Patent GB-844977-A (GB)
Title: Improvements in or relating to anti-collision radio equipment for aircraft use
Inventor: WILD JACK
IPC: G01S13/933
Filed: 1958-02-03
Abstract: 844,977. Pulse radar; transponder apparatus. MARCONI'S WIRELESS TELEGRAPH CO. Ltd. Sept. 17, 1958 [Feb. 3, 1958], No. 3440/58. Classes 40(5) and 40(7). Relates to an aircraft anti-collision secondary radar system wherein each aircraft carries (1) an interrogator transmitting at a radio frequency Fl pulse groups coded in accordance with a predetermined selectable height (e.g. the height of the interrogating aircraft), (2) a transponder transmitting at a radio frequency F2 a reply pulse in response to each received interrogating pulse group coded in accordance with the height of the re-sponding aircraft and (3) a responsor indicating receipt of a reply pulse. According to the present invention the interrogating pulse groups are transmitted at a group recurrence frequency of 10 c/s. and each group comprises two pairs of pulses P1 and P2, Fig. 4(a), the fixed spacing between the pulses of a pair being 4 Ásec and the spacing x between the two pairs comprising a constant t equal to twice the radio propagation time over the maximum intended operating height (e.g. 100 Ásec for a maximum height of 50,000 ft.) plus a variable portion representative of the predetermined selected height. Such interrogating signals will be received by the transponder of a responding aircraft in the form of corresponding direct signals P1<SP>1</SP>, P2<SP>1</SP>, Fig. 4(b) and indirect signals 1P1, 1P2 received after reflection at the surface of the earth. The transponder derives from each direct pulse pair P1<SP>1</SP>, P2<SP>1</SP> a corresponding single control pulse S1, S2 (Fig. 4(c)) which (1) generates a blanking pulse of duration t, Fig. 4(d) which blanks the receiver thereby preventing reception of the indirect signals 1P1, 1P2, and (2) is applied to a circuit producing a delayed pulse H1, H2, Fig. 4(e), the delay x. comprising the constant t and a variable portion representative of the height of the responding aircraft. The pulses S1, 52, H1, H2 are applied to a coincidence circuit which produces an output pulse R, Fig. 4 (f), if x. is equal to x and the pulse R triggers the transponder transmitter to produce a corresponding reply pulse of radio frequency F2 which is received at a responsor on the interrogating craft as a direct pulse R., Fig. 4(g), and an indirect pulse 1R. due to reflection at the surface of the earth. As described with reference to Fig. 1 (not shown) the interrogator aerial system comprises four aerials having directivity patterns as shown in Fig. 3 and the interrogating pulse signals are applied sequentially to the aerials through a switch rotating at À25 c/s. (i.e. about forty consecutive pulse groups P1, P2 applied to each aerial). At the responsor a range timebase synchronized with the interrogator transmitter is applied sequentially to the two Y plates of a CRT through a switch synchronized with the aerial switch so that when the forward aerials are in use the timebase trace extends vertically upwards from the centre of the tube (as shown in Fig. 2) and when the rearward aerials are in use the trace extends downwards. The reply pulses are applied sequentially to the two X plates of the CRT through another switch also synchronized with the aerial switch so that when the starboard aerials are in use the reply pulses deflect the trace to the right (as shown in Fig. 2) and when the port aerials are in use the trace is deflected to the left. The means in the interrogator for producing the delay x comprises a known combination of phantastron and multiar amplitude comparator, the magnitude of the delay x being determined by a resultant D.C. voltage obtained by adding the outputs from (1) a preset potentiometer set in accordance with the constant t, (2) a potentiometer controlled automatically by the altimeter, and (3) a manually controlled potentiometer for use when it is required to interrogate aircraft at a height differing from that of the interrogating craft. Similar means are employed in the transponder for producing the delay x. but in this case the manual potentiometer is omitted. The variable portions of the delays x and x. may comprise 4 Ásec. per 1,000 ft. for heights up to 25,000 ft. and 2 Ásec. per 1,000 ft. for heights above 25,000 ft. In order to reduce mutual interference between a plurality of interrogating aircraft, the frequency of the transmitter trigger oscillator may be "jittered" in a known manner.