Separable Delay And Doppler Estimation In Passive Radar

In passive radar, a network of distributed sensors exploit signals from so-called Illuminators-of-Opportunity to detect and localize targets. We consider the case where the IO signal is available at each receiver node through a reference channel, whereas target returns corrupted by interference are collected in a separate surveillance channel. The problem formulation is similar to an active radar that uses a noise-like waveform, or an integrated sensing and communication application. The available data is first split into batches of manageable size. In the direct approach, the target's time-delay and Doppler parameters are estimated jointly by incoherently combining the batch-wise data. We propose a new method to estimate the time-delay separately, thus avoiding a costly 2-D search. Our approach is designed for slowly moving targets, and the accuracy of the time-delay estimate is similar to that of the full batch-wise 2-D method. Given the time-delay, the coherency between batches can be restored when estimating the Doppler parameter. Thereby, the separable approach is found to yield superior Doppler estimates over a wide parameter range. In addition to reducing computational complexity, the proposed separable estimation technique also significantly reduces the communication overhead in a distributed radar setting.