Patent GB-1219244-A (GB)
Title: Aircraft proximity warning transponder system
IPC: G01S13/78
Filed: 1969-06-02
Abstract: 1,219,244. Pulse radar. BENDIX CORP. 2 June, 1969 [26 June, 1968], No. 27860/69. Heading H4D. In an aircraft (e.g. helicopter) collision warning system, an interrogating aircraft transmits interrogations which include a unique range pulse group and a unique own-altitude pulse group, the intruder receives the interrogations, extracts the altitude data, compares it with its own altitude and responds a predetermined time after receiving the range pulse group with another unique pulse group containing altitude comparison information, and the system is characterized by specified features which include the utilization of jittered timing pulses. In the embodiment all craft carry the same interrogator-transponder apparatus; an interrogating aircraft transmits at a jittered one second rate a range pulse triad (10, Fig. 1, not shown) followed by an altitude pulse triad (12) whose position with respect to the range triad is a linear function of its altitude plus a fixed guard interval to obviate multipath effects; some time (e.g. 69À6 Ás.) after transmission a range gate (14) is opened to permit responses from aircraft within 2000 ft.to be processed; and after that gate closes there is a return to a reply mode for the remainder of the one-second period. In the reply mode an intruder compares received altitude with its own and initiates a "high" or "low" reply if the two aircraft are within + 300 ft. or - 300 ft. respectively, but not closer than 100 ft. in altitude, or a "co-altitude" reply if within  100 ft. Fig. 2 shows the basic counting and some logic functions of the interrogating aircraft: each jittered ("timer") pulse is fed via AND gate 30 (assuming flip-flop 32 is reset) to a delay line 48 which affords a series of pulses: a pulse "0" having zero delay, a pulse "1" of one pulsewidth delay . . . up to pulse "4" and a pulse "7" of a seven pulsewidth delay; and the jittered pulse sets flip-flop 32 thereby causes oscillator 37 to generate a train of clock pulses for application to counter 39 arranged such that a signal appears on line 39a when the count is less than 8, on line 39c at count 18, on line 39d at count 40, on line 39e at count 41, on line 39f at count 48 and on line 39b at count 54. Pulses "0", "3" and "7" provide the transmitted range triad via an AND gate (103, Fig. 3, not shown) opened when there is a signal on line 39a. The signal on line 39c triggers a linear ramp generator 110, Fig. 4, the voltage from which is compared, 114, with a voltage generated by altimeter 111 and when the voltages are equal a signal appears at terminal 118 ; as a result, a signal appears at terminal 59, Fig. 2, to open an AND gate (109, Fig. 3, not shown) to permit transmission of the altitude pulse triad and a one-shot multivibrator 60 is triggered to apply a pulse to delay line 48 to form that triad ("0", "4", "7"). Altitude comparison at the interrogator utilizes outputs "0", "1" and "2" from the delay line, a range gate and shift registers and actuates an appropriate visual alarm, Fig. 5 (not shown). The system is particularly useful for helicopters since they are relatively slow and thus participant density is not a problem. Jittered pulses eliminate interference due to second and third aircraft both outside the range of interest producing a response at a first aircraft; the jittering is greater than the range gate width and at least two successive responses must be received before a collision threat warning is given.