with these regions may hinder the pilot's reaction to a potential threat.

Other systems for aircraft collision avoidance use existing navigation signals. This however means the systems are not set up for the safety aspect and so are liable to become ineffective as traffic increases. Other systems use satellites for navigation and safety but this has not yet been fully developed and places reliance on small numbers of equipment which if any one fails would result in a whole region of air space having limited or no aircraft collision avoidance system.

Most existing systems are designed to use existing transponders developed for use with existing secondary surveillance radar (SSR) systems, and aircraf 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 signals 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 a 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, inevitably relies upon a rotating antenna beam both t provide the bearing of any interrogated aircraft and also to reduce garble which must be the frequent result of an omni-directiona! 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 antennaes, have been proposed but once again reduce but do not remove the difficulties.

Further, as more aircraft are equipped with these systems^' the hardware and frequency allocations become overloaded leading to problems due to excessive traffic. Just like a traffic jam the systems can clog up leading to a lack of aircraft collision warning. The result of such a situation is obvious.

An object of this invention is to propose an aircraft potential collision avoidance system which will require significantly less complexity in relation t equipment needed for its effective operation and hence shall be less expensive and therefore more accessible for all aircraft operators, and secondly may provide additional reliability.

Further, this invention allows for an aircraft potential collision avoidance system in which aircraft on a collision course can take co-ordinated evasive action. Previous systems do not always allow this because of the use of existing hardware designed for SSR.

Another advantage provided by the invention is the inherent parallelism of th system resulting in an overall highe^''; reliability of the system by removing the dependence of the overall system upon a limited number of expensive and critical pieces of equipment.

A main advantage of this invention and which is of major importance to aircra collision avoidance systems, is an accurate estimate of the closing speed of the aircraft.

A normalised difference type algorithm for the purpose of this disclosure is defined as follows:

A first and a second variable are measured with respect to a third variable. A first difference value is calculated by the difference between the values of the said first variable for two values of the said third variable. A second differenc value is calculated by the difference between the values of the said second variable for the two aforementioned values of the said third variable. The normalised difference value is calculated from the difference of the said second difference value from the said first difference value and then normalised by the said first difference value. Finally the normalised differenc algorithm consists of the final step of multiplying the said normalised difference by another variable which may be single valued. This can written i an algebraic form as:

A(C)= the first varibie B(C)= the second varibie

C= the third varibie, C" indicates the first value of C and C" indicates the second value of C D= the fouth varibie

ND= the result of the normalised difference algorithm Then,

G= A(C") - A(C') H= B(C") - B(C')

And finally,

ND= [ ( G - H ) / G ] ^* D

The invention can be said to reside in an aircraft collision avoidance arrangement comprising in each of two aircraft a device adapted to transmit and receive radio signals such that the respective aircraft are adapted to communicate to each other through such radio signals a closing speed of the two aircraft to one another, as calculated by a respective device using its on board clock reference, and characterised in that each device is further adapted to perform an averaging calculation using the respective closing speed calculated from the device on each aircraft.

5 The invention can be further characterised as an aircraft collision avoidance arrangement where the said device of each aircraft is adapted to transmit a signal containing information identifying the time of transmission according t the transmitting device, the said device of each aircraft also being adapted to retain the time according to the receiving device that a first radio εignal is -*! o received and the time information contained in the said first radio signal, the said device of each aircraft being adapted to retain the time according to the receiving device that a second signal is received and the time information contained in the said second radio signal, and further each device is adapte to perform the calculation of the closing speed between the aircraft as follow

1 5 v = ( ΔT_A - ΔT_B ) / ΔT_A* c

Where v = the closing speed between the aircraft •20 ΔTA = the difference between the time of reception of the secon radio signal and the first radio signal according to the receiving device. ΔTB = the time difference between the time as represented in th time information contained within the second radio signal 25 and the first radio signal.

C = the speed of light.

It can also be said of the invention that it is an aircraft collision avoidance arrangement in which the said device of each aircraft transmit for the receptio 30 by the other the closing speed between the aircraft as calculated by the transmitting device and each device is further adapted to calculate an averag of the closing speed between the aircraft transmitted to the other aircraft and the closing speed between the aircraft as received from the other aircraft.

35 Further characterising the invention, the invention relates to an aircraft collision avoidance arrangement in which the said device of each aircraft is adapted to transmit an interrogation radio signal. The invention may be further characterised in an aircraft collision avoidance arrangement where the said device of each aircraft is adapted to transmit a radio signal in response and to a received interrogation radio signal.

In characterising the invention further it can be;*said that it relates to an aircra collision avoidance arrangement in which the said device of each aircraft is adapted to measure the time taken for an interrogation radio signal to be responded according to the device, and perform a calculation of the range between the aircraft with allowance for time delay in the other device betwee the receiving of the interrogation radio signal and the transmission of the responding radio signal. The aforementioned range being calculated by multiplying the time taken for an interrogation radio signal to be responded t after allowance for the said time delay, by the value of the speed of light.

The invention may be characterised in an aircraft collision avoidance arrangement in* which the said device of each aircraft is adapted to determin the error in the time according to each device, at the same instant of time.

Further in characterising the invention it may be said that it relates to an aircraft collision avoidance arrangement in which the said device of each aircraft is adapted to use the said error in the time according to each device t provide a more accurate value of the range between the aircraft.

The invention can alternatively be said to reside in an aircraft collision avoidance arrangement aboard a first aircraft being adapted to perform the following modes of operation:

a) a first mode of operation where the aircraft collision avoidance arrangement on the said first aircraft is adapted to: repeatedly transmit a first radio signal modulated by a cyclic digital pulse sequence which in code form provides information of the identity of th said first aircraft, altitude of the said first aircraft and the time according to a clock aboard the said first aircraft; receive radio signals of the same type as the said first radio signal an obtain therefrom the identity, altitude and time according to the aircraft collision avoidance arrangement of other the aircraft, the received radio signals emanating from another aircraft; have processing means to assess from said received signals whether the said other aircraft from which the said received signal emanated from po a threat of collision and if so calculate by the aforementioned normalised difference algorithm the closing speed between the first aircraft and a secon aircraft, the said second aircraft being the aircraft posing a threat of collision; calculates the said closing speed from the time difference between tw received cycles of the said received signals according to information contained within the said received signal and according to information deriv from the said clock on board the said first aircraft as to when the said two cycles of said received signal arrived; b) a second mode of operation where the aircraft collision avoidance arrangement on the said first aircraft is adapted to: communicate with the aircraft collision avoidance device on the said second aircraft, exchange the said closing speed calculated by each, and using the said closing speed of the first and second aircraft perform an averaging calculation so forming a corrected closing speed; assess the threat of collision between the said first and second aircraft and if above a threshold notifies the pilot of the first aircraft; return to the first mode of operation when the threat of collision passes.

