Radar Interferometry and Tomography for the Exploration of Enceladus’ Surface and Subsurface

Orbital Synthetic Aperture Radar (SAR) interferometry (InSAR) and tomography (TomoSAR) are key techniques for the exploration of terrestrial ice sheets that are used operationally. However, in the context of planetary exploration, these approaches are rather exotic and have not been used yet. In the frame of DLR’s Enceladus Explorer (EnEx) initiative, we propose a multi-modal, multi-frequency orbital radar mission, operating -among others- in a SAR interferometric and tomographic mode capable of delivering high-accuracy and high-resolution topography, tidal deformation, and composition measurements as well as 3-D metric-resolution imaging of the ice crust along tens of kilometers wide swaths. The ice penetration capability of radar signals allows for the exploration of both surface and subsurface features down to hundreds of meters, depending on the used carrier frequency.Multiple SAR acquisitions of the same area are needed to form interferometric and tomographic products. These acquisitions are collected successively following a repeat-pass concept using so-called periodic orbits with repeating trajectories. For the available observation geometries, the baselines between the repeat trajectories need to lie within a few hundreds of meters (i.e., the radar needs to fly within a tube of hundreds of meters). Unfortunately, the low Enceladus mass and its proximity to Saturn commonly lead to instabilities for highly inclined science orbits. We find that published orbit solutions do not exhibit sufficient stability for providing the necessary repeat passes. However, through a grid-search approach in a high-fidelity gravitational model, we identified highly stable periodic orbits that sustain the required repeat characteristic up to hundreds of days. The short repeat periods in the order of 1 to 4 days allow for a fast acquisition of InSAR observations and the formation of tomographic stacks within several days.Based on a representative system, we present global performance simulations for both InSAR and TomoSAR products with a focus on the prominent south polar plume region of Enceladus. The performance of these products depends on several factors, including the system being used, the orbital geometry, the accuracy of the guidance, navigation, and control (GNC), the accuracy of the orbit determination, and the structure and composition of the ice crust, which affects the backscatter characteristics and potential decorrelation effects in the SAR acquisitions. We use an End-to-End (E2E) simulator developed at DLR for generating realistic SAR, InSAR, and TomoSAR products. The E2E is capable of accommodating the designed orbits, the Enceladus topography, deformation models, representative backscatter maps, and decorrelation effects, as well as any relevant instrument, baseline, and attitude errors.