Ground speed sensor

Claim 1

A velocity sensor for sensing the velocity of a vehicle moving over a terrain, the velocity sensor comprising: a transmitter for transmitting a signal with a certain constant transmit frequency toward the terrain; a receiver for receiving a signal reflected from the transmitter to the receiver by the terrain, the reflected signal having a variable reflected frequency dependent upon the speed and direction of the motion of the vehicle with respect to the terrain, the receiver also receiving a cross talk component signal having the same frequency as the transmit frequency, the receiver generating a combined signal having variable reflected frequency and constant transmit frequency components; means for converting the transmit signal to an attenuated, phase-shifted signal; a summing circuit having a first input receiving the combined signal, a second input receiving the attenuated, phase-shifted signal, the summing circuit operating to cancel the transmit frequency component of the combined signal, the summing circuit thereby providing at an output thereof a modified signal with a substantially pure reflected frequency; a first circuit for converting the modified signal to a first voltage having a magnitude proportional to the frequency of the modified signal; a second circuit for converting the transmit signal to a second voltage having a magnitude proportional to the transmit frequency; and a difference circuit receiving the first and second voltages and generating a third voltage representing a difference between the first and second voltages, the first and second circuits having similar components so that the third voltage is substantially uneffected by component drift.