In furthering the alternative description of the invention it can be said that the invention relates to an aircraft collision avoidance arrangement in which the said device is adapted to repeatedly transmit a modulated radio signal, said modulated radio signal is transmitted for a short time and the next modulated radio signal transmitted occur, after a time significantly longer time than the said short time. Also, the duration of the said longer time is pseudo-randomly determined, the maximum duration being fixed. In other words, the statistical average ratio of the said short time to said longer time is between 1/100 to 1/10000000.

It may also be said that the invention is further characterised by the said digit pulse sequence has included coded information of the identity of the aircraft, the altitude of the aircraft, the rate of climb or descent of the aircraft, and the time according to the clock aboard the aircraft.

Further the pulse sequence may contain parity bits for information validating and may be for error correcting. The invention may be further characterised in an aircraft collision avoidance arrangement comprising a transmitting means to transmit a sequence of pulses modulated onto a radio signal, the said sequence of pulses contain in coded form the identity of the aircraft, the time according to a clock aboard th aircraft and parity bits for information validation, and the frequency of the sai radio signal is selected as a function of the altitude of the aircraft. The parity bits may also be used for error correcting.

An aircraft collision avoidance arrangement as discussed above further characterised in providing an aural warning of collision threat. This aural warning may be replaced or used in conjunction with a visual warning displa

The invention can be further characterised in suggesting to the pilot or implementing by means of control of the auto pilot, evasive action.

The invention can be described as a method for detecting a collision potenti between aircraft which comprise 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 in coded form information uniquely identifying the said first aircraft, the altitude or a range of the altitude of the said first aircraft and the time according to a clock aboard the said first aircraft, of the transmission.

Further the invention can be characterised as a method of detecting a collisi potential between aircraft in that the transmitted signal is repeated with intervals which are selected as between successive pulse sequences on a random or pseudo-random basis.

Also it can be said that the invention relates to a method for detecting a collision potential between aircraft as discussed above further characterised in that there is included the further step in that each aircraft upon detection o the said pulse sequence from another aircraft assesses the signal on the basis of the altitude information contained therein, calculations of the closin speed from the time information of the aircraft, and the rate of climb or decen information and so effects a priority for further assessment only if the other aircraft poses a threat of collision. The invention may be further characterised in that if in a received signal the altitude information is detected as being within a selected range indicating an initial collision potential there is effected the next step of uniquely interrogatin the aircraft originating the signal.

The invention may be further characterised in that the interrogation includes exchanging the closing speed between the respective aircraft as calculated b the respective aircraft. Then each aircraft calculates an average of the closing speed originating from both aircraft hence resulting in a more accurate assessment of the closing speed.

The invention can be characterised in a method for detecting collision potential in which the range between the aircraft is calculated from the time taken for an interrogated aircraft response to be received after the interrogation has been commenced.

The invention can be further characterised in a method of detecting aircraft collision potential in that a warning is given to the pilot of an impending collision. The warning may further be characterised in that evasive action is suggested to the pilot, the evasive action being co-ordinated between the potentially colliding aircraft. The invention can also control the auto-pilot of the aircraft so as to avoid any collision.

Whilst through out this specification reference will be made to aircraft, the invention may be applied to a vehicle other than aircraft.

The invention may be further characterised in an aircraft collision avoidance arrangement where the said device of each aircraft is adapted to transmit a radio signal containing information identifying a time of transmission according to the transmitting device, the said device of each aircraft also being adapted to retain a time according to the receiving device that a first radio signal is received and the said information identifying a time of transmission contained within the said first radio signal, the said device of each aircraft being adapted to retain a time according to the receiving device that a second radio signal is received and the said information identifying a time of transmission contained within the said second radio signal, each device being further adapted to calculate two time difference values, the first time difference value being the difference in a time represented by the said information identifying a time of transmission contained within the said second radio signal and that time represented by the said information identifying a time of transmission contained within the said first radio signal, and the second difference yalue being the difference in the retained time of receiving the said second radio signal and the said first radio signal, and means to calculate a closing velocity of the said aircraft.

Further to the last preceding paragraph, the aircraft collision avoidance arrangement is adapted to perform a calculation of a normalised velocity factor obtained by calculating the difference between the said first and the said second difference values and normalising the result by the value of the said first difference value, and further being adapted to calculate the closing velocity of the aircraft by multiplying the said normalised velocity factor and the speed of light.

With regard to the method exhibiting the invention it can also be said that ea aircraft assesses the received pulse sequence in regard to the altitude information and the rate of climb or descent information contained therein an calculates from the time information therein the closing speed of the aircraft, and so effects a priority further assessment only if the altitude detected is within a preselected range of altitudes. If the aircraft are at the same or near altitudes after consideration of the rate of climb or descent information thus indicating an initial potential collision there is effected the next step of the method that being to uniquely interrogating the originating aircraft originatin the signal. This interrogation includes the exchange of the closing speed as calculated by the respective aircraft. From the closing speed of the aircraft a calculated by each aircraft an averaged closing speed of the aircraft can be calculated which is more accurate than the individual original values of the closing speed.

In further characterising the method exhibiting the invention the range between the aircraft may be estimated by calculations based upon the time taken for an interrogation response to be received.

The radio signals used by this system would preferably be in the microwave portion of the electromagnetic spectrum. In preference pulse coded modulation would be used though other forms of modulation can be used.

In preference, the duration of time taken to transmit the pulse coded sequences would be very much less than the average period between cycles of transmission of pulse sequences. This allows the aircraft to be listening for transmission for much more time than sending so not masking incoming signals with outgoing signals. Also, by making the exact duration of the perio between transmission pseudo random within a maximum length of time, it becomes statistical negligible that the transmit signal will mask an incoming signal repeatedly.

The invention may be alternatively be said to reside in an aircraft collision avoidance arrangement in which the said radio signal comprise of a radio frequency carrier modulated by a pulse sequence, the said pulse sequence containing information of the altitude, rate of ascent or descent and the identit of the aircraft originating the pulse sequence, and the said radio signal is repeatedly transmitted at time intervals which are pseudo-randomly determined.

