Synthetic Aperture Radar in Medium and Geosynchronous Earth Orbits: System Concepts and Analysis

Abstract

The increasing availability of Earth observation (EO) data from space has significantly advanced our understanding of Earth's physical, chemical, and biological systems. However, significant gaps remain as climate change and anthropogenic pressures intensify. Addressing many of today's pressing Earth science challenges requires EO data with high temporal and spatial sampling to effectively capture rapidly evolving processes. Microwave remote sensing instruments are central to this effort but face trade-offs among temporal revisit, spatial coverage, and spatial resolution. For instance, scatterometers offer daily global observations with coarse spatial resolution (on the order of tens of kilometers), while synthetic aperture radar (SAR) systems provide high-resolution imagery (meter-scale) but with longer revisit times typically exceeding ten days. SAR constellations help improve revisit times to a few days, but require large constellations in low Earth orbits (LEOs) for large-scale coverage. Medium Earth orbits (MEOs) and geosynchronous orbits (GEOs) offer a compelling alternative to address the spatio-temporal sampling gap for frequent revisit over large geographic regions. SAR missions at these altitudes enable a single spacecraft to deliver global coverage on a multi-daily basis and regional coverage---such as countries, continents, or oceans---on a sub-daily basis. These capabilities make high-altitude SAR platforms good candidates for future scientific missions, including those with interferometric requirements. This dissertation provides an in-depth quantitative analysis of SAR missions in higher orbits, emphasizing critical trade-offs among mission aspects. These trade-offs reveal new operational opportunities and application domains uniquely enabled by monostatic MEO and GEO configurations. Examples include global 3-D deformation monitoring using a single MEO spacecraft at approximately 6000 km altitude and sub-daily coverage of Europe from around 20000 km. Contrary to prior assumptions, the analysis demonstrates that current technologies can support such high-altitude missions, delivering critical EO data with (i) continental to global coverage, (ii) frequent temporal revisit, and (iii) moderate spatial resolution on the order of tens of meters. This dissertation proposes two MEO-SAR mission concepts optimized for multi-daily and sub-daily revisit, along with a novel geosynchronous orbit concept for sub-daily interferometric acquisitions. It also introduces a framework for designing observation scenarios for GEO-SAR missions. Furthermore, it develops a comparative framework for evaluating the relative complexity of high-orbit SAR missions and their equivalent monostatic LEO-SAR constellations, supported by distribution strategies and illustrative mission concepts.