GB1239859A - Doppler radar - Google Patents

Abstract

1,239,859. Doppler radar. MULLARD Ltd. 25 March, 1969, No. 15560/69. Heading H4D. Specification describes a vehicle speed detecting C.W. Doppler radar in which the speed measurement is not commenced until the vehicle has passed the radar locality and is receding at a distance determined by the amplitude of the echo signal, Fig. 3, falling to a predetermined level 7. The circuitry may be part solid state whereby the aerial (S) and said circuitry part may be assembled in a cats-eye in the middle of a traffic lane and the analysing and display means situated at a remote control point and be fed with the output of several of said cats-eye assemblies. In the embodiment of Fig. 4, an X-band Gunn-effect oscillator is amplitude modulated at 10 and the output transmitted at 13. The receiver aerial lobe is such that the amplitude of the echo signal as the vehicle nears the aerial, passes over it and then recedes, is as shown in Fig. 3. The Doppler signal produced by detector 16 is extracted by low-pass filter 17. The amplitude of the echo is detected at 27 by means of the amplitude modulation extracted at 21. Means 27 only gives an output when the amplitude is at the level 7, Fig. 3. The receding part of curve, Fig. 3, is detected by differentiator 23 and diode 26. The outputs of detector 27 and diode 26 occur simultaneously at point 8 and this event is detected at 24. An output from 24 actuates a presence counter 27 and also commences a ¢ sec. pulse from means 28. This gates the Doppler signal at 18 to the analysing and display means. The length of the vehicle may be determined by multiplying the duration of the constant (maximum) part of the amplitude curve by the vehicle's velocity.