the ground speed sensing system 10 for mounting on a vehicle lower frame member 11 includes a conventional ultrasonic ( generally 20 - 200 kilohertz ( khz ) transmitter 12 , such as which continuously transmits an ultrasonic signal , t , at a preferred transmit frequency , f ( t ), of 40 khz . the system also includes a conventional ultrasonic receiver 14 . one such transmitter and receiver is available from the southwest research institute and is described in sae paper 790510 by gorfs and benson . however , an equivalent transmitter and receiver would also suffice . the receiver 14 receives reflected ultrasonic signals , r , with frequency f ( r ), which are reflected from the terrain over which the vehicle travels . if the transmitter 12 and receiver 14 are aimed towards the ground in the forward direction with respect to vehicle motion , then the reflected signal , r , will have a reflected frequency , f ( r ), which will be higher than f ( t ) if the vehicle and system 10 are moving forward . conversely , f ( r ) will be lower than f ( t ) if the vehicle and system 10 are moving in reverse . in actuality , the output of the receiver will contain an f ( r ) component and a &# 34 ; cross talk &# 34 ; component &# 34 ; c &# 34 ; with frequency f ( t ) due to cross talk between the transmitter 12 and the receiver 14 . a signal processing circuit 15 includes a buffer 16 which receives the transmit signal , t , from a pre - amp ( not shown ) of the transmitter 12 . buffer 16 includes a capacitor c1 , three identical resistors , r1 , r2 and r3 and an integrated circuit operational amplifier ic1 . the output of buffer 16 is coupled to an attenuation and phase - shift circuit 18 . circuit 18 includes resistors r4 and r5 , capacitor c2 , an op - amp , ic2 , feedback capacitor c3 , variable phase - adjust resistor r6 and voltage divider resistors r7 and r8 . circuit 18 attenuates and phase shifts the transmit signal , t , and provides a phase - shifted and attenuated output , t &# 39 ;. the t &# 39 ; signal is applied to a variable ( amplitude adjusting ) resistor r9 of summing circuit 20 . summing circuit 20 also includes resistors r10 and r11 and op - amp ic3 . resistor r10 receives the output ( r + c ) of receiver 14 so that the receiver output can be superimposed on the t &# 39 ; signal . preferably , the phase and amplitude of t &# 39 ; is such that it just cancels out the cross talk component c of the receiver output , leaving a voltage or modified signal at 24 which is an amplified signal , r &# 39 ;, having a substantially pure reflected frequency , f ( r ). this reflected frequency f ( r ) is &# 34 ; unmixed &# 34 ; in the communications technology sense of that term in that the reflected frequency f ( r ) is produced by circuit 20 which has no homodyne or heterodyne mixer . this signal r &# 39 ; with the pure reflected frequency , f ( r ), is received by a low pass filter and amplifier circuit 26 . circuit 26 includes capacitors c4 and c5 , a comparator input , bias adjust potentiometer r13 , resistors r12 , r14 and r15 and an op - amp ic4 . the output of circuit 26 is received by a comparator circuit 28 which includes an op - amp , ic5 , and resistors r16 and r17 . this is essentially a &# 34 ; squaring &# 34 ; circuit with hysteresis to eliminate low level noise . the output of circuit 28 is received by a phase - locked - loop ( pll ) circuit 30 . pll circuit 30 includes a resistor r21 , a pair of diodes d1 and d2 , a phase comparator ( pc ) 32 , a low pass filter 34 , ( including resistor r18 and a capacitor c6 ) and a voltage controlled oscillator ( vco ) 36 . the vco 36 preferably includes resistors r19 and r20 , variable resistor r22 and capacitor c7 and is powered by + 8 and - 8 volts . the pll 30 provides a voltage , vg , at 38 , which has a magnitude and sign corresponding to the magnitude and sign of the difference : as an alternative to a pll , a slope detector with a low pass filter characteristic could be used instead . this difference is , in turn , proportional to the ground speed of the vehicle ( not shown ), less a constant . preferably , the voltage , vg , is conditioned by a buffer amplifier 40 which includes a capacitor c8 , resistors r23 and r24 and an differential op - amp ic6 . finally , the voltage , vg is conditioned by a noise - limiting , 2 - pole , low pass filter circuit 42 . circuit 42 includes resistors r25 and r26 , capacitors c9 and c10 and an op - amp ic7 . preferably , filter circuit 42 has a break frequency of 10 hz and a damping factor of 0 . 71 . this provides , at the output of ic7 , an output voltage , vv , with a magnitude representing the speed of the vehicle and with a sign representing the direction of vehicle travel ( forward or reverse ). of course , by providing an offset level , then the direction information could be represented by whether the output voltage is above or below the offset level , not by whether it is positive or negative . an optional feature would be to provide an additional or reference pll 50 identical to pll 30 . this reference pll 50 would receive the transmit signal t from transmitter 12 and would provide a reference output voltage vt which would be proportional to f ( t ), less a constant . this reference voltage vt would be coupled to the (-) input of ic6 via a resistor r26 . the differential op - amp ic6 generates at its output a voltage which is proportional to the difference between the signals applied to its (+) and (-) inputs . in this manner , drift in the transmit frequency or in the plls 30 and 50 would be cancelled and the voltage vv would be relatively uneffected by component drift . suggested component values for the circuit 15 are listed in the following table : ______________________________________capacitorsc1 . 0015 mfc2 , c7 . 001 mfc3 , c5 100 pfc4 , c10 1 mfc8 47 pfc9 3 mfc6 . 01 mfresistors ( ohm ) diodesr1 , r2 , r3 1 m d1 , d2 in914r4 3 . 9kr5 75kr7 , r14 , r17 100kr8 , r21 10kr10 , r16 2 . 2kr11 , r26 22kr12 2kr15 39kr18 6 . 8kr19 390kr20 62kr23 , r26 68kr24 18kr25 4 . 7kr6 , r9 variable 0 - 100kr16 , r13 25k potentiometerr22 variable 0 - 10kicsic1 - 5 lf 356 ( motorola or equiv . ) pl ( 32 ), vco ( 36 ) cd 4046 ( rca or equiv . ) ic6 , 7 mc 34001 ( motorola ) ______________________________________ while the invention has been described in conjunction with a specific embodiment , it is to be understood that many alternatives , modifications and variations will be apparent to those skilled in the art in light of the aforegoing description . accordingly , this invention is intended to embrace all such alternatives , modifications and variations which fall within the spirit and scope of the appended claims .