TDX DEMs have a spatial resolution of 12 m by 12 m at the equator. The absolute vertical accuracy, defined as the uncertainty in height of a point with respect to the World Geodetic System (WGS) 84 ellipsoid caused by random and uncorrected systematic errors, is better than 10 m. The relative height accuracy, defined as the uncertainty between two height estimates caused by random errors, is smaller than 2 m over a 1° × 1° geocell in latitude/longitude (28). The horizontal accuracy, defined as the uncertainty in horizontal position of a point with respect to the WGS84 ellipsoid, is better than 10 m. ICESat-1 laser altimeter data, namely, GLAS/ICESat L2 Global Land Surface Altimetry Data, version 34, GLA14, was used to calibrate the TDX product. Several criteria were used to extract the most reliable pixels. Outliers above 100 m were discarded. Using a laser-specific weighting function, the 10 best ICESat points per 50-km TDX scene were selected and TDX pixels within the ICESat resolution cell were averaged. In addition, the SD of the TDX DEM within the footprint must be less than 1 m. One issue is the unknown penetration depth of X-band radar signals into snow depending on ice structure, dielectric properties, and incidence angle. This issue was solved by selecting areas with homogeneous backscattering characteristics and presumably homogeneous penetration depths based on a Radarsat-1 image mosaic of Antarctica. For each area, a constant offset between ICESat and TDX was calculated. Starting from these fixed blocks, all Antarctica acquisitions were adjusted and calibrated following an inner-to-outer direction.

We generated a time series of time-tagged DEMs using the global TDX product (28) for geocoding and calibration. The SAR processing chain comprises three main steps: (i) spaceborne monostatic TerraSAR-X processing, (ii) bistatic TDX processing, (iii) interferometric combination of images, (iv) phase unwrapping, and (v) phase-to-height conversion and geocoding to a latitude/longitude grid. Movie S1 shows a time series of TDX DEM differences over Thwaites Glacier, West Antarctica. The time-tagged TDX DEM difference accuracy is of the order of 2 m (28). Using Airborne Topographic Mapper (ATM) data over grounded ice, we found a relative height accuracy of 4 m (fig. S4).

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