## 4 Discussion  
Although the constructed DEM had a very good correlation with both the other DEM sources and altimetric observations, it showed a different level of offsets which should be calibrated. The different level of offsets might be caused by the acquisition time when each measurement was completed. Moreover, the glacier surface was continuously flowing according to time, as such it was difficult to make precise estimations to calibrate for the glacier surface. Thus, the constructed DEM may still have an offset, although it would be calibrated with the estimated offset using other observations. The different level of the calculated offset might have resulted from a difference in the penetration depth resulting from a different operational frequency of the electromagnetic wave of the signal. We believe that the CryoSat-2 observations can be better constrained to calibrate the constructed DEM because they were acquired during periods in which the TanDEM-X data was collected. The goal of this study was to evaluate the feasibility of estimating more detailed topographic height along a low slope moving surface, such as a glacier, rather than assessing the accuracy of the derived DEM. The constructed DEM showed very detailed topographic characteristics that could not be presented using other DEMs or height measurements such as altimetry. A few tens of meters of height ambiguity in a conventional repeat-pass radar interferometric acquisition is not sufficient to discriminate topographic change along a low slope area, such as a glacier or an inter-tidal flat. The TanDEM-X Science Phase observation can offer a great opportunity to estimate very detailed topographic height by providing a small height of ambiguity from a large perpendicular baseline. The main reason to reduce the signal-to-noise ratio is to generate a coherent interferogram on the glacier surface. The nearly simultaneous TanDEM-X interferometric pair with only approximately 10 s of short temporal baseline was very advantageous in maintaining the coherence of a rapidly  
Figure 10: IceBridge ATM observations of each of the six flight paths compared to the constructed DEM in this study, presented in Table 2. The color of the scatter plot followed the color of the location of the ATM measurement shown in Figure 9b. The calculated offsets along the yellow and green flight lines (**b,c**) were just \(-1.3\) m and \(-5.1\) m, respectively. Relatively larger offsets were calculated at both the bedrock and upper part of the glacier (**a,d,e,f**). changing surface such as a glacier. In addition to the height of sensitivity, the very high spatial resolution of the TanDEM-X X-band wavelength can be useful to monitor the morphology of a glacier surface such as an ice crack or crevasse. The TanDEM-X dataset used in this study was in StripMap mode, which only has 3 m of pixel spacing. If the SpotLight or Staring SpotLight modes which have 0.5 m to 1 m pixel spacing can be acquired, dramatic topographic features of a glacier surface can be captured. However, the limitation of InSAR application using a large perpendicular baseline remains, as the phase aliasing problem might be experienced in a mountainous area due to a steep slope. The multi-baseline and multi-temporal TanDEM-X observations can improve the accuracy of the constructed DEM. The experiment was conducted to create a DEM using only a single TanDEM-X interferometric pair acquired along a single track and in a single beam-mode (e.g., single incidence angle). We believe that a single-track acquisition is sufficiently good to retrieve the topographic features of most parts of the glacier surface. However, the multiple baseline, incidence angle, and track interferograms can produce an improved DEM by reducing possible artifacts that can occur along the boundaries between the glacier and surrounding bedrock. The phase unwrapping issue can also be mitigated using multiple interferograms. The TanDEM-X Science Phase observation is not operational, thus potential application is currently quite limited.