Patent GB-1453451-A (GB)
Title: Collision avoidance/proximity warning system using secondary radar
IPC: G01S13/933
Filed: 1973-10-22
Abstract: 1453451 Radio navigation LITCHSTREET CO 22 Oct 1973 [22 Dec 1972] 49047/73 Heading H4D To provide at an "own" station, e.g. an aircraft, an indication of proximity and bearing of a transponder at another station, e.g. "another' or "intruder" aircraft, both stations being within the area of a SSR system, the responses of the transponder to interrogations from the SSR are received by the "own" station by direction finding means, whereby bearing of the other station is determined and the timing of the receipt of the responses is analysed by comparison with the time that the interrogation from the SSR reaches the "own" station to determine whether the other station is within the predetermined range of distances from the own station. Such a determination can indicate whether or not the other station is a collision hazard or not. If the receiver of the "own" aircraft Fig. 1, is enabled for a listen in period starting a short time after reception of the interrogation signal from the SSR ground station and terminated a predetermined interval thereafter, the monitored region will be defined as being between two ellipses as shown. This area is further limited as to azimuthal extent by limiting the number of the PRPs during which the listen in periods occur to a number involved in the immediate sweeping of the own and "intruder" aircraft. This twofold definition of the monitored or "listen-in" region gives the shown shaded region. The  "own" aircraft may basically include apparatus of Fig. 2, which as well as the standard transponder 30 includes the direction finder antenna which determines the bearing of the received 1090 MHz responses from the "intruder" aircraft during the time # starting (50) at a time after the reply trigger 48 from transponder 30 determined by a variable delay 44 and terminating (52) at a time determined by a variable gate generator 42. The area under surveillance as well as being limited in range and azimuth as described, may also be limited in altitude by comparing the altitude of the "own" aircraft with the altitude of the "intruder" aircraft the latter being determined by decoding of the response therefrom Fig. 3 (not shown). The direction finder may comprise four antennae having the respective reception lobes of Fig. 6 and producing respective signals after detection which upon being connected in pairs to meters 98, 100, 102, 104, Fig. 10, of the amplitude comparison type, produce indications of the "intruder" aircraft bearing. Each indicator indicates the bearing within a respective 90 degree sector, three out of the four indicators showing a nominal zero indication and one indicator, e.g. that having an enabled light or flag 106 associated therewith, indicating the bearing within its respective sector. When the "own" aircraft responds to an interrogation (a) Fig. 14 with a full response (b) the listen in period starts just after the end of such response. In accordance with international regulations however the full response may be intermittently omitted whereupon the listen in period may start just after the start of the would be full response, i.e. as delineated by a synthetic F1 pulse (e). In one arrangement Fig. 18, the signal transit time T2, Fig. 16, from the other or "intruder" aircraft to the "own" aircraft is determined by using the equation where T D  is the difference between a time that an interrogation would be received if the beam had been directed at the "own" aircraft and the time of the receipt of the transponder reply from the "intruder" aircraft to said interrogation. D, Fig. 16, is the sum of the bearings A and B of the ground station and the "intruder" aircraft with respect to the axis of the "own" aircraft. In the arrangement of Fig. 18 the direction finding assembly 32, 118, of the kind already described, provides at the respective outputs of gated amplifiers 126, 128, indications of the angles A and B. A logic circuit 136 then produces an indication of the angle D from the algebraic difference of the angles B and A, several such determinations of angle D being produced as the interrogating beam sweeps the aircraft, an average of the several measurements being made in means 144. Logic circuits 146, 148 produce indications of cos D and (1 - cos D) respectively. The interrogation from the ground station is recieved at 122 and after decoding (152) causes the switch of a timer 150. The reception at 124 of the corresponding response from the "intruder" aircraft causes the timer 150 to switch off whereby an indication of the time T D  is produced. As for the angle D, several consecutive measurements of T D  are made as the interrogation beam sweeps the aircraft, the average being produced by means 154. The average value of T D  together with the value of 1 - cos D) is fed to a logic circuit 156 wherein the value of T 2  is determined using the above equation. As indicated in Fig. 17 the other aircraft may, with respect to the "own" aircraft, be in one of four quadrants thereby necessitating that the correct signs of T 1 , R 2  and cos D be taken. A discriminator 174 therefore determines erroneous values of T 2  produced by the wrong choice of such signs and eliminates the same such that a series of correct values of T 2  may be averaged over an entire rotation period of the interrogation beam, e.g. four seconds. The average value of T 2  is converted into a range R and is displayed at 178. In addition said range is also fed to a cathode-ray tube 180 together with an indication of the bearing angle B. In a second arrangement not shown the "intruder" aircraft has a transponder which produces responses when interrogated by the own aircraft at the response frequency 1090 MHz. This enables a calculation of the time T 2  using the period between the transmission by the "own" aircraft of this additional interrogation signal and the reception thereat of the corresponding response from the "intruder" aircraft, the bearing of the "intruder" aircraft being determined as before by the described direction finding means.