Context and objectives
This project deals with passive synthetic aperture radar. Passive radars use transmitters of opportunity as signal source and are necessarily bistatic. It should be noted that most, if not all, operational SAR systems are active (they have their own dedicated transmitter) and monostatic (the transmit antenna is collocated with the receive antenna). Passive SAR is to be seen as an enabler of other applications since, due to its low cost, it puts SAR imaging at the reach of lower budgets.
The most straightforward passive SAR system consists in using an existing SAR sensor such as ENVISAT as transmitter of opportunity and having a static (non moving) receiver. The direct and the backscattered signals are received and processed at the receiver to produce recurrent images of a given site at each sensor pass.
The topic is extremely wide. Consequently, we propose to start from our current expertise to develop, in a first step, a passive system with a static receiver, including setup and processing chain, using ENVISAT as transmitter of opportunity. A first objective being to perform a “proof of concept” which will allow us to acquire the required experience in order to be able to address more complex configurations in the near future and pave the way toward potential application for other Belgian research teams.
The receiving system was built and is almost fully automated, allowing to autonomous acquisition when a satellite pass is predicted. In 6 month time, about 23 ENVISAT passes have been acquired.
The aperture synthesis algorithms were developed and tested, both a beam-forming based approach with among others the distinct advantage to provide geocoded data, and a Fourier-transform-based algorithm with the distinctive advantage of being extremely fast.
In addition to this, while not in itself new, what was noticed is that there are a lot of passes (of the Envisat satellite to be precise) but only a few (2 in 6 months) are in a so-called high resolution mode (Image mode). Most of them are in one of the ScanSAR modes (Global monitoring or Wide Swath). In an attempt to exploit these passes, a method consisting in exploiting the signals received from the sidelobes of the transmit antenna was developed. This permits to increase the (azimuth) resolution in ScanSAR (Wide swath) mode. An assessment of the actual performance of this method was performed, showing that more than 60% of the Wide swath passes are suitable for resolution enhancement, yielding a resolution close to that of the Image mode, thus dramatically increasing the revisit time.
|ERM - KMS - RMA - Signal and Image Centre