# Assessment of Hongtu-1 Multi-Static X-Band SAR Constellation Interferometry
## 2 Materials and Methods
### Hongtu-1 Multi-Static SLC Data  
Some characteristics of the SLC data that were used are listed in Table 1. The monostatic backscatter image in slant-range geometry over Yongfeng Town (45.7\({}^{\circ}\)N, 128.6\({}^{\circ}\)E), China, is shown in Figure 1. The area is relatively flat, with elevations between about 120 m.a.s.l and 450 m.a.s.l. The land cover includes agriculture, forest, rivers and small lakes, and some smaller built-up areas and other infrastructure such as roads, powerlines and dams.  
Orbit data are provided separately for each satellite (HT1-A, HT1-B, HT1-C, HT1-D). Geocoding was performed for the monostatic HT1-A image. In the geocoding process, we conducted refinement using matching between a simulated image calculated based on the Copernicus 1\" DEM [25] and the multi-looked and detected SLC. The small refinement offsets of \(<\)10 m obtained confirm the high quality of the orbit data. A more comprehensive  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline Active satellite & HT1-A \\ Passive companions & HT1-B, HT1-C, HT1-D \\ Polarization & HH \\ Look direction & Left-looking \\ Radar frequency & 9.600 GHz \\ Chirp bandwidth & 100 MHz \\ Pulse Repetition Frequency & 4201 Hz \\ Range pixel spacing & 0.75 m \\ Azimuth pixel spacing & 1.69 m \\ Incidence angle & 42.5 deg. \\ Acquisition date & 5 September 2023 \\ \hline \hline \end{tabular}
\end{table}
Table 1: HT1 (Hongtu-1, also called PIESAT-01) strimpap mode SLC characteristics.  
Figure 1: Monostatic HT1-A backscatter image in slant-range geometry.  
investigation of the geometric accuracy of HT1 is carried out in [33]. The bistatic images were not directly geocoded, but co-registered to the slant-range geometry of the monostatic image. The interferometric analysis was performed in the slant-range geometry of the monostatic reference. Results such as height corrections, updated heights, coherence and backscatter were then geocoded.  
For each of the four SLCs, we considered the image spectrum and determined the Doppler centroid. The spectrum was determined from the SLC using 2D FFTs over small image sections. The center of the spectrum in the azimuth direction could then be used to calculate the Doppler centroid. This process was repeated over the entire image to obtain the range and azimuth-dependent Doppler centroid. Using an accurate Doppler centroid value is important to achieve good interpolation quality in the co-registration resampling step.  
Considering some identified strong point-like scatterers, we investigated the point target response and estimated the range and azimuth resolution. The 3 dB range width determined was 1.6 m, and the 3 dB azimuth width was 2.3 m. The peak-to-side lobe ratios estimated were around \(-20\) dB. Using data over a site with known, ideal point targets, such as corner reflectors or transponders, as available in [33], the characterization of the point target response could be further improved.  
The HT1 data used were acquired with an incidence angle of about 42.5 degrees (relative to nadir). As a result of this relatively large incidence angle, the backscattering over smooth water surfaces was very low, lower than the system noise, so the backscatter over water surfaces could be used to determine the noise-equivalent sigma zero (_NESZ_). Considering several water surfaces, we determined that _NESZ_ = \(-22.3\) dB.