Doppler radar speed sensor

A Doppler radar speed sensor for a land vehicle, comprising a radar transmitter/receiver circuit coupled to a transmit/receive aerial system for transmitting from said aerial system first and second beams of radio signals, derived from a common source, in first and second directions respectively both of which intersect the ground, and for processing reflected said radio signals received by said aerial system from said first and second directions to obtain information about the speed of said aerial system in a third direction which is parallel to the ground, said first and second directions making acute and obtuse angles respectively with a given sense of said third direction, said circuit comprising a mixer arrangement for mixing the received signals with a reference signal derived from said common source to produce a beat signal having a pair of Doppler frequency components the frequencies of which are representative of the speeds of said aerial system in said first and second directions respectively, and digital data processing means including a spectrum analyser for determining the speed of the aerial system in said third direction from the spectrum of said beat signal, wherein the sum of said acute and obtuse angles is different from 180.degree. and said mixer arrangement is a single-phase mixer arrangement.