Effects of Cross-Correlated Waveforms on Polarimetric Scattering Parameter Recovery

Pulse compression offers increased range resolution and energy on target without the high peak power required of an unmodulated waveform. In addition, polarimetric radar measurements can provide valuable insights into hydrometeor properties. From a scientific perspective, it is desirable to obtain high resolution measurements of a target's four polarimetric scattering parameters. In contrast to alternating polarization (or frequency-duplexed) transmit mode measurements where the target scattering parameters may decorrelate between measurements, the scheme described herein enables direct measurement of all parameters simultaneously. A fully polarimetric radar system with both polarization orthogonality and coding orthogonality, where two waveforms in an orthogonal set are simultaneously transmitted on separate orthogonal polarization basis vectors, is utilized in addition to pulse compression. In practice, ideal orthogonality cannot be achieved in polarization or waveform coding. Polarization orthogonality is compromised due to radiators invariably transmitting non-zero power on the polarization orthogonal to the intended polarization. True waveform coding orthogonality is impossible because the cross-correlation of different waveforms in an orthogonal set will not result in a zero magnitude return at all time delays. Therefore, the effects of orthogonality degradation on pulse compression output and the resulting scattering parameter retrieval must be characterized. The mathematical basis for the matched filter output of a fully polarimetric radar return in a system with antenna cross-polarization contamination and co-polarization gain mismatch is derived for wideband pseudo-orthogonal waveform sets. Single and multiple target cases are considered. Using linear-frequency modulated (LFM) and P2-coded waveforms as examples of pseudo-orthogonal waveform sets, resolution characteristics, range sidelobe characteristics, and scattering parameter separability are examined as a function of orthogonal polarization degradation and pseudo-orthogonal coding degradation. This analysis is accomplished by examining the characteristics of simulated pulse compression outputs of both single and multiple resolved targets with varying scattering parameters. It is seen that while resolution is minimally affected, sidelobe behavior is moderately affected, and scattering parameter separability is severely degraded as a function of orthogonality degradation.