Further to the last paragraph the invention can also be said to reside in an aircraft collision avoidance arrangement in which the said pulse sequence also contains parity bits for validation and error correcting.

Further to the last two paragraphs the invention can also be said to reside in an aircraft collision avoidance arrangement in which the said time intervals between the transmission of the said radio signal is determined by the sum of a fixed minimum length of time and a pseudo-randomly generated integer, which may be algebraically written as:

ΔT = Tmin + N^*δt

Where ΔT = time interval between transmission of radio signals. Tmin = Minimum time interval between transmission of radio signals. ^'} N = a pseudo-randomly generated integer variable, δt = a small length of time. Further to the last paragraph the invention can also be said to reside in an aircraft collision avoidance arrangement in which the said small interval of time is approximately the same length of time as the time taken to transmit the said pulse sequence, the said small interval of time is very much larger than a time interval due to the relative change in position of the aircraft and due to th difference in the clock speeds of each device, the said minimum time interval is very much larger than the product of said small interval of time and the said integer variable.

Further to the last paragraph the invention can also be said to reside in an aircraft collision avoidance arrangement in which a closing speed of the aircraft is calculated by the following steps: a) determine a first time interval between two of the transmitted radio signals, b) determine a second time interval between two of the received radio signals, c) determine an intermediate factor by determining the difference between the said first time interval and the said second time interval, ^' d) minimise the said intermediate factor by modifying the value of the first time interval as in step c by adjusting the value of the variable integer in integer steps, e) determine a normalised factor by dividing the minimised value o the said intermediate variable by the value of the said first time interval, f) determine the closing speed by multiplying the said normalised factor by the value of the speed of light.

This can be stated in algebraic form as :

V = (Tb - Ta) / Ta ^* C

Where

V = the closing velocity of the aircraft,

Tb = the first time interval between two of the transmitted radio signals, Ta = the second time interval between two of the received radio signals C = the value of the speed of light. Further to the last paragraph the invention can also be said to reside in an aircraft collision avoidance arrangement in which the said device further adapted to exchange via radio communications the closing speed calculated by the device on each aircraft and perform an averaging calculation of the closing speeds to result in a more accurate value for the closing speed of the aircraft.

Further to the last paragraph the invention can also be said to reside in an aircraft collision avoidance arrangement in which the said device of each aircraft is adapted to transmit an interrogation radio signal.

Further to the last paragraph the invention can also be said to reside in an aircraft collision avoidance arrangement where the said device of each aircra is adapted to transmit a radio signal in response to a received interrogation radio signal.

Further to the last paragraph the invention can also be said to reside in an aircraft collision avoidance arrangement in which the said device of each aircraft is adapted to measure the time taken for an interrogation radio signal to be responded according to the device, and perform a calculation of the range between the aircraft with allowance for time delay in the other device between the receiving of the interrogation radio signal and the transmission o the responding radio signal.

Further to the last paragraph the invention can also be said to reside in an aircraft collision avoidance arrangement, in which the said device of each aircraft is adapted to determine the error in the time according to each device, at the same instant of time, that is to determine the difference between the tim according to the clocks aboard each of the aircraft at the same instant of time.

Further to the last but one paragraph the invention can also be said to reside in an aircraft collision avoidance arrangement, in which the said device of each aircraft is adapted to determine the range between the aircraft by determining a response time taken for an interrogation signal to be responde to, making an allowance for the said time delay and multiplying the response time after the due allowance has been applied by the value of the speed of light. Further to the last two paragraphs but one, the invention can also be said to reside in an aircraft collision avoidance arrangement in which the said device of each aircraft is adapted to use the said estimate of the error in the time according to each device to provide a more accurate value of the range between the aircraft. _?

The invention may also be discribed as a method for detecting a collision potential between aircraft which comprise the steps of effecting from a first aircraft and on a repeating basis a transmitted signal on a radio frequency for reception by a second aircraft, the transmitted signal comprising a pulse sequence including in coded form information uniquely identifying the said first aircraft, the altitude or a range of the altitude of the said first aircraft and the rate of ascent or descent of the said first aircraft.

Further to the last paragraph the invention can also be said to reside in a method of detecting a collision potential between aircraft characterised in tha the transmitted signal is repeated with intervals which are selected as between successive pulse sequences on a random or pseudo-random basis.

Further to either of the last two paragraphs the invention can also be said to reside in a method for detecting a collision potential between aircraft characterised in that there is included the further step in that each aircraft upon detection of the said pulse sequence from another aircraft assesses the signal on the basis of the altitude information contained therein and the rate climb or descent information and so effects a priority for further assessment only if the other aircraft poses a threat of collision.

Further to either of the last three paragraphs the invention can also be said to reside in a method for detecting a collision potential between aircraft characterised in that if in a received signal the altitude information is detecte as being within a selected range indicating an initial collision potential there i effected the next step of uniquely interrogating the aircraft originating the signal.

Further to the last paragraph the invention can also be said to reside in a method for detecting collision potential between aircraft characterised in that the interrogation includes exchanging the closing speed between the respective aircraft as calculated by the respective aircraft. Further to the last paragraph the invention can also be said to reside in a method for detecting collision potential in which each aircraft calculates an average of the closing speed originating from both aircraft hence resulting in more accurate assessment of the closing speed.

Further to the last paragraph the invention can also be said to reside in a method for detecting collision potential in which the range between the aircra is calculated from the time taken for an interrogated aircraft response to be received after the interrogation has been commenced.

For better understanding of the invention reference will be made to the accompanying drawing within the following discussion.

The drawings are:

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

FIG. 2 is a block diagram of a second embodiment,

FIG. 3 is an illustration of the carrier frequency used by the first embodiment showing its dependence on altitude, FIG. 4 is a schematic illustrating the radio spectrum to be used by the first embodiment,

FIG. 5 illustrates aircraft ascending and descending at 200 feet/min or more, this being of special concern to the first embodiment,

FIG. 6 illustrates the pseudo random but repeated transmit signal. The time scale has been compressed for illustration purposes the t.2 period being actually much greater than ti ,

FIG. 7 illustrates the pulse sequence as used by the first embodiment,

FIG. 8 illustrates a range finding technique,

FIG. 9 illustrates an alternative protocol for the pulse sequence, FIG. 10 illustrates the antenna characteristics,

FIG. 1 1 illustrates two alternative protocols for the pulse sequences including parity bits,

FIG. 12 illustrates the algorithm conducted by the second embodiment,

FIG. 13 illustrates in block form a receiver suitable for use with the first embodiment,

FIG. 14 illustrates in block form a receiver suitable for use with the second embodiment,

FIGS. 15 and 16 illustrates a technique for assessing collision potential, FIG. 17 illustrates the pulse sequence used in the third embodiment.

