A direct sequence spread spectrum (DS-SS) communications modem for low signal to noise ratio applications and secure communications systems (10). The modulator (22) implements a quadrature-phase shift keying (QPSK) modulation scheme in which a pseudonoise code is supplied to one channel and a pseudonoise code is combined with data to define the other channel. The demodulator (34) receives the modulated signal and utilizes a signal acquisition processor (92) in order to detect the pseudonoise code applied to the first channel. Upon detection and acquisition of the first pseudonoise code, the second pseudonoise code may be determined in order to demodulate the second channel to yield the modulated data. The signal acquisition processor (92) provides numerous parametric measurements to enable fine demodulator tuning and robust acquisition of the pseudonoise spreading codes on the I and Q channels.