Quadratic Phase Coding for High Duty Cycle Radar Operation

Quadratically varying phase codes applied from pulse to pulse can be used to impart a range-dependent frequency shift in the decoded signal of a pulsed radar. Radars employing such codes can operate at extremely high pulse repetition frequencies (PRFs) with overlaid signals from multiple echo trips separated in the spectral domain. When operating at high PRFs, the radar duty cycle can approach 50% in a single-antenna system. High duty cycle operation results in a substantial increase in average transmit power with a proportional increase in signal processing gain as compared to a conventional pulsed radar. The shortest quadratic phase code, or base code, has a length equal to the number of echo trips M that can be unambiguously resolved in the spectral domain. The decoded waveform is essentially free from range sidelobes under ideal conditions. However, amplitude and phase errors associated with nonideal phase coding result in range sidelobes that appear at all echo trips in the decoded signal. These sidelobes can be suppressed by using a composite phase code composed of a periodically repeating base phase code added to a much longer quadratic code with a proportionally slower phase variation. Meteorological data gathered with a Ka-band radar operating at 3.0-MHz PRF at 45% duty cycle are presented. A comparison of these data with data gathered in short-pulse mode at a duty cycle of 0.3% exhibited a 21-dB improvement in the Doppler spectrum signal-to-noise ratio, equal to the ratio of the respective duty cycles.