LAUSR

Abstract Twenty-five years ago, the two flights of the SIR-C/X-SAR Synthetic Aperture Radar (SAR) system on the Space Shuttle Endeavour blazed a trail towards the future with a series of radar system innovations — that nearly every spaceborne SAR flown since then has benefited from, and in some cases improved upon. Many of the SAR techniques adopted by SAR system designers worldwide as part of their toolkit, such as: ScanSAR, Spotlight mode, along-track interferometry, polarization diversity and polarimetry, polarimetric calibration, variable length and bandwidth pulses, and on-board processing, can trace their heritage back to this first-of-a-kind, civil-use SAR system. The electronic steering capability of SIR-C's phased array antenna, combined with the exquisite orbit track control provided by the Space Shuttle, paved the way for systematic mapping of the Earth's topography by the Shuttle Radar Topography Mission, and later by TanDEM-X. Some techniques, such as multi-frequency SAR, multi-frequency repeat-pass interferometry and onboard processing have yet to be fully exploited. The richness of the SIR-C/X-SAR data set has proved to be a treasure trove for opening up entirely new remote sensing techniques, such as Polarimetric SAR Interferometry (or PolInSAR), and GPS, now GNSS, reflections (also known as Signals of Opportunity), which were both demonstrated from archive data, years after the 1994 flights. The ground-breaking legacy of SIR-C/X-SAR lives on in the many SAR systems collecting data in Earth orbit today, and in those planned for the future.