Abstract: The present invention describes a method and apparatus for improving the autocorrelation performance in spread spectrum radar and communications systems by using a tri-state digital correlator to demodulate the received biphase modulated waveform. The tri-state correlator (100) includes a multiplexer (112) having a code input for receiving a tri-state code sequence, and three inputs each for receiving a sequence of data words including a first input for receiving a digitized signal data word, a second input for receiving an inverted digitized signal data word, and a third input for receiving a zero data word. The multiplexer includes an output and a means for producing a sequence of data words at the output, said means presenting at the output exactly one of the three data words appearing at either the first, second, or third input depending on which of the three code states appears at the code input.

Abstract: A method and apparatus at a transmitter (800) initializes (904) pseudonoise (PN) sequence generators (302, 306, 320) to predetermined initial states and thereafter generates (906) time-synchronized PN sequences. A spread spectrum signal is then transmitted (908) by utilizing the PN sequences to encode information. At a receiver (600, 700) a predetermined one of the PN sequences is acquired and tracked (1002), and one of the predetermined initial states is predicted and tracked (1004). A PN sequence generator (608, 714, 722, 730) of the receiver is adjusted (1006, 1014) to generate a selected PN sequence time-synchronized with the predetermined one, and a corresponding one of the PN sequences is despread (1008) by utilizing the selected PN sequence to decode the information.

Abstract: This invention relates to a method and apparatus for controlling a biphase modulator (602, 702) to improve autocorrelation in pseudorandom noise coded systems. The biphase modulator modulates a carrier frequency with one of two phase states responsive to a pseudorandom noise (PN) binary code sequence. The spectrum (610, 710) at the output of the biphase modulator comprises a plurality of spectral lines separated by the code repetition frequency, including a center spectral line (611, 711) and at least one adjacent spectral line (612-615, 712-717). The magnitude of the center spectral line is measured and compared to a reference to produce a control signal which is responsive to the magnitude of the center spectral line. This control signal is supplied to the biphase modulator for maintaining a predetermined magnitude of the center spectral line thereby achieving a desired spectrum output and improving autocorrelation of the system.