Patent GB-1462831-A (GB)
Title: Adaptive proximity indicating system
IPC: G01S13/933
Filed: 1974-02-20
Abstract: 1462831 Radio navigation system LITCHSTREET CO 20 Feb 1974 [27 March 1973] 7784/74 Heading H4D An airborne adaptive proximity indicating system, utilizing interrogations transmitted by standard azimuthally scanning SSR stations and responses thereto from transponders carried by other aircraft, monitors the proximity of said other aircraft in a predetermined volume of space, said volume being varied in dependence on one or more characteristics of the signal environment. Such characteristics are: (a) the rate of interrogations received, i.e. the number of SSR stations in the vicinity; (b) the rate of response received, i.e. the number of aircraft in the vicinity; (c) the strength of the side lobe suppression signals of an interrogating SSR; (d) the rate of time of arrival (TOA) of response measurements, the TOA being measured relative to the reception of the interrogation. For monitoring characteristics (a) and (b) the circuit of Fig. 1 is possible wherein receiver 30 and decoder 32 produces a pulse for each interrogation received and receiver 31 and decoder 33 produce a pulse for each response received. The rate of interrogations and/or responses are determined using counters 38, 39, active for predetermined periods set by timers 40 and 41. The respective interrogation rate and response rate representative pulse counts are fed to respective comparators 46, 47 for determination as to whether they are lower than first respective set values, whence respective Modes A and D are indicated, in between said first and second values, Modes B and E apply, or are higher than said second values, whence Modes C and F apply. The Mode signals are fed to control systems 56 and 57 to separately or jointly affect the start of the listen-in period for the responses, or "min. TOA gate", the end of the listen-in period, or "max. TOA gate", the min. altitude of aircraft whose responses will be received or "min. altitude gate", and the max. altitude of such aircraft or the "max. altitude gate". The volume of space monitored for responses is thus varied in range and altitude. Fig. 4 shows how the variation of the min. TOA gate and max. TOA gate can produce two monitoring volumes-a near, large volume for a Mode G corresponding to a low rate of interrogations or responses, and a farther, smaller volume for a mode H corresponding to a high rate of interrogations or responses. The locus of other aircraft giving constant times of arrival of responses to the same interrogation is an ellipse with one focus at the SSR and the other at the "own-aircraft". Fig. 5 shows how the control sustems of 56, 57 may be used to vary he min. and max. TOA gates as well as the altitude discrimination. Thus an interrogation is received at 84 and PRF selected at 86. A phase-locked generator 88 produces a continuous series of pulses at the interrogation PRF, which pulses are fed through a variable delay 89, Fig. 6 (not shown), controlled according to the determined mode to give a series of pulses delayed from the interrogations by the time to the start of the listen-in period. These pulses in turn trigger variable duration pulses from a generator 90, Fig. 7 (not shown) also controlled according to the mode whereby said variable duration pulses terminate at the end of the listen-in period. The listen-in period defining pulses part enable gate 80 receiving the decoded responses. The own aircraft's altitude is fed to an altitude range selector 99 where it is added to and subtracted from by amounts dependent on the mode, to produce the required monitored altitude band. The responses at 75 are altitude coded, to give the other aircraft's altitude at 101 for comparison at 100 with said band. If the altitude is within said band the gate 80 is fully enabled to pass the responses to an indicator 82. To stop the own aircraft's response to the interrogation from being received, the responses are inhibited at 78 for 24 Ásec. (93). This however stops the monitoring of nearby other aircraft. FAA regulations however allow the own aircraft to disable its transponder for 2500 Á sec. every <SP>1</SP>/ 18 th sec. Thus the 24 Ásec. inhibition of responses may also be stopped for said 2500 Ásec. With a crowded signal environment, i.e. high interrogations and/or responses the transponder inhibition and the 24 Ásec. inhibition is reduced to periods of 100 Ásec. Figs. 9, 10 (not shown). The monitored volume may be varied by changing the azimuth region, Fig. 14, e.g. a narrower region for a sparese Mode H signal environment and a wider region for a crowded Mode G signal environment. To accomplish this, Fig. 15, a pulse train is generated at 88, phase locked to the received interrogations, which