Dr. Michelangelo Villano
From TanDEM-X to Multi-Static SAR Systems
Earth science benefits tremendously from spaceborne synthetic aperture radar (SAR). High-resolution wide-swath SAR systems allow for frequent imaging of the Earth’s surface on a global scale and constellations of low-cost SAR sensors further reduce the revisit time on local areas and guarantee a timely response to natural disasters. SAR becomes even more powerful, if multiple images taken from different positions are combined to form digital elevation models (DEMs) or tomograms that unveil the three-dimensional structure of vegetation, ice, and dry soil. The TanDEM-X mission, the first bistatic radar in space consisting of two satellites flying in close formation, allowed producing a global DEM with unprecedented height accuracy and resolution and, more recently, a “Change DEM”, which highlights the topographic changes occurred over a five-year period on a global scale.
In order to create robust and accurate DEMs or high-resolution tomograms and exploit them to monitor fast dynamic processes, however, it is essential to be able to simultaneously acquire several, sometimes many, images from different viewing angles. This is driving the development of distributed and multi-static SAR systems, for which a number of challenges needs to be addressed, from mutual synchronization to safe formation flight. In order to contain the overall cost of the missions, clusters of smallsats with small apertures can be exploited in combination with innovative processing approaches to replace and enhance large aperture, high power radar systems. A prominent example is the creation of high-quality DEMs from noisy and undersampled data, which represents a paradigm shift from state-of-the-art techniques that demand expensive, ambiguity-free imagery. Furthermore, the simultaneous transmission of orthogonal waveforms from multiple satellites allows inferring unique information about different scattering mechanisms in natural and man-made environments, overcoming in this way an inherent limitation of conventional SAR tomography.
Multi-static systems represent a giant leap for radar remote sensing with a significant impact on numerous applications. They will pose the basis for future advanced Earth observation missions that will offer remarkable societal benefits.