Patent GB-1111702-A (GB)
Title: Pulse radar response simulator
IPC: G01S7/40
Filed: 1966-02-07
Abstract: 1,111,702. Secondary radar simulation. SOCIT NOUVELLE D'ELECTRONIQUE ET DE LA RADIO-INDUSTRIE. 7 Feb., 1966 [10 Feb., 1965], No. 5371/66. Heading H4D. The invention relates to the simulation of a complete air traffic control radar-beacon system of the type defined by I.C.A.O. Specification, wherein a reply from an interrogated aircraft comprises two framing pulses F1 and F2, Fig. 1a, 20À3 Ás apart enclosing 13 possible pulse positions, 1À45 ÁS apart, the positions being occupied according to the coded information to be transmitted. In civil aircraft (Fig. 1g) the seventh (X) position is always kept unoccupied, four modes of reply are available, A, B, C or D, C being reserved for altitude information, and a special identification pulse SP1 is provided 4À35 Ás after pulse F2. In military aircraft, Fig. 1b the seventh position X is always occupied, three modes of reply are available 1, 2 and 3, and the identification pulse SP1 comprises two pulses 20À3 Ás apart, the first occurring 4À35 Ás after pulse F2. An emergency mode is also available to military aircraft. The reply mode required by the interrogation is defined by the pulse spacing of an interrogating pulse pair according to the following table:- The simulation thus allows the simulation of a target to be interrogated by a rotating radar beam, the interrogation changing modes from one pulse repetition period to the next in a cyclic sequence, a minor cycle, e.g. A, B, C, being repeated every third period, and the last mode of said cycle being changed every rotation period of the simulated antenna according to a major cycle, repeated every third rotation period. The simulation thus generally comprises, Fig. 2, a time base generator TBG producing a train of 0À1 Ás pulses for synchronizing the various elements of the simulation, and a target co-ordinate selector and mode sequencer TCM, having inputs defining the range 7, and azimuth 38 of a simulated target, the number of consecutive repetition periods over which the target is to be interrogated 50, and an input 58 allowing the introduction of a garbled response from a target on the same azimuth as the simulated target. A mode sequence selecting input is also provided for the TCM to define the minor and major mode cycles. The TCM produces a response command signal RC simulating the time of the required response, and an interrogation mode signal IM defining the mode and mode cycles to be used. The RC signal and the IM signal are fed into civil responder CIR and military responder MIR. Each responder is supplied with means setting the codes in the various modes, and supplies simulated responses CR and MR such as shown in Figs. la and 1b respectively. The TCM means for producing the RC signal is shown in detail in Fig. 3. The 0À1 Ás pulses from the T.B. are fed as range increment pulses to a counter 2. The increasing digital output of the counter is fed to a decoder matrix where it is converted into a form comparable in comparator 6 with the simulated target range setting in means 7. At the end of a range sweep the counter 2 is reset either from the output of the decoder 4 or from an external source terminal 16. When the count in counter 2 represents the range in setter 7, comparator 6 feeds a pulse to one input of AND gate 8. Similarly an azimuth counter 22 counts pulses produced by an azimuth clock generator 26 and a full count cycle represents 360 degrees. Comparator 36 gives an output when the count in counter 22 represents the azimuth in setting means 38. This output is applied to one input of an AND gate 40 which has a pulse P3 indicating the start of a range cycle applied to its other input. The output of gate 40 sets a bi-stable circuit 42, such that it enables gate 8 and passes the output of comparator 6 to the response command signal output. The number of range sweeps is counted in counter 44, and when this equals that set in azimuth extent setter 50, comparator 48 produces a pulse resetting circuit 48 and blocking further RC pulses. Comparator 56 produces a garbling response command signal RC<SP>1</SP> when the range difference from that in range setter 7 is equal to that set in setter 58. Counter 52 starts counting the TBG pulses at the occurrence of the RC pulses. The output of decoder 4 is applied to the mode sequencer which produces the interrogation mode signal IM and is shown in detail in Fig. 4. The mode sequencer comprises an interrogation pulse gauge circuit 20 which produces the pulse P3 marking the beginning of a range cycle and a pulse PA preceding pulse P3 by 28 Ás. It also produces pulses P 1 (1), P 1 (2), P 1 (3+A), P 1 (B), P 1 (C), P 1 (D) preceding pulse P3 by 3, 5, 8, 17, 21 and 25 Ás respectively and which are fed in parallel to first inputs of respective gates in five banks 88, 90, 92, 94, 96 of AND gates having their other inputs fed from the contacts of respective switches 76, 78, 80, 82, 84. The number of range cycles is counted by counter 60, fed by pulses PA. The output of the counter is fed to a decoder matrix 62 which has three outputs a 1 , a 2 , a 3  respectively connected to the moving contacts of switches 76 and 78 and to first inputs of AND gates 86 and also to contacts 1, 2 and 3 of minor cycle number selector switch 68. With the switch as shown a pulse appears at output a 1  only at the commencement of each range cycle, and is fed to the second input of the gate in bank 88, selected by switch 76. The P i  pulse thus passed is fed via an OR gate of bank 98 to set a particular one of bi-stable circuits 100. The set output of the bi-stable circuit enables a corresponding AND gate in bank 102, such that the P 3  pulse applied to the other inputs of the AND gates 102 appears at the output of the particular AND gate indicating the required mode and constituting the signal IM. With switch 68 connected to contact 2, pulses appear alternately at outputs a 1  and a 2  for consecutive range cycles, resulting in the IM signal comprising a corresponding two mode cycle. With switch 68 connected to contact 3 pulses appear at outputs a 1 , a 2  and a 3  in cyclic sequence. The production of an output at outputs a 1  and a 2  merely causes the production of corresponding P 3  pulses in the IM signal via AND gate banks 88 and 90 respectively. The output at output a 3  however enables AND gates 86. A counter 64 counts the number of simulated antenna rotations and feeds the count to a decoder matrix 66, having three outputs b 1 , b 2 , b 3  respectively connected to the AND gates 86. A major cycle number selector switch 70 is provided similar in action to switch 68. Thus with switch 70 connected to contact 3 outputs appear at outputs b 1 , b 2  and b 3  in cyclic sequence every three antenna rotations. Thus the output at output a 3  is applied to switches 80, 82 and 84 on successive antenna rotations. Thus, if there are n range cycles in each antenna rotation and switches 68 and 70 are both connected to their 3 contact, a sequence of IM signals may comprise:- The civil responder unit is shown in detail in Fig. 5 and comprises code setters 106A-106D in which the required respective responses are preset. A converter 107C converts the code of the altitude response-setter 106C into the required Gilham code. When a response is to be produced, as determined by the simulated target range etc., signal RC appears to set bi-stable circuit 115. This enables gate 114 to pass 0À1 Ás T.BG pulses to a divider counter which produces 1À45 Ás spaced pulses defining the desired positions of the response pulses. Seventeen of these pulses are counted by counter 112 and are fed via decoder matrix 110 to one input of comparators 108A-108D receiving the respective outputs of the code setters at their other inputs. The comparators thus feed the pulse outputs of their respectve code setters to respective AND gates 120A- 120D in synchronism with desired pulse response positions. The IM signals are fed to bi-stable circuits 122 such that one of said bi-stable circuits is set depending on the particular P3 signal being used. The output of said one bistable circuit enables a respective one of the AND gates 120A-120D such that the respective response signal is fed from the comparator to an OR gate 124 and pulse width adjusting circuit 126 to produce the response signal CR. The military responder unit is shown in detail in Fig. 7 (not shown) and is basically similar to the civil responder unit just described.