Comparison of complementary sequences in hybrid phase and frequency shift keying CW radar using periodic ambiguity analysis

Abstract

Continuous waveform (CW) polyphase sequences for radar have a much lower power spectral density (PSD) than pulsed signals but can retain the same target detection capability. The use of different phase values or subcodes to modulate the carrier provides a low probability of intercept (LPI) radar waveform which cannot be seen by a non-cooperative intercept receiver (NCIR). Also, it is a low probability of detection (LPD) waveform due to the low PSD. Frequency shift keying (FSK) radar has a higher PSD but is moved about quickly in frequency over a large bandwidth in which the NCIR cannot follow. Consequently, the FSK (usually a Costas frequency set) remains a LPI signal but not a LPD. To combine the advantages of each waveform, this thesis presents a hybrid FSK/PSK emitter waveform to further the LPI, LPD characteristics. By combining both techniques (PSK/FSK), a high time-bandwidth waveform is constructed that provides better LPI/LPD characteristics than each waveform. The periodic ambiguity function (PAF) is evaluated for three different complementary sequences to modulate a Costas frequency set. The peak time and Doppler sidelobes of the PAF are compared against the P4 polyphase modulation for the Golay complementary sequence (15 dB improvement), the quaternary periodic complementary sequence (16 dB improvement), and the quaternary Golay complementary sequence (18 dB improvement).