Quadratic Phase Coded Radar

This paper describes a method for increasing the duty cycle of a pulsed radar by the application of a quadratically varying phase code to the transmitted waveform. Modern weather radars often employ solid-state power amplifiers (SSPAs) capable of continuous wave operation. While FMCW radars can take full advantage the average power of the SSPAs, pulsed radars typically operate at relatively low duty cycles, with a reduction in the achievable sensitivity that scales with the duty cycle.The Quadratic Phase Code (QPC) technique steps the phase from pulse to pulse by a linearly increasing step. Applying a QPC local oscillator waveform with the reversed sense to the received waveform separates the received signal into N evenly spaced frequencies in the Doppler spectrum. Repeating QPC sequences may be employed, either of length N if N is odd, of length 2N if Nis even, or non-repeating QPC sequences can be generated by slightly perturbing the phase step. Non-repeating codes have the advantage of suppressing spurious sidebands in the Doppler spectrum due to phase quantization errors.

By applying a quadratically-increasing phase code from pulse-to-pulse, the QPC radar avoids the maximum unambiguous range constraint normally associated with pulsed radar. Unambiguous sampling using the QPC method assumes that the magnitude of the Doppler shift of objects viewed by the radar is less than the unambiguous velocity for a given maximum observation range. Assuming this constraint is satisfied, the pulse repetition frequency of the QPC radar is no longer limited by the normal unambiguous range constraint. In the limit, a phase modulated square wave (PMSW) radar can be implemented running at a duty cycle of 50 percent. The range profile of the PMSW radar is under sampled, since every other range gate is blanked to protect the receiver during transmission. However, a PMSW radar can achieve any desired range resolution while still operating at 50 percent duty cycle, only limited by the switching speed of the T/R network and the recovery time of the receiver.

W-band data gathered at pulse repetition frequencies (PRFs) of 1 MHz and higher show close to theoretical improvement in sensitivity as compared to standard short pulse operation. Doppler spectra taken in rain show over 60 dB dynamic range, an improvement of approximately 20 dB as compared to spectra gathered at 10 kHz PRF.