Patent GB-1463388-A (GB)
Title: Method and apparatus for measuring passively range and bearing
IPC: G01S13/933
Filed: 1974-11-25
Abstract: 1463388 Radio navigation LITCHSTREET CO 25 Nov 1974 [31 Jan 1974] 50967/74 Heading H4D When an own location (e,g. an own aircraft) and a transponder (e.g. a second, possibly collision-bound, aircraft) are in the field of at least two SSR transmitters, the time difference T between the arrival at the own aircraft of an interrogation from an SSR transmitter and the arrival at the own aircraft of the response from the second aircraft to said interrogation places the second aircraft on a set loci having a polar (R, #) equation (origined at the own aircraft) of where α is the bearing of the own aircraft from the SSR transmitter. Thus for two SSR transmitters two loci T 1 , T 2  Fig. 1D are produced, crossing at two points, one of which is a false indication of the second aircraft's position. The bearing # of the second aircraft from the own, is given by  where α is now the difference in the bearings of the own aircraft from the two transmitters. This equation assumes that the transmitters are at infinity and that the loci are parabolic. This is an approximation to the true case of the loci being ellipses. The equation gives two values of # and to determine the correct one, use must be made of the relative bearings # of the own aircraft and second aircraft from each of the transmitters. Thus if # own > # second, then lagging # own < # second, then leading. The derivation of # own and # second may be as described in Specification 1,407,091. In the apparatus of Figs. 3A, 3B the interrogations from two SSR stations are received 20 and selected by PRF selectors 24A, 24B and the intervals between the beams of the SSR stations sweeping north and the own station determined by north pulse decoders 26A, 26B, interrogation decoders 28A, 28B and interval timers 30A, 30B. The interval between successive north bursts enables beam rotation timers 32A, 32B to determine the rotation intervals of the beams such that ratio detectors 34A, 34B receiving the outputs of the interval and rotation timers produce indications of the bearing α 1  and α 2 . The required bearing difference α is determined in combining circuit 36. The responses of the second aircraft are received at 22 and fed via identity and altitude decoders 46A, 46B (for blocking further operation of the circuit if the altitude of the second aircraft differs greatly from that of the own) and gates 42A, 42B enabled by the selected corresponding interrogations (selectors 40A, 40B) to stop inputs of interval timers 44A, 44B. The timers are started by the interrogations, such that they produce outputs indicating the time differences T 1  and T 2 . A computer 49, receives the α, T 1  and T 2  signals, and carries out the indicated computation to give two possible bearing values # A1  and # B1 . To determine the lead/lag state for each SSR station lead/lag circuit 47A, 47B receiving the interrogation and response signals, operate as described in Specification 1,407,091 to give positive or negative outputs when the own aircraft is interrogated before or after the second aircraft, respectively. The bearing values # A2  and # B2  relative to the second SSR station are computed by combining # A1  and # B1  with α in circuits 50A, 60B, and all four bearing values fed to sign detectors 52A, 52B, 54A, 54B, to give positive outputs for 0 to 180 degrees and negative from 180 to 360 degrees. Sign comparators 56A, 56B produce positive outputs when the lead/lag signal from circuit 47A has the same sign as the outputs from detectors 52A, 52B respectively, whilst comparators 58A, 58B act similarly for lead/lag circuit 47B and detectors 54A, 54B. Truth gate 60A produces an enabling output for gate 62A when the outputs of comparators 56A, 58A are both positive and truth gate 60B enables gate 62B for positive output from comparators 56A, 58B. Gates 62A, 62B when enabled respectively pass the determined "true" bearing # A1  or # B1  to a computer 64 which, receiving a T 1  indicative signal, calculates the inter aircraft distance R. Bearing, range rate and range are indicated. Where three or more SSR stations are used, the determination of the "true" bearing may be based on a comparison of the various bearings as determined using the three or more stations, instead of the lead/lag logic, Figs. 4, 5A, 5B (not shown).