Abstract : A circuit is described which is capable of canceling first-order Doppler frequency shifts. It has potentialities in the fields of communication, data transmission, and navigation systems for vehicles moving at high velocities. The circuit has the following features: (1) Received and transmitted frequencies at each of the stations involved are different, thus preventing feedback from the transmitting to the receiving antenna at the stations. (2) Amplification of the signals is provided at a frequency which can be chosen to have a convenient value. This factor is important in the case of microwave systems, since high-gain microwave amplifiers are much more expensive than, and not as reliable as, IF amplifiers operating at frequencies in the 1 to 300 mc range. (3) The system is capable of continuous-wave operation and simultaneous transmission and reception at each of the communicating stations.