Quasi-simultaneous measurements of scattering matrix elements in polarimetric radar with continuous waveforms providing high-level isolation in radar channels

This paper presents a new type of sounding signals for polarimetric FM-CW radar and corresponding de-ramping technique for processing, which provide the possibility for simultaneous measurement of all backscattering matrix elements and have high-level isolation between branches of the polarimetric receiver. The radar transmitter forms sounding signal, which has orthogonally-polarized components with orthogonal waveforms. The radar receiver is splitting orthogonally-polarized components of the scattered signals in two parallel receiver channels. Every such channel includes two parallel independent branches, which use wave-form orthogonality of the orthogonally-polarized components of sounding signal to split further signal components for simultaneous estimation of all four complex elements of the polarization backscattering matrix of radar object. The isolation between pairs of such branches in one receiver channel strongly depends on the time interval when scattered signals with orthogonal waveforms occupy the same frequency band. In this paper we propose to use a pair of LFM-signals as orthogonally-polarized components of the sounding radar signal. Both have the same form but are shifted in time relatively to each other. Our analysis shows that such time shift between the components of sounding signal prevent the frequency overlap between polarimetric components of scattered signal. As result, standard de-ramping processing technique can be used in the receiver, providing high level isolation between receiver's branches that simultaneously estimate all elements of the radar target backscattering matrix.

I. Introduction

Modem polarimetric frequency modulated continuous wave (FM-CW) radar combines high performances both in range resolution and sensitivity. Sounding signals with linear frequency modulation (LFM signals) provide high range resolution due to signal compression in the radar receiver. The continuous wave transmission, which has 100% duty cycle, can use low peak power for the same detection performance as obtainable with pulsed transmission [1]. The use of the vector sounding signal with two orthogonally-polarized components allows to measure the complete electromagnetic vector information about observed objects [2]. In general, any radar object (or any radar resolution cell) can be described by an back-scattering matrix (SM) with four time and frequency dependent complex elements, which describe the transformation of amplitude, phase and polarization state of sounding radio wave during its scattering on the radar object.