Referring to FIG. 1 the first embodiment will be discussed. Antenna 13 preferably exhibiting a radiation pattern as shown in FIG. 10, is connected to transmitter 10 and a receiver 11 through directional coupler 12. The directional coupler 12 is to provide isolation between transmitter 10 and receiver 11 and so other similar techniques obvious to those skilled in the art can be used.

The computer module 5 controls the running of the system and performs all necessary calculations. It determines the transmitting and receiving frequencies (FIGS. 3, 4 and 5) the pulse sequence and the repeation of the pulse sequence (FIG. 7 and FIG. 8). Also the computer module 5 performs th encoding of the aircraft identity and other information to be transmitted (FIG. 7). The computer module 5 controls the local oscillator frequencies generat by the frequency synthesiser 9.

The calculation performed by the computer module 5 include determining th relative distance and velocity to other aircraft within^'the range of the system; computes the time and location of a potential collision; computes the most suitable evasive action to avoid the collision; generates the data/information for aural and or visual display units 8; and optionally provide commands to t auto pilot through the auto pilot interface 6 for the evasive action to avoid collision.

Analogue signals from the pressure transducer forming an altimeter 1 , the magnetic bearing transducer 2, and the air speed transducer 3 are converte to digital signals by analogue to digital converters within the computer modu 5.

A signal from the clock 4 is fed into the computer module 5. The clock 4 can be any reasonably accurate type of clock common to the art, preferable digit and particularly accurate over the short term. '

In controlling the frequency synthesiser 9, when the aircraft is flying level, th computer module 5 provides a signal dependent on the aircrafts altitude which the frequency synthesiser 9 interprets as a specific transmit frequency (f^*ι - 27 FIG. 3). If the aircraft is ascending or descending in excess of 200 fe per minute (FIG. 5) the computer module 5 instructs the frequency synthesise 9 to supply the transmitter 10 with a carrier at frequency fo, which is a special alert frequency used by all aircraft whilst changing altitude. The computer module 5 also maintains the correct specific normal altitude frequency whilst ascents or descents are being performed. Once level flight is resumed the alert frequency fo is not transmitted.

The receiver as depicted in FIG. 13 is designed to simultaneously receive signals on four channels, one of the fixed frequencies being designated fo, th other three vary with the altitude of the aircraft. A typical spectral arrangemen is shown in FIG. 4. The lowest varying frequency f[_ corresponds to the altitud range below that of the aircraft, the centre varying frequency fc corresponds t the altitude range of the aircraft and the highest varying frequency fh corresponds to the altitude range directly above the aircraft.

For the special cases when the aircraft is flying at the altitude corresponding t a frequency of fi or f27 then only two frequency signals are sent, that is fι/f2 or f26/f27.

The receiver 7 of FIG. 1 is shown in more detail in FIG. 13. It is a superhetrodyne type using double-conversion.

The first portion of the receiver is standard with the local oscillator frequencies being supplied from a frequency translator 45. The received RF signal is filtered by RF filter 39, amplified by RF amplifier 40 and then mixed with the first local oscillator signal in mixer 41 , the output of which is fed to the first Intermediate Frequency Amplifier 42. The output of the first Intermediate Frequency Amplifier 42 (IF Amp) is split by the signal splitter 43 and then fed to mixers 46 and 47.

The output of mixer 46 is then split into three by the 3 way splitter 44 and each output is separately fed to a second I.F. amplifier 48, 49, 50. The second I.F. amplifiers 48, 49, 50 are tuned to receive the signal corresponding to lowest fl_, centre fc and highest fh frequencies respectively. The output of each I.F. amplifier 48, 49, 50 is fed to a detector 52, 53, 54 respectively. The detected signals are then supplied to the computer module 5. The other output of the signal splitter 43 is mixed in mixer 47, amplified in I.F. amplifier 51 and detected in detector 55 similar to the other signals except tha this part is designed to receive frequency fo.

The aforementioned detectors would be of a type suitable to demodulate the type of modulation used by the system.

The computer module 5 determines the following:

1. For frequency fo:

(a) Aircraft's identification

(b) Altitude and rate of ascent or descent

(c) Direction of flight

(d) Airspeed (e) Time

2. For frequency fc:

(a) Aircraft's identification

(b) Precise altitude of intruding aircraft relative to own aircraft (c) Direction of flight

(d) Airspeed

(e) Time

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 memor circuitry, the Computer Module 5 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 separatin 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. 8. The subject plane receive this transmitted sequence and, in the manner of a transponder, re-transmits the short-duration, high-power pulse within a standard delay time at frequen fc. The re-transmitted, transponder pulse is received by the original sending aircraft and demodulated by receiver 11 of FIG. 1. The Computer Module 5 then calculates the distance separating the two aircraft by solving the following simple mathematical expression:

Dnm = 0.1618 (t - d)

2 where: Dnm = distance in nautical miles separating aircraft

t = total elapsed time in micro-seconds between transmissions and reception of transponder pulse

d = delay time with transponder in micro-seconds

Once the distance and relative velocity are known, they may be added to the already known flight directions and airspeeds 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 5 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 remai 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 time of transmission and reception may be used in a difference type algorithm with the speed of light to determine the closing speed of the aircraft. V = ( ΔT_A - ΔT_B ) / ΔT_A ^* C

Where v = the closing speed between the aircraft

ΔTA = the difference between the time of reception of the secon radio signal and the first radio signal according to the receiving device. ΔTB = the time difference between the time as represented in th time information contained within the second radio sign and the first radio signal. C = the speed of light.

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 th usable for aircraft received frequency fc. In addition to considering the flight parameters mentioned above, it is necessary for the fo algorithm to calculat 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 ordϊnates. 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. 7 and FI 8 would be that illustrated in FIG. 9, wherein only aircraft identification, altitu and time are transmitted during the short-duration burst transmissions depicted in FIG. 6. 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 shortes 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.

The second embodiment will now be discussed. FIG. 2 shows the overall system in block diagram form. The computer module 16 has inputs from the altimeter 14, the clock 15 and the receiver 22. The clock 15 and altimeter 1 can be the same as clock 4 and altimeter 1 described earlier. Analogue to digital converter are used where necessary and here are included along with memory in the computer module 14.

The computer module 14 performs the same general function as for the first embodiment.