pulses are counted at 197 to give an indication of the azimuth of the SSR's beam. The counter 197 is reset to zero a short time (e.g. 1 Ásec., (delay 210) after the reception of each interrogation pulse, whereby during the dwell of the beam on the own aircraft, the counter alternates between 0 and 1. When the beam is not on the own aircraft, the count indicates its azimuth and is fed to a comparator 198 which also receives (208), (209) counts representing the desired azimuths of the beam giving the required azimuth region, the comparator thereby producing an enable pulse started and stopped by the determined equalities, for enabling gate 196 to pass the responses. A count equivalent to the width of the beam is determined at 201 and stored at 205 after having been divided by 2 for mode G, or 6 for mode H at 204. This stored number, indicating the extent by which the azimuth region is wider than the beam width is applied directly to comparator 208 (line 209) to provide the "stop enable pulse" signal, and to subtracter 207, wherein the "pre-interrogation" number is obtained by subtraction from the number in buffer 206 representing 360 degrees minus the beam width. The subtraction thus provides the "start enable pulse" signal on line 208. The generator 88 may synchronize with the side lobe suppression signals when the main beam is not sweeping the own aircraft-simpler circuitry is thereby possible, Fig. 16 (not shown). When the own aircraft is in an area of little or no SSR activity (i.e. mode G) additional circuitry may be used to transmit interrogation signals to the other aircraft whereby a slant range is directly measured from the responses. If the other aircraft has an "X" pulse decoder, a 1090 MHz transmitter and "X" pulse encoder may be used, otherwise a low power 1030 MHz transmitter at a very low repetition rate (e.g. 5-10 per sec.) is used. The additional monitored region is circular, Fig. 18 (not shown). Band pass filters may be used to select the interrogations of a particular SSR in the case of a crowded SSR environment, (Mode H), Figs. 20, 21 (not shown). As well as modes being decided on the basis of the rates of interrogation and of responses, e.g. Mode A#Low int, Mode B# medium int, mode C#high int; mode D#low resp, mode E# medium resp, mode F#high resp. other modes may be decided when certain combinations of the above modes occur, e.g. mode I when a mode C occurs with a mode D. These additional modes also control the min. TOA, &c., Fig. 22 (not shown). In addition the modes can be determinable in dependence on the altitude of the own aircraft on the assumption that the altitude of an aircraft can be correlated with its speed, number of aircraft in the vicinity and number of SSR stations visible. The modes can be made determinable on the altitude alone, Fig. 23 (not shown), or the altitude in combination with some other feature of the signal environment, e.g. rate of interrogation, Fig. 24 (not shown). The modes can also be made determinable on the side lobe suppression (SLS) level of the SSR, using a SLS decoder as described in Specification 1,379,693, Fig. 25 (not shown). The total number of response TOA's passively determined in a set time, e.g. 4 sec., may be compared with a minimum, and if said minimum is not reached, the active slant range measurement may be resorted to. If the minimum is exceeded, the largest TOA is selected, Fig. 26 (not shown). In the TOA measuring arrangement of Fig. 28 a ramp generator and averaging element 290 is controlled by a received interrogation from 30 (or an SLS pulse when the SSR beam is not sweeping) and a received response from 31. Four agreeing TOA's (due to the same SSR) must be obtained from 290 for a result to be produced on output 292. When being interrogated by a large number of SSR's, outputs from 290 due to the different SSR's are sequentially stored at 294 by means of sequentially enabled gates 296 the sequence lasting about one beam rotation-e.g. 4 to 5 sees., and an indexing counter being stepped by sweeps from the plural SSR's to give outputs at each step enabling a corresponding gate. A logic 298 receives from the stores, the average TOA measured during sweeps by beams from corresponding SSR's and determines the maximum TOA, which maximum is fed to an indicator 304 during a crowded environment or Mode V. If a sparse environment or Mode U prevails, active ranging has to be resorted to. The system of Fig. 15 may be modified for use in the air to air range solicitation as described in Specification 1,407,091, Fig. 29 (not shown). The TOA gate may be adjusted, e.g. in increments of 50 Ásec. having a 50 Ásec. width, by means of a 50 Ásec. interval tapped delay line fed with the interrogation pulse or the output of a PRF repeater synchronized therewith, Fig. 30 (not shown).