Passive synthetic aperture radar imaging with single frequency sources of opportunity

In this paper we consider passive airborne receivers that use backscattered signals from sources of opportunity transmitting fixed-frequency waveforms. Due to its combined passive synthetic aperture and the fixed-frequency nature of the transmitted waveforms, we refer to the system under consideration as Doppler Synthetic Aperture Hitchhiker (DSAH). We present a novel image formation method for DSAH. Our method first correlates the windowed signal obtained from one receiver with the windowed, filtered, scaled and translated version of the received signal from another receiver. This processing removes the transmitter related variables from the phase of the Fourier integral operator that maps the radiance of the scene to the correlated signal. We next use the microlocal analysis to reconstruct the scene radiance by the weighted-backprojection of the correlated signal. This imaging algorithm can put the visible edges of the scene radiance at the correct location, and under appropriate conditions, with correct strength. Additionally, it is an analytic reconstruction technique which can be made computationally efficient. We show that the resolution of the image is directly related to the length of the support of the windowing function and the frequency of the transmitted waveform. The image reconstruction method is applicable with both cooperative and non-cooperative sources of opportunity using one or more airborne receivers. We present numerical simulations to demonstrate the performance of the image reconstruction method and to verify the theoretical results.