A pulse sequence as shown in FIG. 1 1 is supplied by the computer module 1 to the transmitter 20. FIG. 11 shows two forms of pulse sequence including in the first case aircraft identity 25, aircraft altitude 26, rate of climb or descent 2 time of transmission 28 according to the clo.ck is and parity bits 29. In the second case the sequence is identity and parity 30 combined, aircraft altitude 31 , aircraft rate of climb or descent 32 and the time of transmission 33 according to the clock 15. The sequence is preferably pulse coded modulate onto a carrier frequency provided by the local oscillator 12. The output of the transmitter 20 is fed via the directional coupler 23 to the antenna 24. The directional coupler provide isolation between transmitted signals from transmitter 20 and the receiver 22.

Received signals from the antenna 24 are fed via the directional coupler 23 t the receiver 22 which is shown in more detail in FIG. 14. The received pulse sequences is supplied by the receiver 22 to the computer module 14.

As in the first embodiment the computer module 16 performs calculations and determines if any threat of collision exists. If a potential collision is detected a warning to the pilot is provided by the aural and visual display 19 and evasiv action can be instructed to the auto pilot via the auto pilot interface 17.

Considering the receiver 22 in more detail. A block diagram is given in FIG. 14. The receiver is of the common superhetrodyne type. The received signal is fed into a RF filter 56 then amplified by radio frequency amplifier 57 and then mixed with a signal from the local oscillator 61 in by mixer 58. The intermediate frequency signal from the mixer 58 is amplified by the intermediate amplifier 59 and then the pulse sequence in detected for the computer module 14 by the detector 60.

FIG. 12 illustrates the main process carried out by the system. There are two modes of operation. The first consists of block 34 and block 35 in a cyclic fashion dependent on whether a potential collision is detected. Block 34 consists of the following steps: transmit pulse sequence (FIG. 1 1 ); listen for other signals, from received signals calculate if a threat is poised by the other aircraft based on the relative altitude and the rate of climb or descent and upon a calculation of the closing speed. Block 35 asks if there has been a threat of collision detected. If not so then the computer module 14 repeats th steps in block 34. If yes the computer module 14 proceeds with the steps in blocks 36, 37 and 38. Block 36 includes the transmission of an interrogation sequence including the identification of the aircraft, the identification of the aircraft posing a threat, an estimate of the closing speed.

In block 37 the computer module 14 calculates the range between the aircraf and an accurate estimate of the closing speed of the aircraft, based on the average of the closing speed estimates calculated by each aircraft in block 3 (in block 38). From the information now available to the computer module 14 assesses the threat of collision and if applicable further communicates with the other aircraft to provide warning to the pilot and evasive action which is c ordinated with the other aircraft, to the auto pilot.

The range between the aircraft may be determined by one of the following means:

A radar pulse, intended to be reflected by the metallic surfaces of the other plane is transmitted. This radar pulse, traveling at the precise speed of light, returns to the sending plane within a total time of about 6.7 micro-seconds fo each kilometre of separation. The computer uses this time interval to determine the distance which separates the two planes and can, by comparing the rate of any change in distance, also determines the rate of closure between them.

Alternately the time taken for a response to the interrogation pulse sequence is a function of the distance between the aircraft and a delay in the other aircraft between reception and transmit. That is the interrogation sequence i essentially a two-way communication between aircraft. The identities of both aircraft are necessary to ensure a unique "channel" between the aircraft, i.e. ensure that there is no confusion with signals passed between otl^~9r pairs of aircraft. The range between aircraft is determined by, ^'■ R = (ΔTl - ΔT^D) / c

where R is the computed range, ΔT¹ is the time between initiating the interrogation and receiving a reply, ΔT^D is a constant allowing for delays in t receiver processing time and c is the speed of light.

Since the interrogation and response contain similar data, in the case when the interrogated aircraft also requires fresh range data, asking th l<e interrogating aircraft to treat the interrogated aircraft's response a as an interrogation saves one signal between the aircraft.

If an interrogation fails, i.e. the interrogating aircraft does not receive a reply, fresh interrogation is re-schedule a random time later. This random delay before re-trying is to ensure that interrogations from different aircraft do not repeatedly overlap.

The following description demonstrates a preferred technique for an aircraft determine the relative position of another aircraft on a collision course, usin 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 take into account climbing and descending aircraft. Furthermore, in the following description aircraft A determines the relative position of aircraft B, although the roles can easily be reversed.

In FIG. 15, aircraft will obtain from the pseudo-random transmissions of aircr 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. 15 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. 16. 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. 16. As a consequence it is possible for aircraft A to uniquely determine the position of aircraft B relative t 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 indicat a potential collision situation.

Accordingly there are proposed means whereby the closing speed between 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.

The closing speed calculation is critical. If it is inaccurate the computer module 14 will perceive the other aircraft in a different location, the result being obviously unsatisfactory. According to this invention it has been discovered that any error caused by differing frequencies of clocks on respective aircraft can be effectively canceled making an assessment of closing speed potentially more accurate.

According to this invention then, there is in respect of each participating aircraft, means so that both the sending and receiving aircraft will effect calculation of a closing speed, and upon this being calculated, transmit the respective information to the other aircraft and there are means with each aircraft such that the two measurements will be averaged to calculate and provide an assessment of closing speed.

The result of this is that any error resulting from differences between the clock rates can be made to be self canceling so that the assessed closing speed can be accurate within a tighter tolerance.

For a further explanation of this, it will now be further described as follows.

Assume that the clocks on each respective aircraft are running at slightly different rates (i.e. one of the clocks is slow). If the clock on the assessing aircraft measures an elapsed time T, then the clock on board the other aircraft will measure an elapsed time T + dT, where dT is the difference in time determined by how much slower (or faster) the clock on the threat aircraft is running. The error in closing speed is given by-

dT error in closing speed = X speed of light

T

(computed by assessing aircraft).

There are two techniques for combating this problem. The first is simply to make dT very small by using an extremely stable clock. The second involves canceling the error. This can be done by observing that the closing speed of the assessing aircraft, as computed by the threat aircraft, will be in error by-

-dT error in closing speed = X speed of light

T (computed by threat aircraft)

Thus when the closing speeds, as computed by the assessing aircraft and the threat aircraft are averaged, these errors will cancel and the result will be the correct closing speed regardless of whether a clock on board an aircraft is slow or not.

