A homodyne communication system comprising an interrogating unit and a plurality of responsive remote units wherein a continuous wave RF signal is transmitted from the interrogating unit to illuminate a remote unit. The remote unit causes a return (reply) signal to be transmitted or reflected back to the interrogating unit. A sample of the transmitted (interrogation) signal is mixed with the return signal. Amplitude nulls in the mixer output signal at given relative phase relationships between interrogation and reply signals, are substantially eliminated by selectively shifting the relative phase of the signals. Also disclosed are a homodyne communication system utilizing phase modulation to impress FSK signaling on the reply signal and an automatic vehicle location system utilizing the present invention.