# Assessment of Hongtu-1 Multi-Static X-Band SAR Constellation Interferometry
## 3 Results
### Height Update
Location A in Figure 6 indicates a mining area--as confirmed with optical imagery found in Google Earth. The observed height difference of more than \(-20\) m relates to lowering of the surface height by the mining activity during the period between the dates of the two datasets. Other examples of such temporal height changes include landfill sites, rock falls and landslides, glacier ice melt, and volcanic and seismic activity. Identifying such changes is well supported by a multi-static SAR constellation such as HT1. Having a well-defined date for the information is a clear advantage over using data acquired over a longer time period. The next HT1 acquisition could be used to map the height change occurring in this well-defined period between the two acquisitions. Positive height differences, mostly in the range of 5 m to 10 m, are observed over forests. This means that the HT1-based height is higher than the 1\" Copernicus DEM height. An example of this is location B in Figure 6. The single-pass interferometric height of a forest is expected to lie somewhere between the ground and the top of the canopy, depending on various factors, including the used radar frequency and the incidence angle, and possibly also the baseline length. The interferometric Tandem-X data used for Copernicus DEM generation and HT1 are both operated in the X-band--so the frequency is the same. Using different incidence angles may be an explanation for the different heights observed over forest. The incidence angle of the HT1 acquisition is 42.5 deg. The Tandem-X data used for DEM generation is also acquired at lower incidence angles, but still above 30 deg. Based on the available information, it is difficult to provide a definitive explanation for the significant positive height difference observed over the forest. Seasonal effects, along with the used baselines and the estimation process, may also be relevant factors. We also observe examples of forest areas with negative height differences in the range of \(-5\) m to \(-10\) m which correspond to forests present in 2019 and then cut before the HT1 acquisition. This clearly shows that the Copernicus DEM height of a forest is not the ground elevation. Consequently, the height differences in forest areas have to be interpreted with caution, as both values lie somewhere within the canopy and do not perfectly represent neither the ground elevation nor the canopy top. The calculated height difference is clearly not a direct measurement of the canopy height. For the forest stand at location B in Figure 6, the average height difference calculated was 7.4 m with a standard deviation of 3.3 m. Applying spatial filtering reduces the standard deviation--median filtering with 7 \(\times\) 7 pixels reduces the standard deviation to 1.5 m. The standard deviation value is partly related to phase noise and partly to the forest structure. There are also small relative height differences around one meter observed for some agricultural fields (e.g., at location C in Figure 6). The plausible explanation is that these are fields with crops such as maize or rice which may still not have been harvested in early September. Being able to discriminate a 1 m height difference between such a field and neighboring fields may also be used to characterize the precision of the method. It indicates that meter-scale height changes can be identified for homogeneous areas with a size in the order of 100 m.  
Interesting features in the resulting height difference include the relatively long and narrow linear structures with height corrections of several meters (see D in Figure 6). These features correspond to hedges and trees located along roads and water canals. In the 1\" Copernicus DEM with a spatial sampling of about 30 m, we do not see these features. This clearly demonstrates the finer spatial details obtained using the HT1 dataset.