Advancement of SAR Imaging Techniques for the Observation of Terrestrial and Planetary Snow and Ice

Abstract

Radar remote sensing is an essential tool for observing Earth's cryosphere and has revolutionized our understanding of the state and dynamics of glaciers, ice sheets, and snow covers in the context of a changing climate. Beyond terrestrial snow and ice, radar imaging is a crucial technology for future exploration missions to the so-called icy moons of the giant planets, of which Saturn's moon Enceladus has recently been identified as the key target for investigating habitability on other worlds. Especially synthetic aperture radar (SAR) imaging has been extensively used for monitoring snow covers as well as the extent and dynamics of glaciers and ice sheets. Besides basic SAR imagery, SAR interferometry (InSAR) and tomography (TomoSAR) provide unparalleled measurement capabilities for the observation of Earth's cryosphere. Although modern radar remote sensing techniques like InSAR and TomoSAR are considered standard in Earth Observation (EO), they have not yet been adopted for the exploration of icy moons due to increased system complexity and strong orbit perturbations. However, these techniques have been recently identified as key developments for future exploration missions to Saturn's moon Enceladus. Upcoming Earth Observation (EO) SAR missions will acquire SAR, InSAR and TomoSAR data incorporating advanced capabilities, by: i) operating at lower frequencies (e.g., in the P- and L-band), resulting in considerable signal penetration into snow and ice covers, ii) providing very high spatial resolution, and/or iii) acquire as a satellite constellation to provide multi-aspect observations. The radar signal penetration capability at lower frequencies allows to image structures and processes within or underneath the snow and ice cover. Besides these opportunities, the penetration of the signals results in position ambiguities of imaged features, as well as biases and distortions in InSAR and TomoSAR products. An additional dimension of information in SAR observations of snow and ice that has not received much attention in the past is the propagation effect on the SAR signals when penetrating in the snow and ice volumes. The aim of this thesis is to improve SAR, InSAR and TomoSAR imaging techniques for snow and ice observation in the frame of future EO and planetary missions by developing novel approaches for exploiting and compensating SAR signal propagation effects, as well as enabling InSAR and TomoSAR for the exploration of icy moons. This thesis presents several novel concepts grouped into four research objectives. First, it describes the information content in single SAR images regarding snow and ice volume properties and introduces new single-image retrieval approaches that can be applied independently of polarimetric, interferometric, or tomographic information. These single-image approaches are highly relevant in scenarios where interferometric or tomographic information is unavailable (e.g., in planetary exploration missions), as well as for calibrating interferometric and tomographic products over ice sheets and glaciers. The remaining research objectives focus on advancing SAR interferometric and tomographic techniques for snow and ice observation. The second objective assesses the feasibility and potential of using repeat-pass InSAR and TomoSAR for exploring icy moons, particularly within the context of an Enceladus mission scenario. Despite the strong orbit perturbations around Enceladus, highly stable repeat-pass orbits are designed to meet the stringent conditions for InSAR and TomoSAR. This assessment is adopted in a mission proposal currently being developed at the Jet Propulsion Laboratory (JPL), targeting repeat-pass InSAR observations of Enceladus for deformation and topography mapping. The third objective addresses the significance of commonly ignored propagation effects in elevation measurements of ice sheets and glaciers using InSAR. These propagation effects result in considerable geolocation errors of meters to tens of meters beyond the well-known penetration bias. Several adapted processing approaches are developed to accommodate the propagation effects in terms of range and phase offsets, representing an important step toward a robust penetration bias calibration in InSAR elevation products. The final objective tackles the limitations of current differential InSAR (D-InSAR) techniques for retrieving snow parameters. A novel explanation of temporal decorrelation over snow-covered areas is provided, linking snow density changes to the decorrelation of SAR signals caused by changes in the wavenumber within the snow volume. Additionally, methods to mitigate the 2-pi phase ambiguity of the interferometric measurement are developed by exploiting multiple D-InSAR acquisitions with different squint angles, which can also serve as a direct measurement of snow density. The upcoming Harmony mission by the European Space Agency (ESA) is a suitable candidate to implement these developed concepts due to its large squint diversity among the satellite constellation. This thesis demonstrates the significant potential of synergistically developing terrestrial and planetary radar remote sensing. It advances the state-of-the-art of SAR imaging techniques for observing glaciers, ice sheets, and snow covers, as well as for exploring icy moons.