# Assessment of Hongtu-1 Multi-Static X-Band SAR Constellation Interferometry
###### Abstract  
In 2023, the Chinese company PIESAT launched the multi-static X-band SAR constellation Hongtu-1 (HT1). HT1 consists of the active monostatic SAR sensor HT1-A and the three additional passive SAR receivers HT1-B, HT1-C and HT1-D. The passive sensors are arranged as a cartwheel in a circle around the active sensor. For our SAR interferometric investigation, we were able to use a multi-static HT1 recording. After a brief introduction of HT1, we describe the processing performed. Based on the phases of the six single-pass interferometric pairs, we calculated height differences relative to the Copernicus DEM. Larger deviations were observed mainly for mining areas and for forest areas. Thanks to the simultaneous acquisition of the interferometric pairs, the high spatial resolution and the good signal quality, the necessary processing was relatively easy to perform. Besides the interferometric phase, we also investigated possible applications of multi-static coherence. Forest can be recognized by its reduced single-pass coherence values. Based on our results, we expect that the multi-static HT1 coherence and its dependence on the interferometric baseline can be used to estimate parameters such as forest biomass. Hongtu-1, HT1; PIESAT; SAR constellation; interferometry; cartwheel; bistatic; multi-static; DEM generation; height changes; single-pass coherence; multi-static coherence product +
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0000-0001-6600-3899-2233]Cited for 0000-0001-6600-3333-2333]Cited for 0000-0001-6600-3333-2333]Cited for 0000-0001-6600-3333-2333-23333-23333-23333-23333-23333-23333  
interferometric pairs recorded at different times is that the surface may change over time. On the one hand, the terrain surface changes, for example, due to mining and construction activities, deforestation and forest growth, volcanic activity or the melting of glaciers and ice shelves; on the other hand, seasonal changes in vegetation and snow cover lead to temporary changes in the observed terrain heights. An attractive way to avoid these problems is to use a constellation of satellites that allows the simultaneous acquisition of several interferometric pairs with different spatial baselines. This idea, proposed around the year 2000 as the \"cartwheel\" mission, was described in [26] and was subsequently evaluated by the European Space Agency (ESA) as a possible mission concept. However, the idea was not realized. Operating one or several passive companion satellites was also proposed in [27; 28; 29], and more recently, the ESA decided to move forward its tenth Earth Explorer mission HARMONY [30]. Nevertheless, the main application addressed with these bi- and multi-static mission proposals is not DEM generation, but developing applications for bistatic data acquired with significantly larger spatial baselines, e.g., of interest to obtain different displacement components using repeat-pass interferometry. In the meantime, however, the cartwheel idea was taken up by the Chinese company PIESAT, developed further and put into practice with the launch of the Hongtu-1 constellation in 2023. The actively transmitting satellite is also part of the constellation and not a pre-existing satellite that has already been planned or realized, as in most of the other systems proposed. In addition, the navigation data measurements have improved and there is more experience with the precise control and correction of the satellite orbits, as several satellites are successfully operated in very narrow orbital tubes. With its commercially available SAR/InSAR software, version mid-2024, Gamma Remote Sensing AG, Gumligen, Switzerland [31] supports the processing of SAR data acquired by various SAR satellites. In order to adapt the software to Hongtu-1 and evaluate its potential, we received from PIESAT the multi-static Hongtu-1 data on which this work is based. After a brief description of the Hongtu-1 constellation and the data used, in Section 2, we describe the processing performed. The results are then presented in Section 3.