Patent GB-1250678-A (GB)
IPC: G04G7/02
Filed: 1969-08-08
Abstract: 1,250,678. Radionavigation. BENDIX CORP. 8 Aug., 1969 [16 Sept., 1968], No. 39700/69. Heading H4D. Relates to a clock synchronizing arrangement for a co-operative radio aircraft collision avoidance system (CAS) of the type in which each participating station (ground or airborne) carries a clock unit comprising a main counter 10, Fig. 1, whose time cycle comprises repeating epochs of three seconds duration, each epoch comprising 2000 time slots each of duration 1500 Ás. and the time cycles of all the stations being maintained in absolute synchronism with a standard (master or world-wide) time cycle by means of a two-speed sync. counter 12; each ground station transmits a coded ground epochstart signal at the commencement of each oddnumbered epoch and each airborne station transmits a differently coded air epoch-start signal at the commencement of each evennumbered epoch and also transmits during an allotted own time slot on a radio frequency assigned to said slot:- (1) a "Doppler burst " range pulse whose time of reception at another station gives the distance between the two stations (i.e. one-way ranging); so-called " qualified " aircraft carrying high stability clocks transmit range pulses of duration 200 Ás. and other aircraft carrying less stable clocks transmit clock pulses of duration 190 Ás., (2) other flight data signals, e.g. altitude and altitude rate, for use in a known manner by other participating aircraft to evaluate collision threats. The range pulse transmitted by each airborne station is also used as an interrogating sync. request signal which when received by a ground station or by a qualified airborne station controls the two-speed sync. counter 12 at the interrogated station to produce a variably delayed coded sync. reply signal which when received at the interrogating aircraft controls the sync. counter 12 at that aircraft to synchronize the main counter 10 in a known manner. Since the time quality of a ground station is superior to that of an airborne station units, 26 27 are provided to inhibit sync. reply signals from airborne stations when sync. reply signals are being received from ground stations. According to the present invention at each qualified airborne station the sync. reply signal produced by the sync. counter 12 is applied to the transmitter 16 through a gate 36 which is enabled by a probabilistic counter 40 during each of a plurality of randomly spaced "Home" periods occurring in each time slot, the time interval between adjacent "Home" periods being dependent on the number N of qualified airborne stations in the area and the average duration of each Home period being dependent on the quality of the clock time at the interrogated station expressed as an inverse function of the elapsed time T since said station was synchronized by a ground station. Thus when an airborne station is being synchronized by other qualified airborne stations, all the other qualified airborne stations participate in a random probabilistic manner in the sync. control of the requesting station so that no one qualified station monopolizes the sync. control. In the probabilistic counter 40 the number N of 200 Ás. Doppler burst range pulses received from qualified aircraft during any one epoch is counted by a binary air carrier counter, Fig. 2A (not shown), and the number N + 1 is stored in a register and applied in parallel form to a comparator 85, Fig. 2B, together with the instantaneous count output n from a variablecount cyclic binary counter comprising stages 75-81 fed through a gate 83 by clock pulses from a poor quality free-running clock oscillator 59 of average recurrence frequency 1 MHz (corresponding average bit duration #t=1 Ás.), the clock 59 pulses being also applied to the first stage 70 of a three-stage non-cyclic binary Home counter 70, 71, 72. If the interrogated aircraft receives a sync. reply signal from a ground station, the resultant output from a decoder 22, Fig. 1, is applied via a terminal A to reset another clock 67, Fig. 2B, which then produces first and second enabling signals which are applied respectively to normally closed gates 68 and 69 at the end of respective predetermined time periods T1 and T2 (T2>T1). Initially the three stages 70, 71, 72 of the Home counter are turned ON, the resultant output from stage 72 at terminal B inhibits the sync. reply gate 16, Fig. 1, and stage 71 enables the clock 59 pulse gate 83. When the count n in the variable-count counter reaches the value N+1 the comparator 85 produces an output pulse which resets the counter and turns the Home counter stages OFF thereby enabling the sync. reply gate 36 and inhibiting the clock 59 pulse gate 83. If the elapsed time T since the interrogated airborne station was last synchronized by a ground station is less than the first predetermined time period T1 then the clock 67 gates 68 and 69 are closed and the third Home counter stage 72 is turned ON (to again inhibit the sync. reply gate 36) by the third clock 59 pulse after the reset pulse from the comparator 85 so that the gate 36 is enabled for a period 3#t and the probability that this aircraft will respond to any particular sync. request is 3/(N+ 1). If T1<T<T2 the clock 67 gate 68 is enabled to pass a voltage V which cuts out the first stage 70 of the Home counter so that the third stage 72 is now turned ON by the second clock 59 pulse (i.e. gate 3.6 enabled for a period 2#t giving a reply probability of 2/(N+ 1)) and similarly if T>T2 clock 67 gate 69 is enabled thereby cutting out the first and second stages 70 and 71 and the sync. reply gate 36 is enabled for a period #t (i.e. reply probability 1/(N + 1)). Since the clock 59 pulse gate 83 is enabled when the second stage 71 of the Home counter is turned ON, when the enabled period of the sync. reply gate is i#t the inhibited period of the clock pulse gate 83 is (i - 1)#t. The numerical value of the reply probability i/(N +1) can be adjusted by varying the number of stages in the variable count counter and/or the Home counter.