## 1 Introduction  
As a new technology of microwave remote sensing, Global Navigation Satellite System Reflectometry (GNSS-R) has advantages such as abundant signal source, low cost, low power consumption, all-day, all-weather, and it can acquire various global ocean dynamics and environmental parameters such as Sea Surface Height (SSH), sea surface wind field, sea ice, and ocean salinity with high spatiotemporal resolution [1]. In 1993, [PERSON] first proposed the feasibility of using Global Positioning System (GPS) reflected signals for altimetry [2]. In 1994, [PERSON] detected the GNSS-R signal for the first time in the data of a receiver used in an aircraft test in July 1991 [3]. In 1996, NASA first proposed that GNSS-R can be applied to sea state remote sensing and developed a delay mapping receiver [4; 5; 6]. In 2000, [PERSON] and [PERSON] gave a theoretical model (Z-V model) of sea surface reflection signals [7]. From 1997 to 2002, the [PERSON] team, the Jet Propulsion Laboratory (JPL) and the European Space Agency (ESA) conducted several shore-based and air-based GNSS-R altimetry experiments, many of which achieved accuracy at the centimeter level [8; 9; 10; 11; 12]. In 2003, [PERSON] et al. [13] gave systematic analysis of elevation measurements using GPS reflected signals. For the space-based GNSS-R, in 2002, [PERSON] [14] detected the reflected signal of GPS in the data collected by the Spaceborne Imaging Radar C-band (SIR-C). From 2001 to 2004, [PERSON] found GPS reflection signals in the data collected by the Low Earth orbit (LEO) satellite of the Challenging Minisatellite Payload (CHAMP) mission [15; 16]. In 2003 and 2014, the GNSS-R receiving equipment by the British Surrey Satellite Technology Ltd. were loaded on the UK Disaster Monitoring Constellation (UK-DMC) mission and the TechDemoSat-1 (TDS-1) mission, with observations of the sea surface wind field obtained [17; 18]. In 2016, [PERSON] [19] carried out the first spaceborne observation of SSH using GPS-R data from the TDS-1 satellite. In 2016, NASA launched the Cyclone Global Navigation Satellite System (CYGNSS) mission consisting of eight small satellites with the goal of tracking and monitoring the mid-low latitude surface wind field [20; 21]. To date, the application of the high-temporal resolution observation capability of the GNSS-R satellites for sea surface wind research is relatively more often [17; 18; 20; 21], but the application of spaceborne GNSS-R high-spatial resolution observation capability for global SSH research is relatively rare [19]. Our research team of navigation and detection based on the information of aerospace-aeronautics-marine integration ([[http://www.qxslab.cn/ndia/](http://www.qxslab.cn/ndia/)]([http://www.qxslab.cn/ndia/](http://www.qxslab.cn/ndia/))) of Qian Xuesen Laboratory of Space Technology has carried out preliminary exploration research on the theory, method, and key technology of improving underwater gravity matching navigation accuracy based on the GNSS-R altimetry constellation principle. Underwater gravity matching navigation is an effective method to correct the drift error of the submersible inertial navigation system [22], therefore, the construction of a global, high-spatial-resolution and high-accuracy ocean gravity field reference map is the key to determining the accuracy of underwater navigation [23]. The current conventional means of obtaining the global ocean gravity field is to convert the SSH obtained by the satellite radar altimeter into gravity anomaly, while this study aims to improve the specular reflection point position accuracy for the GNSS-R satellite altimetry, and thus to support the obtention of the high-spatial-resolution and high-accuracy ocean gravity fields. The multi-channel GNSS-R receiver can utilize the abundant signal source of more than 100 satellites of the four international GNSSs to acquire multiple GNSS-R signals simultaneously.