Method and apparatus for improved auto-correlation and range correlation in pseudo-random noise coded systems

A method of improving out of range rejection in a pseudo-random noise coded system comprising the steps of:

• sampling a transmitted signal comprised of a carrier frequency phase modulated with a first pseudo-random noise code sequence, said first pseudo-random noise code sequence having two phase states with a first relative phase difference, the sampling step producing a sampled transmitted signal;
• generating a first error signal based on said sampled transmitted signal;
• adjusting said first relative phase difference between said two phase states of said first pseudo-random noise code sequence based on said first error signal to maintain a first predetermined phase offset from said first relative phase difference, said first predetermined phase offset based on a code length of said first pseudo-random noise code sequence;
• and
• combining a received signal with a reference signal having a spectral characteristic of said first pseudo-random noise code sequence to obtain a correlation range response.

A pseudo-random noise coded system for improving discrimination between returns from a target and returns from clutter comprising;

• an radio frequency generator for generating a carrier frequency;
• a transmit bi-phase modulator for generating a transmit signal comprised of said carrier frequency phase modulated with a first pseudo-random noise coded sequence, said transmit bi-phase modulator coupled to said radio frequency generator and having a transmit phase control port and a transmit output port;
• a transmit antenna for transmitting said transmit signal and coupled to said transmit output port of said bi-phase modulator;
• a receive bi-phase modulator for generating a reference signal comprised of said carrier frequency phase modulated with a second pseudo-random noise coded sequence, said second pseudo-random noise coded sequence being a delayed version of said first pseudo-random noise coded sequence, said receive bi-phase modulator coupled to said radio frequency generator and having a receive phase control port and a receive output port;
• a receive antenna for receiving a received signal, said received signal being a Doppler shifted version of said transmit signal;
• and
• a correlator for producing a Doppler frequency from said received signal and said reference signal, said correlator having first, second and third ports, said first port coupled to said receive bi-phase modulator, said receive antenna coupled to said second port of said correlator.

A pseudo-random noise coded system for improving discrimination between returns from a target and returns from clutter comprising:

• a transmit bi-phase modulator for generating a transmit signal comprised of a carrier frequency phase modulated with a first pseudo-random noise (PN) code sequence, said first PN code sequence having two code states resulting in two phase states of said transmit signal that have a first relative phase difference of approximately one-hundred eighty degrees;
• a transmit feedback control loop for sampling said transmit signal and producing a transmit feedback signal based on a magnitude of spectral lines of said transmit signal, and wherein
• said transmit bi-phase modulator has means for adjusting said first relative phase difference based on said transmit feedback signal to maintain a first predetermined phase offset from said first relative phase difference of said two phase states;
• and
• a correlator for combining a received signal with a reference signal to obtaining a correlation range response, said reference signal including said carrier frequency phase modulated with a second PN code sequence, said second PN code sequence being a delayed version of said first PN code sequence.