# First In-Orbit Validation of Interferometric GNSS-R Altimetry: Mission Overview and Initial Results
## 2 Mission Overview
### Interferometric Waveforms Description  
The iGNSS-R interferometric waveform is the main measurement employed by the NSSC iGNSS-R altimeter for high-precision interferometric altimetry. They are 1D delay correlation power mappings (DMs) which correspond to the 0 Doppler cuts of the traditional 2D delay-Doppler correlation power mappings. The interferometric waveform is generated at a frequency of 5 Hz (i.e., one interferometric waveform every 0.2 s). Each interferometric waveform has a coherent integration time of 1 millisecond and undergoes 200 times of incoherent accumulation to reduce noise. The waveform consists of 448 time delay sampling points with a sampling rate of 112 MHz. The time delay between adjacent sampling points corresponds to a spatial distance of approximately 2.6767 m, covering a total range window of about \(\pm\)600 m. The frequency bands are L1 and L5 for GPS, B1 and B2 for BDS, and E1 and E5 for the Galileo system. The main parameters used by the NSSC iGNSS-R altimeter to form the interferometric DMs can be found in Table 1.  
Since its launch in 2023, the NSSC iGNSS-R altimeter has collected a vast amount of interferometric waveforms from the BDS, GPS, and GAL systems. Figures 2-4 present the typical coherent and incoherent interferometric waveforms collected on the sea ice surface and open ocean surface, corresponding to BDS, GPS, and GAL, respectively. \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameters** & **Values** \\ \hline Frequency & GPS L1/L5, BDSB1/B2, GAL E1/E5 \\ \hline Beam & 16 (4 direct L1/B1/E1, 4 direct L5/E5/B2, 4 reflected L1/B1/E1, 4 reflected L5/E5/B2) \\ \hline DMs & 5 Hz 448 delay tags \\  & Delay coverage: \(\pm\)600 m \\  & Delay tag interval: 2.6767 m (112 MHz) \\ \hline Coherent time & 1 ms \\ \hline Incoherent times & 200 \\ \hline Band width & B1: 32 MHz, B2: 44 MHz \\  & L1: 26 MHz, L5: 20 MHz \\  & E1: 32 MHz, E5: 44 MHz \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Main parameters of NSSC iGNSS-R altimeter interferometric waveforms. Figure 1: Onboard configuration of NSSC iGNSS-R altimeter. (**a**) Upward-facing antenna array with multiple beams simultaneously tracking distinct GNSS satellites. (**b**) Reflected signal acquisition geometry, illustrating specular reflection paths and cross-track observation swath. Figure 3: Characteristic spaceborne GPS dual-frequency iGNSS-R DMs. (**a**) Coherent GPS L1 DM over sea ice. (**b**) Coherent GPS L5 DM over sea ice. (**c**) Incoherent GPS L1 DM over open sea, with incidence angle of 8.6\({}^{\circ}\) and wind speed of 8.2 m/s. (**d**) Incoherent GPS L5 DM over open sea, with incidence angle of 8.6\({}^{\circ}\) and wind speed of 8.2 m/s. Figure 2: Characteristic spaceborne BDS dual-frequency iGNSS-R DMs. (**a**) Coherent BDS B1 DM over sea ice. (**b**) Coherent BDS B2 DM over sea ice. (**c**) Incoherent BDS B1 DM over open sea, with incidence angle of 22.9\({}^{\circ}\) and wind speed of 8.7 m/s. (**d**) Incoherent BDS B2 DM over open sea, with incidence angle of 22.9\({}^{\circ}\) and wind speed of 8.7 m/s. As is evident from Figures 2-4, the utilization of GNSS broadband ranging codes in iGNSS-R yields two characteristic improvements in DMs: (1) steepened leading-edge slopes across all observed waveforms; and (2) multi-peak structures attributable to the incorporation of multiple code types. Particularly for sea ice observations, the DMs exhibit enhanced coherence in reflected signals. The spaceborne-acquired waveforms exhibit remarkable consistency with the land-based experimental waveforms reported in [44]. To more clearly illustrate the differences between GNSS-R interferometric waveforms and traditional GNSS-R DDM measurements, Figure 5 demonstrates the simultaneous acquisition of both DDM waveforms and dual-frequency interferometric waveforms from the same GPS PRN 3 satellite, obtained through the interferometric altimetry mode and the DDM measurement mode of the iGNSS-R altimeter, respectively.