# Assessment of Hongtu-1 Multi-Static X-Band SAR Constellation Interferometry
## 4 Discussion  
The Hongtu-1 Multi-Static X-band SAR Constellation permits simultaneously acquiring one monostatic and three bistatic SAR scenes. With processing bandwidths of 100 MHz in the range direction and 3775 Hz in the azimuth direction, it provides high-resolution SAR imagery. Thanks to the simultaneous acquisition of the scenes and interferometric baselines much smaller than the critical baseline, the coherence level is generally very high, with reduced coherence levels for volume scatterers (e.g., forest areas) and very low coherence levels for areas with very low backscattering, below the NESZ, where noise dominates (e.g., smooth water surfaces and radar shadow). Having six simultaneously acquired single-pass interferometric pairs available has significant advantages for the interferometric processing performed to retrieve height differences relative to a pre-existing DEM. Thanks to the single-pass nature of the pairs,  
Figure 10: Image of 9 km \(\times\) 9 km section of geocoded, HT1 multi-static coherence product. RGB composite of average coherence (red), average backscatter (green) and the backscatter variability between multi-static scenes (blue). there is no temporal decorrelation, no deformation phase and almost no atmospheric path delay phase. Having multiple pairs available facilitates phase unwrapping and permits optimizing the results (reducing noise, reducing the presence of ambiguity errors). Deriving the height difference relative to an existing DEM, here shown relative to the 1\" Copernicus DEM, is relatively straightforward. Using only data acquired in a single acquisition means that the results refer to a single date. This is of high interest to study multi-temporal developments in terrain height changes (e.g., for volcanoes, mines and land-fill areas) or surface cover such as forest, other types of vegetation, ice and snow. The HT1 data investigated demonstrated promising potential for mapping height changes at high spatial resolution (e.g., 5 m) or for updating DEMs at such spatial resolution. And it would be possible to do this at short temporal intervals, e.g., every 15 days. Determining the height difference compared to the 1\" Copernicus DEM heights, derived from Tandem-X data acquired between December 2010 and January 2015, permitted identifying height changes related to mining and landfill that look very plausible. Furthermore, positive height differences up to 10 m were observed for forest stands. Here, we think the change does not so much relate to a significantly changing forest height, but rather, to different characteristics between Tandem-X and HT1, such as the flatter incidence angle of HT1. There are also a few forest areas with negative height differences, relating to forests which were cut between the Tandem-X and HT1 acquisitions. For more open areas (e.g., agricultural fields, grassland) the height corrections determined are very small (<1 m). This good correspondence is understood as an indication that both the Copernicus DEM and the determined updated height are of good quality. But it cannot be interpreted as a validation of the results. Besides updating the existing DEM to a newer date, an important aspect is that the updated DEM was generated with a higher spatial resolution. In addition to the interferometric phase, the coherence is also of interest. On one hand, the coherence serves as a quality parameter for the interferometric phase and is used to calculate weights in phase unwrapping. On the other hand, it relates to the characteristics of the target. For forests, the single-pass coherence becomes lower with increasing forest height as well as with increasing baseline length. Consequently, the coherence can be used to retrieve forest parameters, particularly if it is available over the same forest at the same time with different baselines. We expect similar behavior for other volume scatterers such as ice, snow and dry sand, and recommend investigating the related potential of X-band multi-static phase and coherence data.