The invention provides a system for discriminating between modulated backscattered signals based upon transmission-to-reception time delay. The system employs a carrier, phase-modulated with a modulating signal, having characteristics which provide unity output when correlated with itself with zero time shift and a substantially lower output level when correlated with itself with a significant time shift. The system works with a backscatter-modulator located a finite distance from the transmitter for receiving and modulating the backscatter of the carrier. A receiver/detector receives and detects the modulated backscattered phase-modulated signal, the detector being phase-sensitive and having as a reference signal the transmitted phase-modulating carrier signal, and having as an input signal the phase modulated backscattered carrier. The detector provides an output signal whose averaged amplitude is substantially dependent upon the degree of phase correlation between the phase of the modulation returned input signal and the reference signal. Finally, the output signal from the detector is selectively processed only when its averaged amplitude is above a threshold level. In a preferred embodiment, the transmitted signal is a spread spectrum signal.