Furthermore, once the correct closing velocity is known to the assessing aircraft it is possible to deduce dT. It is then possible to correct future estimates of the closing velocity without needing to obtain the closing speed computed on board the other aircraft. Since it is relatively simple to ensure that clocks are very stable for a period of a few minutes, accurate estimates of closing velocity can now be made solely from the beacon signals of the threat aircraft.

From the process just outlined the range between the aircraft can be predicte and checked.

After a^'new beacon is received the following calculations can be performed

S_c ^* = S_cs + E _ss

R new = R old + ^"Tfc> ^* Sc

to estimate the range and closing speed between interrogations (T^ is again the time between beacon broadcasts). The advantage of using these estimated values is that interrogations need to be performed less frequently and the system can still function whilst re-trying a failed interrogation.

A third preferred embodiment of the invention uses the same constituents as the second preferred embodiment with the following differences.

The pulse sequence is illustrated in Fig. 17 and consists of a bit stream uniquely identifying the aircraft 62, the altitude of the aircraft 63 and the rate of ascent or descent of the aircraft 64. ^' ; A potential collision is determined if a received signal provides information that the transmitting aircraft is at the same altitude as the receiving aircraft or will be from assessment of the rate of ascent or descent within a space of time determined by the algorithm being used. Once a collision threat has been 5 determined the closing speed of the aircraft is determined.

The pulse sequence is transmitted at intervals defined by:

ΔTB = tmin + N^*δT o Where tmin = a minimum time interval,

N = an integer variable which is pseudo-randomly determined, δt = a small time interval, the length of which is comparable to the time required to transmit the pulse sequence.

5 The value of the minimum time interval is between 0, 1 and 1 second in duration.

The time interval between the transmission of pulses has a fixed component tmin to which is added the pseudo-random component N^*δT. 0

Provided N^*δt is small compared to tmin for a maximum value of N then the time interval is relatively constant. Further, provide N has a maximum value which is large, then the effects of the deterministic nature of tmin is of little concern with regard to repeated garbling of received signals. 5

The closing speed of the aircraft can be determined by:

V = [(Tb - Tr) / Tb] ^* C where Tb = time between two broadcasts of the pulse sequence Tr = time between two receptions of the pulse sequence

V -= closing speed of the aircraft C = the speed of light

Which can be rewritten as:

V = [(tmin + Kδt) - (tmin +Qδt+)] / (tmin + Kδt) ^* C

where K and Q are integers, and A = the difference in the time intervals of Tb and Tr due to the distance between the aircraft increasing or decreasing and the differences in the clock speeds aboard the individual aircraft.

5 The equation may be simplified to:

V = [{(K-Q) δt - A} / {tmin +Kδt}] ^* C

from which it can be seen that in K is made equal to Q and Kδt is very small 1 o compared to tmin then the closing velocity is given by:

V = [(-A) /tmin] ^* C

The above calculation process is easy to perform using a digital computer ^■j 5 especially if δt is much bigger than A. Clearly, the case here for A being a positive value is that of the aircraft increasing the distance between them. Th value of A will be very small due to the speeds at which aircraft can travel.

Once the closing velocity has been calculated the aircraft exchange the 20 values determined by each for the closing velocity and average the two value to eliminate the error in the values due to differences in the clock speeds of the aircraft. This has been more fully explained earlier.

It is obvious to anyone skilled in the art that alternative circuit and analogous 25 processes would all fall within the spirit of the invention.

As can be seen from this discussion the invention described herein will provide air travel with greater safety. The accurate determination of the closing speed between aircraft iε of paramount importance to this system as 30 with all other like systemε. The invention alleviateε the problems with existin systems by providing a simple, economic and elegant system.

It goes without saying that this invention could be applied to vehicles other than aircraft.

Claims

The Claims defining the invention are as follows:

1. An aircraft collision avoidance arrangement comprising in each of two aircraft a device adapted to transmit and receive radio signals such that the

5 respective aircraft are adapted to communicate to each other through such radio signals a closing speed of the two aircraft to one another, as calculated by a respective device using its on board clock reference, and further characterised in that each device is further adapted to perform an averaging calculation using the respective closing speed calculated from the device on o each aircraft.

2. An aircraft collision avoidance arrangement as in the preceding claim, where the said device of each aircraft is adapted to transmit a signal containing information identifying the time of transmission according to the 5 transmitting device, the said device of each aircraft also being adapted to retain the time according to the receiving device that a first radio signal is received and the time information contained in the said first radio signal, the said device of each aircraft being adapted to retain the time according to the receiving device that a second signal is received and the time information contained in the said second radio signal, and further each device is adapted to perform the calculation to form an estimate of the closing speed between ^* the aircraft as follows:

Where v = the closing speed between the aircraft

ΔTA = the difference between the time of reception of the second radio signal and the first radio signal according to the receiving device.

ΔTB = the time difference between the time as represented in the information contained within the second radio signal and the first radio signal. C = the speed of light.

3. An aircraft collision avoidance arrangement as in either of the preceding claims in which the said device of each aircraft transmit for the reception by the other closing speed between the aircraft as calculated by the transmitting device and each device is further adapted to calculate an averag closing speed between the aircraft transmitted to the other aircraft and the closing speed between the aircraft as received from the other aircraft.

4. An aircraft collision avoidance arrangement as in any one of the preceding claims in which the εaid device of each aircraft iε adapted to tranεmit an interrogation radio signal.

5. An aircraft collision avoidance arrangement as in claim 4 where the said device of each aircraft is adapted to transmit a radio signal in response to a received interrogation radio signal.

6. An aircraft collision avoidance arrangement as in claim 5 in which the said device of each aircraft is adapted to measure the time taken for an interrogation radio εignal to be reεponded according to the device, and perform a calculation of the range between the aircraft with allowance for time delay in the other device between the receiving of the interrogation radio signal and the transmiεsion of the responding radio signal.

7. An aircraft collision avoidance arrangement as claimed in claim 2, in which the said device of each aircraft is adapted to determine the error in the time according to each device, at the same instant of time, that is to determine the difference between the time according to the clocks aboard each of the aircraft at the same instant of time.

8. An aircraft collision avoidance arrangement as in claims 6 and 7 in which the εaid device of each aircraft iε adapted to use the said estimate of the error in the time according to each device to provide a more accurate value of the range between the aircraft.

