Patent GB-888975-A (GB)
Title: A system for detecting the excessive rate of approach of an obstacle to a vehicle
IPC: G01S13/931
Filed: 1960-02-23
Abstract: 888,975. Doppler radar; frequency changer circuits. BENDIX CORPORATION. Feb. 23, 1960 [March 6, 1959], No. 6355/60. Classes 40(6) and 40(7). Relates to a C.W. Doppler radar in which the sense of the relative radial velocity of a reflecting object is determined by heterodyning the reflected signal separately with phase quadrature components El, E2, Fig. 2, of the transmitted signals to give phase quadrature Doppler frequency signals Ep1, Ep2, Fig. 7, and shifting the phase of one of the Doppler signals, e.g. Epl, by 90 degrees so that the shifted signal Ep1 is in phase or in phase opposition with the unshifted signal depending on whether the object is approaching or receding. As shown, the phase-quadrature Doppler signals Epl, Ep2 are applied to respective multi-stage distorting amplifiers 42, 40 producing square wave outputs. The output Ep2, Fig. 5, from amplifier 40 is applied to one control grid of a thyratron 44 and the output Ep1 from amplifier 42 is differentiated by a CR network 54, 56 and the resultant peaky waveform, Fig. 6, is applied to another control grid of the thyratron 44 so that the thyratron strikes only when the waveforms, Figs. 5 and 6, are in phase as shown thereby energizing a coil 60 to actuate a control circuit 62 when the object is approaching. The time constants of the input and inter-stage RC couplings in the amplifiers 40 and 42 are such that the amplitudes of the outputs therefrom are proportional to the relative speed of the object and the thyratron 44 is biased so that it strikes only when the approach speed of the object exceeds a critical value. When the radar is mounted on the front of a motor vehicle the control circuit 62 may actuate a warning and apply the brakes when the -critical speed is exceeded and when the radar is mounted on the rear of the motor vehicle the control circuit 62 may actuate a flashing rear light to warn the driver of the overtaking vehicle. In a modification. Fig. 8 (not shown), a rheostat controlled by the speedometer is included in the anode circuit of the thyratron 44 so that as the actual speed of the vehicle increases the critical relative speed at which the circuit 62 operates is decreased. The phase-quadrature components El, E2 of the transmitted signal are produced by a waveguide assembly comprising two T junctions interconnected by a straight member 33 of length n# containing an attenuator and a twisted U-shaped member 34 of length (2n+1)<SP>#</SP>/ 4 . The common transmitting and receiving aerial 29 and the transmitter klystron oscillator 20 are coupled through arms 26 and 24 to the first T junction and crystal mixers 30 and 32 are located at opposite ends of the cross member 20 of the second T junction. In operation outputs from the oscillator 20 are applied (1) to the aerial 29, (2) through the member 33 to give out-ofphase signals 30a, 32a, Fig. 2, in the two halves of the cross member 28 and (3) through the U-shaped member 34 to give in each half of the cross member 28 a signal 34a, Fig. 2, in phase quadrature with the signals 30a and 32a and which combines therewith to produce the phase quadrature signals E1, E2. The echo signal is applied through the U-element 34 to the two halves of the cross member 28 and is mixed in the crystals 31 and 30 respectively with the signals El and E2 giving the phase quadrature Doppler signals Ep1 and Ep2 such that Ep2 leads or lags Epl by 90 degrees, depending on whether the reflecting object is approaching or receding.