## 2 Principles of GNSS Remote Sensing Technique
### Spaceborne Missions
The receiver had obtained a large amount of 2D DDM data that nowadays are often utilized to retrieve the ocean winds. CYGNSS (Cyclone Global Navigation Satellite System) [44] was launched by NASA in 2016 and is the first GNSS-R mission specifically designed to monitor ocean surface winds, particularly in TC regions, using GNSS signal reflections. In addition to governmental and institutional projects, commercial companies have also entered the field of GNSS-R. Spire GNSS-R consists of a series of CubeSats operated by Spire Global, utilizing GNSS-R technology for Earth observations that includes atmospheric, oceanic, and land surface parameters [45]. HydroGNSS is a mission funded by the European Space Agency (ESA), expected to launch by late 2024 or early 2025 [46]. China has also made significant strides in this area. Since 23 September 2013, the FY-3 series of satellites, equipped with GNOS occultation receivers, have been gradually launched [47; 48]. The data collected by these satellites are now utilized by both the China Meteorological Administration (CMA) and the European Centre for Medium-Range Weather Forecasts (ECMWF) for operational assimilation. FY-3/GNOS-II [49], launched in 2021-2023 and operated by the CMA, is the first operational mission that combines GNSS RO and GNSS-R measurements from multiple GNSS constellations. The payload design that integrates GNSS RO and GNSS-R is shown in Figure 6[50]. This setup marks the first international realization of integrated GNSS RO and GNSS-R remote sensing, enabling the simultaneous acquisition of three-dimensional observational data on the ionosphere, atmosphere, and ocean. To date, more than 5 million occultation event data products have been accumulated, with over 15 million various profile products provided. These data products have passed quality assessments by numerous domestic and international operational and research institutions. The atmospheric occultation refractivity inversion accuracy of the systems compatible with BeiDou and GPS is better than 1% from 10 km to 30 km. These products play a crucial role in global and regional weather forecasting, particularly in regions with high-speed tropical and temperate cyclones, where they exhibit high accuracy. The global ocean surface wind speed data from GNOS-II has been operationally assimilated into the CMA's GRAPES NWP system since May 2023[25; 51], demonstrating a neutral to positive impact on model analysis and forecasting, thereby enhancing the accuracy and reliability of weather forecasts. Except for FY-3, China cooperated with Russia and launched the YH-1/Phobos-Grunt[52] in 2011 for Mars exploration. It used the RO technique to determine the density of electrons in the Martian ionosphere. As for BF-1A/B[53] launched in 2019, people have retrieved ocean surface winds, the sea surface height, and soil moisture using the Earth reflected delay Doppler maps from it. Tianmu-1[54] is a commercial constellation with 23 satellites launched in 2023-2024 that provides global, all-weather atmospheric, surface, and ocean surface parameter data. Table 1 shows spaceborne GNSS RO and GNSS-R missions in chronological order. Figure 6: The structure of the spaceborne integrated GNSS remote sensors (SIGRS) for FY-3E/GNOS- II[50].