We surveyed the Thwaites Glacier grounding zone using CSK SAR acquisition. CSK is a constellation of four low Earth orbit satellites each carrying an X band SAR antenna (3.1 cm wavelength) enabling a finer resolution (3 m) and a better sampling rate (up to 176.25 MHz) of ground displacements. Each satellite had a repeat cycle of 16 days. Shorter repeats were achieved using the constellation. The shortest interferometric time period between two successive acquisitions was 1 day when using satellites CSK2 and CSK3, which we used here. CSK SAR data were assembled by concatenating 9 × CSK STRIPMAP-HIMAGE consecutive overlapping frames, each covering a 40 km by 40 km swath, at a 3-m resolution in both the azimuth (along-track) and range (cross-track) directions. The incidence angle averaged 26.27° across the swath. We analyzed scenes in a single-look complex format. Polarization of the electromagnetic waves is HH (horizontal transmit and receive). We used an orbit co-registration and pixel offsets to maximize coherence in image pairs. We applied a multilooking of 8 in both range and azimuth directions to improve phase coherence. We used 18 images (9 CSK pairs) acquired between February 2016 and September 2017 to produce pixel offset velocity maps and differential interferograms (DInSAR), revealing changes in ice velocity and vertical tidal displacements (figs. S1 to S3). We did not combine image pairs 48 days apart to avoid contaminating the DInSAR results with long-term changes in horizontal velocity. We generated velocity maps and analyzed the results to verify this hypothesis a posteriori (fig. S3). The precision with which we detected grounding lines also depends on the amplitude of the tidal signal. A larger differential tidal signal is preferable for delineating grounding lines. In total, 60% of our DInSAR pairs include small differential tidal displacements; hence, they are not used in the final analysis (figs. S1 and S2). Because the baseline separation between CSK2 and CSK3 is large, we also need to remove the topographic component of the interferometric phase. To do this, we used the time-tagged TDX DEM acquired closest in time with the CSK data (15). Using multiple grounding line measurements, we identified a grounding zone, i.e., an area over which the grounding line migrates back and forth with changes in oceanic tide (fig. S1).

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