Abstract: In a Doppler (frequency shift) spacecraft tracking system, a transponder mounted on the spacecraft including, two or more, receivers connected to spaced-apart antennas for receiving an RF tracking signal transmitted from a ground based station regardless of the attitude, or position, of the spacecraft. The intermediate frequency (IF) signals from the receivers are combined to provide a continuous third IF signal at the proper frequency and at a phase dependent upon the signals received, which third signal is utilized to control a transmitter for retransmitting coherent reply carrier frequency signals to the ground based station.