Patent GB-1500448-A (GB)
Title: Secondary radar for vehicles
IPC: G01S13/76
Filed: 1975-05-02
Abstract: 1500448 Secondary radar K VOCKENHUBER and R HAUSER 2 May 1975 [3 May 1974] 18594/75 Heading H4D [Also in Division B7] The Specification describes a secondary radar system which comprises a first and a second station in each of which is a pulse transmitter and a receiver for receiving pulses transmitted from the other station, the second station effecting transmission of a pulse after receiving a pulse transmitted thereto from the first station, the first station comprising a means for computing from the time interval between transmission of a pulse therefrom and reception of a response from the second station the distance between the stations, and means for supplying the 1st station transmitter a coded signal representative of the measured distance which signal is transmitted to and decoded by the 2nd station. Alternatively, the distance information may be communicated between the stations by measurement at the second station of the delay between transmission of its response signal and reception of the next (non-coded) interrogation signal from the first station at which it is triggered on reception at the response signal. The operation of primary station 1 is as follows. Low frequency clock pulse generator TIG, e.g. 100 Hz astable multivibrator triggers TX pulse generator SIG to drive laser diode LD which emits short infra-red light pulses SIO through optical system towards secondary station 2 which transmits response signals after a known delay time #. Current pulses SIE from laser LD also trigger delay circuit VZG which after a delay time # enables window pulse generator ZFG. Response signals from the secondary station are received via optical system EO by photo-diode FD and are converted to a symmetrical double pulse, limited, amplified and fed to zero crossover trigger NGT, which is firstly triggered after delay # by window pulse ZF and thereafter secondly triggered when the middle flank of the fully limited output signal from receiver EV crosses zero provided the received signal in a partially limited state is detected by trigger TR to exceed a predetermined threshold. The delay between the first and second triggerings of NGT is proportional to the transit time of the interrogating and response signals and is hence proportional to the distance between the stations. Range calculator ERA produces at its output the analogue range measurement AM which also controls the frequency of VCO, which frequency is of the order of kHz, to be proportional to the distance. VCO triggers TX pulse generator SIG which thereby causes laser LD to transmit signals at a p.r.f. proportional to the distance measurement which signal is received at the secondary station and decoded by integrator INT. The operation of the secondary station is similar to that of the primary station. Received interrogation signals are received by photo-diode FD amplified and detected by receiver EV and the triggers TR and NGT, the signal from which, coincides with a received pulse EIO, is delayed a time # and then via TX pulse generator SIG triggers secondary laser diode TX to transmit a response signal, and is also fed to window pulse generator ZFG which disables secondary station 2 after reception of the interrogating signals to prevent reflected secondary response signals being detected in station 2. A single circuit (Fig. 2, not shown) may be used in both primary and secondary stations and includes switches to change its circuit configuration between the interrogator circuit and the transponder circuit. In Fig. 5, two vehicles travel along a track 5. At the front of each vehicle is a primary transmitter 1 and a transponder 2 is mounted at the rear. The transmitted energy may be bounced off reflective surface 7 to reach the co-acting vehicle around curve 6. Alternatively Fig. 7, current rail 8a has a lateral slit 9 through which guide pins 10 project from the vehicle chassis and support travelling trolley 13 on which is mounted primary transmitter 1. The rail 8a is box shaped with polished internal surfaces to assist propagation of the transmitted energy.