Patent GB-991362-A (GB)
Title: Stationkeeping radar system
IPC: G01S13/933
Filed: 1961-07-24
Abstract: 991,362. Pulse radar transponders. SIERRA RESEARCH CORPORATION. July 24, 1961 [Sept. 26, 1960], No. 26719/61. Headings H4D and H4L. The invention relates to a plurality of radar units which comprise a time division multiplex system wherein each unit operates both as a primary unit and a secondary radar interrogating transmitter during a time-slot uniquely assinged to the unit from a repeating sequence of similar timeslots, and in which each unit operates as a transponder to the interrogations from the remaining units in the system during the remaining time-slots in the sequence, each unit comprising synchronized clock means to initiate the primary radar and interrogating functions within the time-slot assigned to the unit, and control means to permit unit to function as a transponder only on receipt of signals during the remaining time-slots. Each of the radar units may be located in a respective one of a plurality of aircraft, e.g. helicopters, carrying out a cooperative manoeuvre. e.g. anti-submarine warfare, each unit serving to indicate the location of the other aircraft and of other targets. As described, the system comprises eight mobile radar units, each containing a 4 kc/s. oscillator 52, Fig. 2, driving a ring counter 50 composed of five bi-stable stages so that the duration of one cycle of counting is 8 m/s., one unit serving as the master unit synchronizing the counters of each of the remaining (slave) units once in each 8 m/s. cycle. In each unit the counter 50 connects with a logic selector 58 which comprises a diode matrix arranged to select from the bi-stable stages of the counter a series of signals within a 1 m/s. time-slot unique, in each 8 m/s. counting cycle, to that unit, and these signals actuate a modulator keyer 40 and a switch driver 72. In the slave units ganged switches 53 and 53a are operated to the " S " position so that the counter 50 can receive synchronizing pulses from a decoder gate 56, whereas in the master unit the switches are operated to the "M" position to isolate the counter 50 from the decoder gate 56 and allow the logic selector 58 to pass once each cycle the synchronizing signal to the modulator keyer 40 for radiation to the slave units. In each unit the switch driver 72 is operated to close for the duration of the unit's time-slot a switch 22 to connect a directional aerial 10 to the receiver mixer 23, the aerial 10 being continuously rotated in azimuth. During the remainder of the 8 m/s. counting cycle a second switch 44 connects an omnidirectional aerial 42 to the receiver so that the unit is in a condition to receive any interrogation signals transmitted by the remaining units. The modulator keyer 40 keys the modulator 20 of a transmitter 18 connected through T-R arrangements to the directional aerial 10, and also keys the time base generator 32 of a P.P.I. display so as to trigger the time-base only within the duration of the time-slot, so that there is displayed in each unit only information received during its time-slot in the form of transponder replies to its interrogations and echoes from its primary radar pulses. The pulse waveforms radiated by the master unit in its discrete time-slot are as shown in Fig. 4, in which the synchronizing signal radiated at the commencement of the time-slot is coded as two 1 Ás. pulses separated by an interval of 5 Ás. This signal is received in each slave unit via the omnidirectional aerial 42 and is passed to the decoder gate 56 which is operated only by such a coded signal to feed a synchronizing pulse to the counter 50. Every unit radiates during its time-slot the remaining signals shown in Fig. 4, one being a 1 Ás. pulse occurring 750 Ás. after the beginning of the time-slot to serve both as a primary radar search pulse and as an interrogating pulse, the other being an " arm " signal coded in the form of two 1 Ás. pulses with an interval of 10 Ás. between them and being emitted 250 Ás. prior to the interrogation pulse for the remaining units to identify to prepare them to recognize the succeeding radar pulse as an interrogation signal. The identification in each unit is performed by an arm pulse decoder 60 which responds only to such coded signals to operate a train of two one-shot multivibrators 62 and 64 which apply to a beacon reply gate 66 a 40 Ás. enabling pulse D, Fig. 4, during which the interrogation signal succeeding the coded arm signal should be received. The received interrogation signals are applied from the amplifier 26 via an amplifier 99 to the beacon reply gate 66 which is biased by a potentiometer 98 to pass only signals of amplitude greater than a predetermined level during the 40 Ás. enabled period in order to obviate spurious initiation of the transponder function due, e.g. to false interrogations by energy in the side lobes of the directional aerial 10, and by reflection of the main lobe of an interrogating beam from an adjacent aircraft. Any interrogation signal passed by the beacon reply gate 66 actuates the modulator keyer 40 so that the transmitter 18 energizes the directional aerial 10 for the radiation of a transponder response to the interrogation. Any spurious response of a unit to its own interrogation signal (which would swamp any radar echoes received by reflection of this signal) is precluded by a blanking signal applied to the arm pulse decoder gate 60 from a oneshot multivibrator 100 to disable it for 100Ás. during and after the emission of each pulse signal from the transmitter 18, the multivibrator 100 being triggered for this purpose by an output from the modulator 20. The 100Ás. blanking pulse is also applied to the synchronizing pulse decoder gate 56. Gain control.-An output from the system logic selector 58 actuates once every millisecond multiplex and demultiplex logic circuits 70 and 68, Fig. 2, which form respectively switches 70a and 68a, Fig. 5, to connect in sequence for the duration of each of the seven time-slots appropriate to the other units, a different one of seven A.G.C. circuits 81 to 87 between the video output of the amplifier 26 and the gain control input of the IF portion of the amplifier 26. As shown in Fig. 2, the coded arm pulse component of the output from amplifier 26 is selected by the arm pulse decoder gate 60 and applied to the appropriate A.G.C. circuit via an amplifier 61. As shown in Fig. 5, each A.G.C. circuit comprises a rectifier 93, an R.C. circuit 92, 91 of long time constant equal to the rotation period of the directional aerial 10, e.g. 4 secs., and a cathode follower 90 such that during a first time slot corresponding to said circuit a gain control signal is produced of amplitude proportional to the maximum amplitude of the interrogating signal which occurs when the main lobe of the interrogating beam is directed at the unit. This control signal is stored in the RC circuit and during the next (second) corresponding time slot it is applied through the switch 70a to adjust the sensitivity of the I.F. portion of the amplifier 26 to a level 3 d.b. below that corresponding to the maximum of the previously received interrogation signal so that any received signals below this level are not accepted by the beacon reply gate 66 to produce a transponder reply signal. During the second time slot the amplitude of the control signal is reset to the new maximum value of the interrogation signal to control the sensitivity of the amplifier 26 during the subsequent (third) time slot and so on. The A.G.C. circuits are gated off during transmission of a transponder reply signal by an output from the modulator 20 applied through a shaper 20a to prevent production of a spurious control signal due to leakage from the transmitter.