Inter-period compensation of the unambiguity range degradation in polarimetric FMCW radar with time-shifted dual-orthogonal signals

This paper describes a new type of sounding signals for polarimetric FMCW radar and the corresponding signal processing technique. This technique coordinates continuous transmission of two bi-cyclic LFM-signals on orthogonal polarizations and applies an advanced de-ramping processing on receive. The first signal consists of two identical LFM-signals; the second one has time shift relatively first and is composed by the same LFM-signal and its copy having negative sign. The time between the components of sounding signal prevents the frequency overlap between polarimetric components of scattered signal in the branches of every polarimetric radar channel. Thanks to this frequency gap between the signals in every time instant, a high-isolation level in polarimetric FMCW radar channels can be provided. However, the use of two LFM-signals having the same form but a relative time shift can decrease the unambiguity range, because the delayed signals scattered by far-located objects (interfering signals) can be determined as useful signals scattered by close-located objects in a cross branch in a polarimetric radar channel. The use of the proposed bi-cyclic LFM-signals and the additional summation units in the de-ramping filter allows the suppression of these interfering signals due to the phase sign change of the beat signals. As result, the developed de-ramping processing technique can be used in polarimetric FMCW radar receiver providing high-level cross-channels isolation without unambiguity range degradation for estimation of all elements of the radar target scattering matrix.