Synthetic aperture radar (SAR) is a powerful remote sensing tool that provides enhanced day and night imaging capabilities. It consists of the acquisition of multiple radar echoes to emulate a larger antenna aperture (hence the name synthetic aperture), thus producing high resolution images useful for various applications such as Earth monitoring, reconnaissance, and target tracking. SAR systems typically generate large amounts of information, most of it in the form of complex data that are long to process and difficult to compress.
Real-time onboard SAR image generation could provide local access to processed information, reducing the data throughput requirements and the related transmission bandwidth. Furthermore, for observation missions or unmanned aerial vehicles (UAVs), onboard analysis of images could provide important feature identification clues, help select the appropriate images to be transmitted to command center, and consequently assist with image and bandwidth management.
This makes the optronic processor an attractive choice for interplanetary spaceborne and UAV/persistent platforms. Not only can the design be made lightweight and compact, it also has fast processing speeds and a large dynamic range.
In the context of a project for the European Space Agency (ESA-ESTEC), we have worked closely with the ESA team to develop a prototype of a real-time optronic processor for the generation of SAR images from SAR raw data. The real-time SAR optronic processor architecture provides inherent parallel computing capabilities that can be used advantageously for SAR data processing. Envisat ASAR data provided by ESA were used to generate SAR scenes and test the optronic processor prototype.
The prototype uses 1920 x 1080 pixel spatial light modulators (SLMs) to input the complex SAR raw data to the SAR optronic processor that then simultaneously performs azimuth and range compression. The output image is then stored on a digital camera. The SLM and camera refresh rates are compatible with the data generation rate of Envisat, thus providing real-time capabilities. This means that the rate of data generation by the SAR system is matched without bottleneck by the processing speed of the SAR optronic processor.