Signal discrimination system

A system for preferentially receiving modulated backscattered signals based upon a phase shift of the modulation of the phase-modulated carrier signal resulting from the transmission-to-reception time delay, comprising:

• a means for phase modulating a carrier with a pseudo-random coded sequence having characteristics which provides unity output when correlated with itself with zero time shift and a substantially lower output level when correlated with itself with a time shift of at least a significant fraction of a pulse width of said coded sequence, said modulation producing a direct sequence, spread spectrum signal;
• a means for transmitting said phase-modulated carrier;
• a backscatter modulating means located at a finite distance from said transmitting means for receiving and modulating the back-scatter of said phase-modulated carrier signal;
• a means for receiving and detecting said modulated back-scattered phase-modulated carrier, said detecting means being phase-sensitive and having as a reference signal said transmitted phase-modulated carrier signal, and having as an input signal said modulated backscattered phase-modulated carrier signal, said detecting means providing an output signal whose averaged amplitude is substantially dependent upon the degree of phase correlation between said input signal and said reference signal;
• and
• a means for selectively processing the output signal from said receiving and detecting means only when its averaged amplitude is above a threshold level, whereby only signals from a selected location are processed and the remainder being rendered incapable of being received.