ASYNCHRONOUS COLLISION AVOIDANCE SYSTEM

1,265,823. Pulse radar. McDONNEL DOUGLAS CORP. 24 March, 1969, No. 15383/69. Heading H4D. In an aircraft collision avoidance system the radio signal is modulated by the output of an altitude encoder 22, Fig. 1, received signals are demodulated to compare " other " with " own " aircraft altitude, frequency-sensitive means determine range rate between " other " and " own " aircraft and a reply signal generator responsive to the frequency-sensitive means transmits a reply signal at a precise instant relative to the receipt of the signal from the other aircraft whenever they are at substantially the same altitude and are closing. In the embodiment a relatively long pulse 28, Fig. 2, is followed by a pulse 30, the interval between the pulses being proportional to the altitude of the interrogating aircraft; a co-operating aircraft (provided that the altitude and closing conditions are met) replies with a modulated pulse after a delay: when the two aircraft remain the same distance apart (as evidenced by zero Doppler shift) there is an arbitrary delay T w but for a closing range rate R and an arbitrary " warning " time T the reply pulse time position is given by T early = 2RT/C, where C is the propagation constant. The modulation of the reply pulse is coded for identification and may also carry an instruction to climb or dive to the interrogating aircraft. The interval T A = T w +2/C(R-RT), where R is the distance between the aircraft: a warning signal is generated only when T A