Synthetic Aperture Hitchhiker Imaging using Non-Cooperative Sources of Opportunity

A hitchhiker is a passive radar receiver that relies on sources of opportunity to perform radar tasks.1–4 In this paper, we consider a synthetic-aperture radar (SAR) system with static non-cooperative transmitters and mobile receivers traversing arbitrary trajectories and present an analytic image formation method. Due to its combined synthetic aperture and hitchhiking structure, we refer to the system under consideration as synthetic aperture hitchhiker (SAH). Our approach is applicable to cooperative and/or non-cooperative and static and/or mobile sources of opportunity. Conventional SAR processing involves correlation of the received signal from a receiver with the transmitted waveform as a first step of the image formation. For passive SAR, however, the transmitted waveform is not necessarily known. Instead, we use spatio-temporal correlation of received signals. Given a pair of receivers, the spatio-temporal correlation method compares the received signals to identify a target within the illuminated scene. We combine this with microlocal techniques to develop a filtered backprojection (FBP) type inversion method for passive SAR.5 Combined correlation-FBP inversion method does not require the knowledge of the transmitter locations. Furthermore, FBP inversion has the advantage of computational efficiency and image formation under non-ideal conditions, such as arbitrary flight trajectories and non-flat topography.