## 2 Principles of GNSS Remote Sensing Technique
### Spaceborne Missions  
The development of GNSS-RO can be traced back to the late 1960s. Initially based on occultation methods used to explore planetary atmospheres, NASA's Mariner 10 mission employed this method to study the atmosphere of Venus [26]. When GPS was launched in 1978, the first GPS meteorology experiment announced RO detecting's growth [27]. The application of occultation observation technology in meteorology started with the 1995 GPS/Meteorology experiment. This experiment marked a significant leap from theory to practice by pioneering the use of the GNSS limb sounding to probe Earth's atmosphere [28]. From April 1995 to March 1997, the GPS/MET [29] satellite collected tens of thousands of occultation profiles. With NASA's help, the first satellite Orsted [30] from Denmark launched in 1999. It had the receiver Turbo-Rogue for GPS signals and could  
Figure 5: GNSS-R ocean surface wind observations of FY-3E GNOS-II for Typhoon Mawaron 25 May in 2023 [25]. also obtain the temperature and moisture information vertically through RO detecting. South Africa also launched the Sunsat [31] with Orsted at the same time. By May 2000, the experiment had generated over 80 technical reports and scientific papers, providing the first solid evidence of the accuracy and potential benefits of GPS occultation data. Building on this success, Germany launched the CHAMP [32] mission in July 2000, further advancing the field. From February 2001 to October 2008, the CHAMP satellite delivered around 230 vertical occultation profiles daily, which were crucial for geosciences and atmospheric research. In November 2000, the SAC-C satellite [33], a collaborative effort among Argentina, the U.S., and several other countries, aimed to conduct global limb sounding of the Earth's neutral atmosphere and ionosphere. Following this, in March 2002, the U.S. and Germany launched the GRACE [34] mission, significantly improving geoid measurements from the meter scale to the centimeter scale. The same year, Australia launched FedSat [35]; it received GPS signals to detect the ionospheric electron density and the atmosphere. Germany continued to contribute to the field with the launch of the TerraSAR-X [36] satellite in June 2007, equipped with a high-precision dual-frequency occultation receiver for more accurate orbit determination and atmospheric sounding. Meanwhile, the U.S.-Chinese Taiwan collaboration led to the launch of the COSMIC mission in April 2006, which became a key provider of real-time, high-quality occultation data, crucial for NWP systems [37]. Its successor, COSMIC-2, was launched in June 2019, further enhancing observations, particularly in tropical regions. In addition to these advancements, the European MetOp [38] series of polar-orbiting satellites equipped with GRAS receivers have been providing approximately 600 occultation profiles daily. These data are integrated into NWP systems, alongside satellite microwave and infrared observations, to improve weather forecasts [39]. In addition, Spain's PAZ [40] has great significance for storm forecasting using the polarimetric RO technique. The information near the freezing level could potentially be useful for NWP models when assimilating heavy precipitation data. In the 1990s, people found that the observation could rely on the GNSS-reflected signals. [PERSON] proposed the Passive Reflectometry and Interferometry System (PARIS) [41], which had developed into a pristine GNSS remote sensing technique. Both the reflected signal and direct signal are received by the satellite receiver, and the delay of time or phase between them will be used for interferometry. Then, according to the spatial relation of the satellite, receiver, and specular highlight, we can retrieve the characteristics of the Earth's surface, such as sea surface altimetry, which is based on calculating the time delay between GPS direct signals and reflected signals [42]. [PERSON] used the TDS-1 (Technology Demonstration Satellite-1) carrying the receiver SGR-ReSI launched in 2014 for ocean winds [43].