# Application and Evaluation of the Gaofen-3 Satellite on a Terrain Survey with InSAR Technology
## 3 Interferometric Data Processing
### Interferometry Processing and Interferogram Flattening  
After fine registration and spectral offset filtering were conducted, an interferogram was generated Figure 3a,c.  
There are two ways to generate a DEM using Gaofen-3 interferometric data. One is to use InSAR technology to calculate the terrain directly through the interferometric phase. The other is based on open-source data such as SRTM data and using the differential InSAR technique to process the elevation. In general, the latter can reduce the fringe density and is conducive to phase filtering and unwrapping. However, the resolution of the Gaofen-3 SAR data, in this case, was different from that of an open-source DEM, such as SRTM. For areas with large terrain reliefs, the difference in interferometric  
Figure 2: Improved high-accuracy interferometric synthetic-aperture radar co-registration method used in this study. DEM: digital elevation model; SCR: signal-to-clutter-ratio.  
phases contained dense fringes caused by different degrees of response of the two kinds of data to the surface details. In addition, to verify the ability of Gaofen-3 satellite interferometric data to obtain the ground elevation more objectively, a DEM was produced directly through InSAR in this study, rather than using an external DEM for differential interferometry.  
We subsequently performed interferogram flattening, and the results are shown in Figure 3b,d. The coherence of the flattened interferogram was calculated using a \(5\times 5\) pixel window, and the obtained coherence map is shown in Figure 4a; Figure 4b is the corresponding coherence probability histogram.  
Based on the coherence analysis, the coherence of the Gaofen-3 interferometric data of the Dengfeng area was 0.413, and the standard deviation of the distribution was 0.186. Partial regions had geometric decorrelation due to terrain reliefs. The 29 days of time interval also resulted in certain degrees of time decorrelation.  
Figure 3: Interferograms of the experimental Gaofen-3 satellite data in the Dengfeng area, China: (**a**) interferogram, (**b**) flattened interferogram, (**c**) local part of the interferogram, (**d**) local part of the flattened interferogram.  
Interferograms of Sentinel-1 of the same area with six days and thirty-six temporal baseline were derived to make comparisons with Gaofen-3. Figure 5a,c shows the interferogram and coherence map with the temporal baseline of six days, and the average coherence coefficient is 0.67. Figure 5b,d shows the interferogram and coherence map with the temporal baseline of thirty-six days, and the average coherence coefficient is 0.23.  
It can be seen from the Sentinel-1 data, the interferograms after six days appear to be less noisy than the interferograms derived from Gaofen-3. However, the phase decoherence is more serious for the interferograms after thirty-six days. Long temporal baseline could be the main source of phase noise due to the change of the ground features.  
Figure 4: Coherence analysis of the flattened interferogram with (**a**) coherence map and (**b**) coherence probability histogram.  
Figure 5: _Cont._