9. An aircraft collision avoidance arrangement aboard a first aircraft being adapted to perform the following modes of operation:

a) a firεt mode of operation where the aircraft collision avoidance arrangement on the said first aircraft is adapted to: repeatedly transmit a first radio signal modulated by a cyclic digital pulse sequence which in code form provide^' 3 information of the identity of the said first aircraft, altitude of the said first aircraft and the time according to a clock aboard the said first aircraft; receive radio signalε of the εame type aε the εaid firεt radio εignal and obtain therefrom the identity, altitude and time according to the aircraft 5 collision avoidance arrangement of other the aircraft, the received radio signa emanating from another aircraft; have processing means to assess from said received signalε whether the said other aircraft from which the said received signal emanated from, pose a threat of collision and if so calculate by the aforementioned normalised 10 difference algorithm the closing speed between the first aircraft and a second aircraft, the said second aircraft being the aircraft posing a threat of collision; calculates the said closing speed from the time difference between two received cycles of the εaid received εignals according to information contained within the εaid received signal and according to information derive ■ 5 from the said clock on board the said first aircraft as to when the εaid two cycles of said received signal arrived; b) a second mode of operation where the aircraft collision avoidance arrangement on the said firεt aircraft is adapted to: communicate with the aircraft collision avoidance device on the said 20 second aircraft, exchange the εaid cloεing speed calculated by each, and using the said closing speed of the first and second aircraft perform an averaging calculation so forming a corrected closing speed; asεeεs the threat of collision between the said first and second aircraft and if above a threshold notifies the pilot of the first aircraft; 25 return to the first mode of operation when the threat of collision pasεes.

10. An aircraft collision avoidance arrangement as in claim 2 in which the said device is adapted to repeatedly transmit a modulated radio signal, said modulated radio signal is transmitted for a short time and the next modulated

30 radio signal transmitted occur, after a time significantly longer time than the said short time.

1 1. An aircraft collision avoidance arrangement as in claim 10 where the duration of the said longer time is pseudo-randomly determined, the maximum

35 duration being fixed.

12. An aircraft colliεion avoidance arrangement aε in claim 10 and or 11 where the statistical average ratio of the said εhort time to εaid longer time iε between 1/100 to 1/10000000.

13. Aε in any of the claimε 9, 10, 11 or 12 an aircraft colliεion avoidance arrangement further characteriεed by the εaid digital pulse sequence has included coded information of the identity of the aircraft, the altitude of the aircraft, the rate of climb or descent of the aircraft, and the time according to the clock aboard the aircraft.

14. An aircraft collision avoidance arrangement as in claims 9 or 13 and being ^'adapted such that said digital the pulse sequence further contain parit bitε for information validating.

15. An aircraft coliiεion avoidance arrangement aε in claim 14, and being adapted εuch that the εaid parity bitε are uεed for information validating and error correcting.

16. An aircraft colliεion avoidance arrangement compriεing a tranεmitting meanε to tranεmit a εequence of pulεeε modulated onto a radio εignal, the εaid εequence of pulεes contain in coded form the identity of the aircraft, the time according to a clock aboard the aircraft and parity bits for information validation, and the frequency of the said radio signal is selected as a functio of the altitude of the aircraft.

17. An aircraft collision avoidance arrangement aε in claim 16, being adapted such that the said parity bits are used for information validation and correction.

18. An aircraft collision avoidance arrangement as in any one of the claim 9 to 17 further characterised in providing an aural warning of collision threat.

19. An aircraft collision avoidance arrangement as in claim 18, being adapted such that the said warning is displayed in a visual form.

20. An aircraft collision avoidance arrangement further characterised in suggesting to the pilot or implementing by means of control of the auto pilot, evaεive action.

21. A method for detecting a collision potential between aircraft which comprise the stepε of effecting from a firεt aircraft and on a repeating baεis a transmitted signal on a radio frequency for reception by a second aircraft, the transmitted signal comprising a pulse sequence including in coded form information uniquely identifying the said first aircraft, the altitude or a range of the altitude of the said first aircraft and the time according to a clock aboard the said first aircraft at the time of the transmission.

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

23. 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 of the said pulse sequence from another aircraft assesseε the εignal on the baεiε of the altitude information contained therein, calculationε of the closing speed from the time information of the aircraft, and the rate of climb or decent information and so effects a priority for further asεesεment only if the other aircraft poses a threat of collision.

24. A method for detecting a collision potential between aircraft as in any one of the preceding method claims further characterised in that if in a received signal 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.

25. A method for detecting collision potential between aircraft as in the last preceding claim further characterised in that the interrogation includes exchanging the cloεing speed between the respective aircraft as calculated b the respective aircraft.

26. A method for detecting collision potential as in the last preceding claim in which each aircraft calculates an average of the closing speed originating from both aircraft hence resulting in a more accurate assessment of the closing speed.

27. A method for detecting collision potential as in claim 26 in which the range between the aircraft is calculated from the time taken for an interrogate aircraft reεponεe to be received after the interrogation has been commenced.

28. A method of detecting aircraft collision potential as in any of the preceding method claims further characterised in that a warning is given to th pilot of an impending collision.

29. A method of detecting aircraft colliεion potential aε in the preceding claim further characteriεed in that evaεive action is suggeεted to the pilot, the evaεive action being co-ordinated between the potentially colliding aircraft.

30. A method of assessing collision potential avoidance subεtantially aε deεcribed in the εpecifϊcation with reference to and aε illuεtrated by the accompanying diagramε.

31. An aircraft colliεion avoidance arrangement aε εubεtantially described in the specification with reference to and aε illustrated by the accompanying diagrams.

32. An arrangement as claimed in claims 9 to 22 applied to a vehicle other than aircraft.

33. A method as claimed in claims 23 to 31 applied to a vehicle other than aircraft.

34. A method of detecting aircraft collision potential as in claim 24 in which the auto pilots of the aircrafts are instructed as to evaεive action required.

35. An aircraft collision avoidance arrangement as in claim 1 , where the said device of each aircraft is adapted to tranεmit a radio εignal containing information identifying a time of transmission according to the transmitting device, the said device of each aircraft also being adapted to retain a time according to the receiving a device that a first radio signal is received and the said information identifying a time of transmission contained within t e said first radio signal, the εaid device of each aircraft being adapted to retain a time according to the receiving device that a second radio signal is received and the said information identifying a time of transmission contained with the said second radio signal, each device being further adapted to calculate two time difference values, the first time difference value being the difference in a time represented by the said information identifying a time of transmiεsion contained within the said second radio εignal and that time represented by the said information identifying a time of transmiεεion contained within the said first radio signal, and the second difference value being the difference in the retained time of receiving the said second radio εignal and the εaid first radio signal, and means to calculate a closing velocity of the said aircraft.

