# Assessment of Hongtu-1 Multi-Static X-Band SAR Constellation Interferometry
## 3 Results
### Coherence  
In addition to the interferometric phase, the coherence is also of interest. It characterizes the phase noise and is therefore useful, e.g., in phase unwrapping. But beyond this, the coherence relates to the characteristics of the target. Figure 8 shows the multi-static coherence for three pairs with short to long baselines. For much of the area, the coherence is high as there is no temporal decorrelation. However, we clearly observe reduced coherence levels over forests--this in spite of the simultaneous acquisition of the set of multi-static SLCs. The reason for the reduced coherence is volume decorrelation [46]. With an increasing baseline, the average coherence level over a given forest stand decreases (Figure 9). This behavior is well known and can be used to estimate forest parameters such as the forest stand height or biomass. Simultaneously obtaining multiple single-pass coherence images with different spatial baselines is very attractive for these applications as it permits us to prevent confusing baseline-related effects with effects caused by temporal changes in the targets, such as seasonal changes, leaf-on leaf-off effects and vegetation growth.  
Figure 8: Images of 9 km \(\times\) 9 km sections of geocoded, detrended, multi-static HT1 X-band coherence. The sensor combination and the estimated perpendicular baseline are indicated below the image. The SAR backscatter is used as image brightness.  
Figure 7: A 4.2 km \(\times\) 3.3 km section of the 1′′ Copernicus DEM (oversampled to the sampling of the updated DEM, which is about 3.3 m, (**left**)), and the DEM updated with the multi-static HT1 data (**right**) is shown with shaded reliefs.  
The backscatter is very low over water surfaces and in radar shadow areas; in fact, it is below the NESZ. Here, the radar signal is dominated by noise, and therefore, the coherence level is very low.  
The coherence of repeat-pass SAR interferometric pairs with relatively short temporal separation provides useful landcover information, as demonstrated using ERS-1/2 Tandem acquisitions at the C-band with one-day time intervals [40]. Combining the coherence, the average backscatter and the backscatter change between the two acquisitions into an RGB color composite provides a color image that permits visually recognizing the main landcover classes [40]. This RGB composite is often called a \"coherence product\". As for the C-band repeat-pass case, the multi-static X-band coherence is lower over forest than over open unvegetated or sparsely vegetated areas--which permits defining a \"multi-static coherence product\" with characteristics similar to the repeat-pass coherence product. Figure 10 shows a section of the multi-static coherence product generated with the HT1 data. For the red channel, the average of the six estimated coherence values was used, with linear scaling. For the green channel, the average backscatter over the four mono- and bistatic acquisitions was used with logarithmic scaling for visualization. For the blue channel, the standard deviation of the four backscatter values was used with logarithmic scaling for visualization. Forests appear in a green color thanks to the reduced red and blue values. Water surfaces appear blue thanks to the low red and green values. The blue value (backscatter standard deviation) is relatively high as noise dominates the HT1 backscattering over water. The same characteristics are also observed for radar shadow, e.g., visible in Figure 10 behind forests. Orange colors are observed over agricultural fields, relating to high coherence (red), intermediate backscatter (green) and low backscatter standard deviation (blue). Similar characteristics are observed for other open surfaces such as river banks. Built up areas with high coherence (red), high backscatter (green) and low backscatter standard deviation (blue) appear in yellow, with high spatial variability of the backscatter.  
Figure 9: Baseline dependence of average multi-static single-pass coherence over forest stand B (see Figure 6).