Abstract
Synthetic aperture radar (SAR) focusing will be a fundamental step in the analysis of the radar sounding datasets collected by the Radar for Europa Assessment and Sounding: Ocean to Near-surface (REASON) instrument as part of NASA's upcoming Europa Clipper mission. Due to the flyby trajectory of the mission, REASON data acquisition will be distinct compared to other space-borne radar sounders, and therefore, require a tailored SAR focusing strategy. Here, we present a SAR focusing architecture based on the delay Doppler approach employed in US SHAllow RADar (SHARAD) data analysis with the following modifications for REASON data idiosyncrasies; an interpolation to a constant ground track interval to account for REASON's variable PRF; and an adaptive Doppler centroid estimation to account for the flyby geometry. The ability of our modified delay Doppler SAR focusing approach to focus space-borne datasets as well as its specific feasibility for REASON are demonstrated using both SHARAD and MARSIS datasets. In addition, we present a quantitative quality control framework based on pixel power probabilities and demonstrate how it can be leveraged to quantify the effects of SAR focusing and differentiate focused results generated with different processing parameters. Finally, we revisit and discuss the assumption of depth-independent SAR focusing implicit in the choice to construct a REASON SAR focusing approach based on the delay Doppler method and provide a comparison with depth-dependent SAR focusing for a simplified acquisition geometry.
Highlights
THE past decades have seen active radar sounding measurements become a valuable tool in the study of planetary surfaces and subsurfaces [1]–[5]
A flyby trajectory and desire to maintain a requisite Doppler bandwidth introduces key characteristics that differentiate future Europa Clipper REASON data from those produced by existing space-borne radar sounders
We developed and demonstrated a modified delay Doppler synthetic aperture radar (SAR) focusing approach that accounts for these idiosyncrasies by way of 1) range line interpolation to a constant ground track interval and 2) Doppler centroid estimation based on the predicted mid-aperture surface point target response
Summary
THE past decades have seen active radar sounding measurements become a valuable tool in the study of planetary surfaces and subsurfaces [1]–[5]. A wide variety of SAR focusing strategies exist for use with both space-borne and airborne datasets [2]–[4], [6]–[9] All of these strategies involve the creation of synthetic apertures within which Doppler information is leveraged to recombine energy reflected from surface and subsurface targets as the radar moves along its track. Because Europa Clipper is always either approaching or receding from Europa over the course of a flyby, unlike for LRS and SHARAD, the Doppler frequencies of targets in the middle of an arbitrary synthetic aperture will not be zero (the sole exception being for a synthetic aperture centered on the position of closest approach) The combination of these two factors is distinct to REASON and must be reflected in the design of a suitable SAR focusing algorithm.
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