36. An aircraft collision avoidance arrangement as in the last preceding claim adapted to perform a calculation of a normalised velocity factor obtained by calculating the difference between the said first and the said second difference values and normalising the result by the value of the said first difference value, and further being adapted to calculate the closing velocity of the aircraft by multiplying the said normalised velocity factor and the speed of light.

37. An aircraft collision avoidance arrangement as in claim 6, in which the said device of each aircraft is adapted to determine the range between the aircraft by determining a reεponεe time taken for an interrogation εignal to be responded too, making an allowance for the said time delay and multiplying the response time after the due allowance by the value of the speed of light.

38. An aircraft colliεion avoidance arrangement aε in claim 1 in which the εaid radio signal comprise of a radio frequency carrier modulated by a pulse sequence, the said pulse sequence containing information of the altitude, rate of ascent or descent and the identity of the aircraft originating the pulse sequence, and the said radio signal is repeatedly transmitted at time intervals which are pseudo-randomly determined.

39. An aircraft collision avoidance arrangement as in claim 38 in which the said pulse sequence also contains parity bits for validation and error correcting.

40. An aircraft colliεion avoidance arrangement aε in claim 38 or claim 39 in which the εaid time intervalε between the transmission of the said radio signal iε determined by the εum of a fixed minimum length of time and a pseudo-randomly generated integer, which may be algebraically written as:

ΔT = Tmin + N^*δt

Where ΔT = time interval between tranεmiεεion of radio εignalε

Tmin = Minimum time interval between tranεmiεεion of radio εignals.

N = ^' a pseudo-randomly generated integer variable, δt = a small length of time.

41. An aircraft collision avoidance arrangement as in claim 40 in which th εaid small interval of time is approximately the same length of time as the tim taken to transmit the said pulεe sequence, the said small interval of time is very much larger than a time interval due to the relative change in poεition o the aircraft and due to the difference in the clock εpeedε of each device, the said minimum time interval is very much larger than the product of said εmal interval of time and the εaid integer variable.

42. An aircraft colliεion avoidance arrangement aε in claim 40 in which a closing speed of the aircraft is calculated by the following stepε: a) determine a firεt time interval between two of the tranεmitted radio εignalε, b) determine a εecond time interval between two of the received radio εignalε, c) determine an intermediate factor by determining the difference between the said first time interval and the said second time interval, d) minimise the said intermediate factor by modifying the value of the first time interval aε in εtep c by adjuεting the value of the variable integer in integer steps, e) determine a normalised factor by dividing the minimised value the said intermediate variable by the value of the said first time interva f) determine the closing speed by multiplying the said normalised factor by the value of the speed of light.

This can be stated in algebraic form as : V = (Tb - Ta) / Ta ^* C

Where V = the closing velocity of the aircraft,

Tb = the first time interval between two of the transmitted radio signals, Ta = the second time interval between two of the received radio signalε C = the value of the speed of light.

43. An aircraft collision avoidance arrangement as in claim 42 in which the said device further adapted to exchange via radio communications the closin speed calculated by the device on each aircraft and perform an averaging calculation of the closing speeds to result in a more accurate value for the closing speed of the aircraft.

44. An aircraft collision avoidance arrangement as in claim 43 in which the said device of each aircraft is adapted to transmit an interrogation radio signal

45. An aircraft collision avoidance arrangement as in claim 44 where the εaid device of each aircraft is adapted to transmit a radio signal in responεe to a received interrogation radio signal.

46. An aircraft collision avoidance arrangement as in claim 45 in which the said device of each aircraft iε adapted to meaεure the time taken for an interrogation radio signal to be responded according to the device, and perform a calculation of the range between the aircraft with allowance for time delay in the other device between the receiving of the interrogation radio signal and the transmission of the responding radio signal.

47. An aircraft collision avoidance arrangement as claimed in claim 46, in which the said device of each aircraft is adapted to determine the error in the time according to each device, at the same instant of time, that is to determine the difference between the time according to the clocks aboard each of the aircraft at the same instant of time.

48. An aircraft collision avoidance arrangement as in claim 46, in which the said device of each aircraft is adapted to determine the range between the aircraft by determining a reεponεe time taken for an interrogation εignal to be reεponded to, making an allowance for the εaid time delay and multiplying th reεponεe time after the due allowance haε been applied by the value of the εpeed of light.

49. An aircraft collision avoidance arrangement as in claims 46 and 47 in which the said device of each aircraft is adapted to use the said estimate of the error in the time according to each device to provide a more accurate value of the range between the aircraft.

50. A method for detecting a collision potential between aircraft which comprise the steps of effecting from a first aircraft and on a repeating basis a transmitted signal on a radio frequency for reception by a second aircraft, the transmitted signal comprising a pulse sequence including in coded form information uniquely identifying the εaid firεt aircraft, the altitude or a range o the altitude of the εaid firεt aircraft and the rate of aεcent or deεcent of the sai first aircraft.

51. A method of detecting a collision potential between aircraft as in the la preceding claim further characterised in that the transmitted signal is repeate with intervals which are selected as between successive pulse sequences o a random or pseudo-random basis.

52. A method for detecting a collision potential between aircraft as in eithe of the last two preceding claims further characterised in that there is included the further step in that each aircraft upon detection of the said pulse sequenc from another aircraft asεeεεeε the εignal on the baεis of the altitude information contained therein and the rate of climb or descent information an so effects a priority for further assessment only if the other aircraft poseε a threat of collision.

53. A method for detecting a collision potential between aircraft as in any one of claimε 50, 51 and 52 further characteriεed in that if in a received εign the altitude information iε detected aε being within a εelected range indicati an initial collision potential there is effected the next step of uniquely interrogating the aircraft originating the signal.

54. A method for detecting collision potential between aircraft as in the last preceding claim further characterised in that the interrogation includes exchanging the closing εpeed between the respective aircraft as calculated b the respective aircraft.

55. A method for detecting collision potential as in the last preceding claim in which each aircraft calculates an average of the closing speed originating from both aircraft hence resulting in a more accurate asseεεment of the closing speed.

56. A method for detecting collision potential as in claim 55 in which the range between the aircraft is calculated from the time taken for an interrogate aircraft response to be received after the interrogation has been commenced.

57. A method of detecting aircraft collision potential as in any of the preceding method claims further characterised in that a warning is given to th pilot of an impending collision.

58. A method of detecting aircraft collision potential as in the preceding claim further characterised in that evasive action is suggeεted to the pilot, the evaεive action being co-ordinated between the potentially colliding aircraft.