## 3 Data  
The TanDEM-X (TerraSAR-X add-on for Digital Elevation Measurement) is a German X-band radar mission operated by German Aerospace Center (DLR). This mission is a space-borne constellation of two X-band (3.1 cm wavelength) radar satellites [18; 19]. The twin satellites, TerraSAR-X (TSX; launched on 06/15/2007) and TanDEM-X (TDX; launched on 06/21/2010), fly in a close helix orbit formation to acquire interferometric data [20]. Single images can be acquired in pursuit monostatic mode in which the satellite acts as both transmitter and receiver. In the bistatic mode of the satellite constellation, one of the satellites transmits the signal and both of them receive the backscattered signal, leading to two images being acquired simultaneously. The transmitting satellite is referred to as \"active\" and the other one as \"passive\". The bistatic, single-pass configuration enables interferometric analysis specifically for elevation measurements without the limitations of changing surface conditions as in repeat-pass interferometric modes of previous missions (e.g., ERS-1/2).  
Columbia Glacier had been defined as a super test site of the TanDEM-X mission, and hence data were acquired every 11 to 22 days during 2011\(-\)2014, but less frequently afterwards. The area was covered by ascending and descending orbit passes enabling the coverage of a majority of its catchment (\(\sim\)60% of the main branch and \(\sim\)90% of the west branch). In addition, two monostatic TSX image pairs from September to October 2009 were also available. The active channel of monostatic and bistatic repeat acquisitions is utilized for the velocity measurements, while both channels are exploited for bistatic interferometric SAR processing (InSAR) to generate TanDEM-X DEMs. The bistatic scenes are composed of Coregistered Single Look Slant Range Complex (CoSSC) images. CoSSC images consist of already registered master (active channel) and slave (passive channel) Slant Look Complex (SLC) images. The multi-looked intensity images, generated from master SLCs, are used to manually delineate the glacier front position. A list of the data and its specifications is given in Table 1.  
The United States Geological Survey (USGS) digital elevation model (DEM) of 1957 is primarily used in the first iteration of geocoding the TanDEM-X DEMs. This DEM is the r  
\begin{table}
\begin{tabular}{c c c c c} \hline
**Acquisition Time** & **Orbital Pass** & **Depression Angle (\({}^{\circ}\))** & **Az.(m) \(\times\)** & **SLRg Resolution (m)** & **Product** \\ \hline
2009 & Ascending & 58.98 & 2.06 \(\times\) 0.90 & SV \\
2009 & Descending & 54.55 & 1.89 \(\times\) 1.36 & SV \\
2011–2016 & Ascending & 49.63 & 2.0 \(\times\) 0.90 & SV, EC, AC, MF \\
2011–2016 & Descending & 46.82 & 1.84 \(\times\) 0.90 & SV, EC, AC, MF \\ \hline \end{tabular}
\end{table}
Table 1: Specifications of TerraSAR-X and TanDEM-X stripmap mode datasets used for interferometric (InSAR) processing and intensity offset tracking (SV: Surface Velocity, EC: Elevation Change, AC: Area Change, MF: Mass Flux). The acquisition dates of the TerraSAR-X/TanDEM-X images used for deriving surface velocity, for generating TanDEM-X DEMs and for mapping glacier fronts are listed in Tables S1 and S2 in the supplementary material.  
topographic map at a scale of 1:63,360 provided by the USGS. Bed topography of Columbia Glacier, available from [PERSON] et al. (2012) [13], was derived by a mass conservation approach. The Cannery Creek weather station (ID- GHCND:USC00501240) is located about 27 km from Columbia Glacier. It continuously records daily minimum and maximum temperatures, snowfall (Figure 2a,b) and other meteorological variables.