Polarimetric Antenna Diversity for Improved Reference Signal Estimation for Airborne Passive Radar

Passive radar utilizes signals from broadcast, communication, or other transmitters of opportunity for the applications of target detection and imaging. For these applications knowledge of the transmitted signal for the correlation with the surveillance signal is a crucial factor. If digital signals are exploited for passive radar, it is possible to estimate and restore the transmitted signal. The error rate in the estimated transmitted signal is inversely related to the signal-to-noise ratio of the direct signal. The greater the amount of errors in the reconstructed signal, the worse become the correlation results in terms of a higher noise level due to side lobes and unwanted correlation artifacts. Therefore it is preferred to acquire a copy of the transmitted signal as clean as possible. The reference signal estimation capability can be improved by exploiting antenna diversity. In this paper we present the technique maximal ratio combining, which exploits the reception of multiple antennas to optimize the reference signal estimation. Here we also exploit both linear (vertical and horizontal) polarizations on receive to further improve the estimated reference signal. We show the improvement by using real data from an airborne passive radar system and by comparing the results of the antenna diversity approach to the results when using the reference signal from one antenna only.