## Materials and Methods
### Methods  
Data Processing  
The InSAR technique and radargrammetry are generally used to generate DEMs using a pair of SAR observations. We chose the InSAR application using consecutive TanDEM-X SAR observations, which is a well-known technique to construct precise DEMs [23]. This was because the InSAR technique uses phase information, which is very sensitive to topographic height, and does not need ground control points to register two SAR images. The TanDEM-X mission was launched to generate global InSAR-derived DEMs as referred to in the introduction section. Thus, the InSAR application is a very promising tool to generate high resolution DEMs with TanDEM-X observations. However, two important steps are essential for the processing of InSAR pairs with a large geometric baseline. The first is application of common band filtering in the range direction to compensate for geometric decorrelation because of the large geometric baseline results in spectral decorrelation in range direction. The other is careful application of the unwrapping procedure. During conventional TanDEM-X SAR interferometric processing under normal operation, the height of ambiguity ranging from 30 m to 45 m might not produce a severe interferometric phase aliasing at a steep slope area, such as a high mountainous area. However, a small height of ambiguity because of a large perpendicular baseline can produce an undesirable interferometric phase aliasing even in a low mountainous area. Although there is no interferometric phase aliasing in a low slope area such as a glacial surface, it might be useful to update existing topographic information by adding a differential interferometric phase. \begin{table}
\begin{tabular}{l c c c} \hline \hline  & **CryoSat-2 Radar Altimeter** & **IceBridge ATM Laser Altimeter** \\ \hline Acquisition & January 22, 2015 (D \({}^{\star}\)) & January 24, 2015 (D) & May 5, 2015 12:35:55 \\ Date & January 26, 2015 (D) & January 28, 2015 (D) & May 5, 2015 12:45:06 \\  & January 30, 2015 (D) & February 7, 2015 (A) & May 5, 2015 12:50:02 \\  & February 9, 2015 (A) & February 11, 2015 (A) & May 5, 2015 12:56:20 \\  & February 13, 2015 (A) & February 15, 2015 (A) & May 5, 2015 13:06:14 \\  & February 18, 2015 (D) & February 20, 2015 (D) & May 5, 2015 14:38:29 \\  & February 22, 2015 (D) & February 24, 2015 (D) & \\  & February 26, 2015 (D) & February 28, 2015 (D) & \\  & March 8, 2015 (A) & March 10, 2015 (A) & \\  & March 12, 2015 (A) & March 14, 2015 (A) & \\  & March 19, 2015 (D) & March 21, 2015 (D) & \\ \hline \hline \end{tabular} \({}^{\star}\) A: Ascending orbit, D: Descending orbit.  
\end{table} 
Table 2: Acquisition date of the CryoSat-2 and IceBridge Airborne Topographic Mapper (ATM) altimeter datasets. We processed the TanDEM-X COSAR SLC acquisitions using the Gamma software package, as in Reference [48]. It is possible to utilize other interferometric SAR software (e.g., ROI_PAC [49], DORIS [50], etc.) which has the capability to generate topographic interferograms. Precise coregistration at a sub-pixel scale was required to reduce the decorrelation effect during data processing [51]. Since a large geometric baseline results in wavenumber shifts in the range direction, common range spectral filtering should be applied [52]. Although the difference in the Doppler central frequency of \(-51.55\) Hz (\(35.33\) Hz in the TerraSAR-X image and \(-16.22\) Hz in the TanDEM-X image) was not sufficiently large to result in severe Doppler decorrelation, we applied azimuth spectral common band filtering [53]. After common band filtering was applied in both the range and azimuth directions, the flat Earth phase was removed from the raw interferogram. The phase ramp, which was determined by the baseline components of the two SAR sensor geometries, was subtracted. In the case that the systematic residual fringes remained, additional phase removal using fringe rate estimation by two-dimensional fast Fourier transform (FFT) should be applied.