Patent GB-835185-A (GB)
Title: Aircraft collision warning system
IPC: G01S13/933
Filed: 1958-05-15
Abstract: 835,185. Collision warning systems. BENDIX AVIATION CORPORATION. May 15, 1958 [May 31, 1957], No. 15700/58. Class 97(2) [Also in Group XL(c)] In a system for preventing collision between a first aircraft and other aircraft, each craft transmits to other craft within a predetermined range, e.g. 20 miles, coded' pulse signals of random recurrence frequency controlled by a random trigger generator 3, Fig. 1, said signals indicating the craft's "address" (i.e. identity), predicted altitude at a selected future time (e.g. 45 secs. hence), heading and airspeed. The first craft receives these signals from the other craft in a receiver 16 and applies the received identification signals via a modulator 9 to its transmitter 1 during each recurrence period thereof for transmission as acknowledgment signals. When the altitude of the first craft differs from that of another craft, i.e. a "target" craft, by less than 1,000 ft., an altitude signal comparator 18 operates to apply the headings #1, #2, Fig. 2, and airspeeds V1, V2 of the first craft P1 and the target craft P2 to a computer 21, Fig. 3 (not shown), which computes (1) the velocity V of the first craft P1 relative to the target P2 and (2) the angle # 1 between the true velocity vector V1 and the relative velocity vector V of the first craft. The relative bearing # of the target craft P2 from the first craft P1 is also determined by a radio direction finder 36, 37, 38 and if the values of # and #1 as compared in an angle comparator 34 differ by less than a predetermined amount accepted as the limit beyond which a probability of collision is negligible, the comparator 34 operates to cut off the random trigger generator 3 and to open a gate 40 thereby triggering the transmitter 1 in response to the receipt of each set of signals from the target aircraft. If the target aircraft carries identical equipment, a similar operation will have taken place therein and an interrogatortransponder relation will be established between the two craft so that the recurrence frequency of the target craft signals received by the first craft will be inversely proportional to the distance r, Fig. 2, between the two craft and a signal proportional to r is produced by a generator 44. This signal is applied to the computer 21 together with the bearing signal # to give (1) the time t of nearest approach of the first craft to the target craft (t=r/V) and (2) the miss distance c=r sin (#-#) , the quantities c and t being fed to a device 58 which gives a warning if the miss distance c is below an acceptable value when t is equal to or less than one minute. If c is still less than an acceptable value at t=30 secs., the warning device 58 actuates an indicator 60 which indicates one of four predetermined avoidance actions depending on the values #1 and the relative altitudes which are also applied to the indicator 60. The radio direction-finder 36, 37, 38 may utilize the phase difference between signals received by two spaced aerials 37, 38 one of which may be common with the receiver aerial 17. Since the successful operation of the system in any area depends upon active co-operation between all craft in the area, then if it is undesirable to provide collision warning on certain types of craft, e.g. light aircraft, such craft may carry less complicated equipment. As shown in Fig 5 such equipment may comprise a unit 61 for storing the craft identity, altitude, airspeed and heading signals, the output from the storage unit 61 being applied to the transmitter 1 through a readout gate 62 controlled by the output from a random trigger generator as applied through a normally-open second gate 76. Signals from all craft within the predetermined range are received at 16 and when the crafts own identity signal is received in acknowledgment of receipt of said signal by another craft, a decoding unit 78 operates to close the second gate 76 and to control the read-out gate 62. After a period of 1 m. sec. the random transmission is again initiated by a reset unit 80.