Patent GB-1101340-A (GB)
Title: One-way range and azimuth system for stationkeeping
IPC: G01S11/08
Filed: 1965-12-15
Abstract: 1,101,340. Radio navigation. SIERRA RESEARCH CORPORATION. 15 Dec., 1965 [21 Dec., 1964], No. 53249/65. Heading H4D. General description.-A radio navigation system is described whereby each of a plurality of mobile vehicles 1-8, Fig. 1, is supplied with a PPI indication of the relative positions of all the other vehicles. Each vehicle is provided with nominally synchronized clocks defining a plurality of equal consecutive time slots with individual allocation of each vehicle to a particular time slot. At the beginning of its allocated time slot the vehicle radiates omnidirectionally a coded pulse pair " Code II ", Fig. 3, which is picked up by a rotating directional antenna on each of the other vehicles to provide one way range information together with antenna direction azimuth information, for use in said PPI display. One of the vehicles 1, is designated the " master " vehicle, and is allocated to time slot 9, Fig. 4. At the beginning of its time slot the master vehicle radiates a " Synch A" pulse pair on its directional beam, and causes the follower vehicle then illuminated by said beam to radiate omnidirectionally when its own allocated time slot occurs, an interrogating " Code I " pulse pair. This is repeated by the master vehicle to give on the follower vehicle an indication of the true range to the master vehicle in the form of a number of pulses registered in a counter started and stopped by the interrogating and reply pulse pairs respectively. On receiving the next " Synch A " pulse pair from the master vehicle, the count is continued until a count is reached representing the time duration of one time slot. When this happens the clock is reset to zero, and is now in absolute synchronism with that on the " master " vehicle. Detailed description.-Figs. 2a, 2b show a follower and/or master unit. With switches 25a-f in positions F as shown, the unit acts as a follower unit and with the switches in positions M, the unit acts as a master unit. The clock comprises a 14 stage bistable counter 22 and a logic board 23 receiving the outputs of the bistable circuits. The counter receives pulses at 6 Mc/s. from oscillator 20 and frequency doubler 21 and counts up to 1536 Ás before repeating the cycle. The logic board divides this count up into nine consecutive time slots of 170<SP>2</SP>/ 3  Ás, Fig. 4. A switch 24 is turned to the time slot number allocated to the follower vehicle i.e. 3. In the master unit (switches 25a-f in position M), a signal on line 24b passes via switch 25b to cause a pulse coder 26 to generate the code pulse pair " Synch A" at the beginning of the ninth time slot. This energizes a transmitter 35 rotating at 200 r.p.m. The omnidirectional antenna 36 is only used when a signal appears on line 33a. The transmitted pulse pair is shown as T-A in Fig. 4, and is received at the follower vehicle (which it is assumed is illuminated by the master beam) as pulse pair R-A. From the receiver of the follower vehicle the " Synch A " pulse pair passes via noise rejecting 6V threshold detector 38 to a 10 Ás delay 39 and 4 Ás tapped delay line 40. The " Synch A #pulse pair comprises two pulses 10 1/3 Ás. apart, so that the trailing pulse will coincide at the input of AND gate 43 with the leading pulse extracted at the 1/3 Ás. tapping of the delay line 40. An output therefore appears at the output of gate 43 which is fed to the " off " input of a bistable circuit 102. A signal then appears on line 102b which enables a pulse coder 28, such that at the beginning of the third time slot a code I pulse pair is radiated omnidirectionally to the master vehicle (T-I in third line of Fig. 4). Simultaneously on line 28a a pulse starts a counter 90 counting the 3 Mc/s. pulses from a 2 : 1 frequency divider 98. At the master vehicle the code I pulse pair is received as R-I and produces an output from gate 45. This produces a response on PPI 152 and is fed via 30 Ás delay 87 to the transmitter 31 for radiation back to the follower unit (T-I), At the follower unit it is received and produces an output from gate 45 which, via switch 25d stops counter 90. The count in counter 90, compensated for the 30 Ás delay 87 thus represents the true distance to the master vehicle. At the beginning of the ninth time slot on the master unit, a #Synch A " pulse pair is again received from the master vehicle and the resulting output of gate 43 restarts counter 90 which adds pulses, at 6 Mc/s. on line 99, to the count already accumulated, until a total count of 1024 is reached. A count of 1024, 6 Me/s. pulses gives the width of one time slot 17C<SP>2</SP>/ 3  Ás. When this total is reached a pulse is sent on line 96a to counter 22 to set it to zero. The " clock " in the follower vehicle is now in absolute synchronism with that of the master vehicle. This synchronism is verified by the pulse from the last stage of counter 22 arising simultaneously at unit 101 with the resetting pulse on line 96a. Unit 107 then feeds a pulse to the ON input of bistable circuit 102 whereby coder 29 is enabled instead of coder 28. If initially the follower clock is very much out of synchronism with the master, e.g. up to 62 Ás out, then a " Synch B " pulse pair transmitted by the master vehicle will arrive at follower AND gate 94 within a 62 Ás pulse initiated by the output of counter 22. Gate 94 provides a coarse resetting pulse on wire 96a. The " code II " pulse pairs produced by coder 29 are transmitted omnidirectionally in the third time slot and provide the inputs for the PPI indicators of the other vehicles. The " Synch B " pulse pair is the " code II " pulse pair of the master vehicle. When a pulse pair has been received in a time slot the resulting pulse from gates 43- 46 passes via OR-gate 49 to set bistable circuit 48. This removes an enabling input from gate 42, and so stops the passage of any further signals within that time slot. The circuit 48 is reset, and the enabling input replaced, by a signal from counter 22 marking the beginning of the next time slot. To overcome side lobe reception each time slot is provided with an AGC signal which sequentially controls the amplification of receiver 34 in dependance on the amplitude of the signal received in the respective time slot in the preceding frame. A second method of synchronizing clock 22 is described, involving variation of the frequency of oscillator 20, Fig. 6 (not shown).