Passive synthetic aperture radar imaging of ground moving targets

Abstract

Synthetic aperture radar (SAR) is an imaging technique that uses antenna motion and spatial diversity to coherently combine received waveforms and form images of a scene of interest. This thesis focuses on the SAR problem of passive imaging of moving targets. A passive radar imaging system uses small, mobile receivers that do not radiate any energy. The received signals are obtained from the scattered waves due to illuminating sources of opportunity such as commercial television, radio, and cell phone towers. A system of passive receivers results in significant cost and manufacturing advantages. Additionally, since the passive radar systems do not radiate any energy, they offer stealth benefits.

The first part of this thesis considers the passive synthetic aperture moving target imaging problem using relatively wideband RF sources of opportunity such as DVB-T and WiMax signals. No knowledge of the transmitter locations or waveforms is assumed, so a correlation-based technique is used to backproject the data based on time difference of arrival between pairs of receivers. Velocity estimation is performed by forming images based on a range of hypothesized velocities, and analyzing the degree of focus of each image. We show that images with the correct hypothesized velocities will yield focused reconstructions of the corresponding moving target. In addition, a detailed performance analysis is developed for this method.

The second part of this thesis addresses the case of ultra-narrowband transmitters of opportunity. These signals of opportunity are common, but they do not possess high enough bandwidth to provide the range information to use with conventional SAR methods. Instead, an approach called Doppler-SAR is used, where high resolution position images are formed by using the high Doppler resolution information in the ultra-narrowband waveforms. We first develop an analogous method to the case of unknown transmitter parameters by using correlation to backproject the frequency difference of arrival between pairs of received signals. Next, a novel technique to combine the Doppler imaging approach with the displaced phase center antenna approach is developed. Two channels are used to receive signals from a scene, and their Doppler shifts are compared to remove stationary clutter. Finally, the along-track interferometry clutter suppression method is also combined with Doppler SAR, and the theoretical results of the Doppler case are compared to the well known wideband case.