Passive bistatic SAR imaging — Challenges and limitations

In this article, a concept of passive bistatic SAR (PB-SAR) imaging using a spaceborne SAR radar transmitter of opportunity is presented. This was proposed for the first time in the beginning of this century by Krieger et al and Griffiths et al. Because passive imaging is completely covert and relatively cheap, as it does not require its own transmitter, and the rapid growth of the computational power of processors has made it possible, the topic has attracted the attention of many research institutes. Recently, several bistatic radar experiments have been carried out using ground stationary receivers and spaceborne illuminators, and the results have shown new capabilities of SAR imaging. In most of these trials, cooperative illuminators were used and placed on satellite platforms developed by the country conducting the experiment, means that the receiver has exact a priori information about the transmitter's parameters (e.g., carrier frequency, signal parameters, chirp duration, pulse repetition frequency [PRF], etc.). To obtain a passive SAR image when the parameters are roughly known, additional techniques for signal analysis and parameter estimation have to be applied, which make the processing more sophisticated. In this article, the challenges and limitations of the PB-SAR imaging are discussed, and the differences in the signal processing chain in mono- and bistatic configurations are indicated. Finally, the recent results of real passive SAR images are presented. The results were obtained by the Warsaw University of Technology (WUT) in 2013, using their passive radar demonstrator mounted on the ground. As the transmitter of opportunity, the TerraSAR-X satellite was used.