# First In-Orbit Validation of Interferometric GNSS-R Altimetry: Mission Overview and Initial Results
## 1 Introduction  
Sea surface height (SSH) is a crucial parameter in the fields of ocean dynamics, climate research, marine ecology, and resource management, as well as marine engineering [1]. Satellite altimetry is the most effective means of obtaining global SSH data. Since the United States launched the first ocean satellite radar altimeter in 1973, more than 20 radar altimetry satellites have been successfully launched, providing valuable data support for marine scientific research and applications [2]. Traditional radar altimetry satellites use a radio-point observation mode, which, despite its high precision and resolution, is fundamentally constrained by sparse spatial sampling characteristics. In recent years, altimetry technology has gradually evolved towards wide-swath measurement. A typical example is the Surface Water and Ocean Topography (SWOT) satellite launched by the United States in 2022 [3]. Global navigation satellite system reflectometry (GNSS-R) altimetry is another novel wide-swath altimetry technique that measures surface heights using the signals from global navigation satellite systems (GNSSs) that are reflected off the Earth's surface [4]. Extensive peer-reviewed research has validated GNSS-R as an effective technique for retrieving diverse geophysical parameters, including the following: (i) oceanographic variables--surface wind speed [5, 6], significant wave height [7, 8], and sea surface height [9, 10]; (ii) cryospheric characteristics--ice extent [11, 12] and thickness [13, 14]; and (iii) terrestrial hydrologic measures--soil moisture [15, 16]. However, in the field of GNSS-R altimetry, the altimetry accuracy based on the local code correlation mode is at the meter level [10, 17, 18, 19, 20, 21], which is far from meeting the requirements for studying marine phenomena. GNSS-R carrier phase altimetry technology calculates the time delay difference between the reflected and direct GNSS signals based on their phase tracking results, and the accuracy of satellite-based GNSS-R carrier phase altimetry can reach the centimeter level [22, 23, 24]. [PERSON] et al. used GPS and Galileo observations from the Cyclone Global Navigation Satellite System (CYGNSS) satellite to conduct grazing carrier phase sea surface altimetry, achieving an accuracy of 3 cm/4.1 cm (median/mean) at a 20 Hz sampling rate and centimeter-level accuracy at a 1 Hz sampling rate, which is comparable to dedicated radar altimeters [25]. [PERSON] et al. used initial grazing angle GNSS reflection data from Spire satellite observations and performed altimetry inversion with dual-frequency phase measurements. The root mean square difference between the sea ice area and the sea surface height model was 3 cm after bias removal [26]. However, the conditions for continuous carrier phase tracking of GNSS reflection signals are extremely stringent, requiring that the GNSS reflection signals be dominated by the coherent component. In practice, low-elevation GNSS signals are usually utilized to reduce the impact of sea surface roughness on the continuous carrier phase tracking of GNSS reflection signals, which greatly limits the scope of application [24]. Since [PERSON] first proposed the concept of interferometric GNSS reflection (GNSS-R) in 1993, iGNSS-R altimetry, utilizing the direct cross-correlation method between GNSS direct signals and reflected signals, which enables centimeter-to-decimeter-level altimetry by fully exploiting the broadband ranging code characteristics of GNSS signals, has been highly anticipated [27, 28, 29, 30]. Theoretical analysis indicates that GNSS-R interferometric altimetry has the capability of centimeter-decimeter-level height measurement [4, 27, 28, 29], and can be used for mapping ocean mesoscale sea surface heights [31], climate research [32, 33], and tsunami research [34, 35]. In 2010, [PERSON] et al.