height of the bulldozer blade is controlled by extending or retracting cylinder 21 the true groundspeed sensor package 15 determines the true groundspeed of the tractor 14 and can also determine if there is a loss of traction between the track 16 and the soil 17 referring now to fig2 the detailed operation of the draft power sensor system will be described vehicle velocity is sensed by means of determining the doppler shift of a transmitted ultrasonic beam caused by the velocity of the earthmoving equipment transmitter 46 generates a nominal 40 khz signal which drives an ultrasonic transmit transducer 41 the ultrasonic beam emitted by transmit transducer 41 is reflected from the soil 42 and is received by receive transducer 43 the received signal is amplified in preamplifier 44 and then further amplified and synchronously detected in synchronous detector 45 the output of synchronous detector 45 is the difference frequency between the nominal 40 khz signal transmitted by transmit transducer 41 and the nominal 40 khz doppler shift signal received by receive transducer 43 the doppler shift signal is directly proportional to the velocity of the tractor 14 over the soil 17 as shown in fig1 this resulting signal is amplified and band limited in band pass amplifier 47 and further amplified in buffer amplifier 48.