Spaceborne Synthetic Aperture Radar Technologies for Frequent and Global Earth Monitoring

Abstract

Synthetic aperture radar (SAR) represents a fundamental means for Earth monitoring from space. An inherent limitation of SAR, however, is that the azimuth resolution constrains the swath width and thus hinder frequent observations at a global scale. This limitation can be overcome by staggered SAR, an acquisition mode that uses digital beamforming together with continuous variation of the pulse repetition interval and has been implemented in the recently-launched NASA-ISRO SAR mission. Besides complex systems, novel ambiguous SAR modes can circumvent the swath/resolution constrain without the need for digital beamforming and are effective for dedicated applications, such as maritime surveillance and ground deformation monitoring, also thanks to the exploitation of waveform diversity and innovative postprocessing techniques. The coherent combination of SAR images, taken from different angles, unlocks further opportunities, such as the generation of accurate digital elevation models and high-resolution tomograms that unveil the three-dimensional structure of vegetation, ice, and dry soil. Distributed systems based on clusters of small satellites flying in formation enable simultaneous collection of such multi-angular images with a significant impact on numerous applications. An affordable and versatile approach for their demonstration is based on swarms of drones equipped with lightweight radar sensors. These advances are forerunners for the development of future groundbreaking Earth observation missions that will offer remarkable societal benefits.