# First In-Orbit Validation of Interferometric GNSS-R Altimetry: Mission Overview and Initial Results
## 1 Introduction
were the first to demonstrate the suitability of interferometric techniques for altimetric purposes using the developed PAssive Reflectometry and Interferometry System Interferometric Receiver (PIR) and GPS simulator equipment [36]. In 2011, [PERSON] et al. conducted a two-day shore-based experiment in the Netherlands using high-gain antennas, a calibration system, and a receiver with interferometric correlation capabilities, verifying the feasibility of interferometric correlation technology. The experiment successfully calculated the water surface height and compared it with the measurements from a radar altimeter, demonstrating, for the first time, that interferometric altimetry outperforms local code tracking in terms of height measurement accuracy [37]. Subsequently, [PERSON] et al. conducted an airborne experiment in which two high-gain antennas were used to receive direct and reflected signals, respectively, and obtained airborne interferometric data from the GPS for the first time [38]. In 2016, [PERSON] et al. developed a software-defined interferometric receiver (SPIR), which first sampled raw intermediate frequency signals collected by direct and reflection antennas using a high-speed sampling clock. During data processing, it processed the interferometric correlation results of all GNSS satellites in the same time period, enabling simultaneous tracking of multiple GNSS satellites [39]. Since 2014, the Technical University of Catalonia (UPC) has been developing a microwave interferometric reflectometer (MIR), which is an airborne multi-constellation multi-beam dual-band GNSS reflectometer, with beam-steering capabilities to automatically track the direct and reflected GNSS signals from the GPS and Galileo [40].The device has successfully received interferometric correlation waveforms from different GNSS satellites [40] and been validated in airborne experiments [41]. From 2011 to 2017, several satellite-based interferometric GNSS-R altimetry missions were proposed. In 2011, Martin-Neira introduced a satellite-based GNSS-R interferometric altimetry solution called the Passive Reflectometry and Interferometry System In-Orbit Demonstration (PARIS-IOD) [4]. After the proposal of the PARIS-IOD satellite mission, the European Space Agency (ESA) proposed the establishment of a GNSS-based observation device on the International Space Station (ISS), mainly for ocean height detection, as well as for the detection of sea surface roughness and wind speed. This led to the GNSS Reflectometry, Radio Occultation, and Scatterometry on the International Space Station (GEROS-ISS) project, which achieved synchronous observation of GNSS reflection and occultation and continuous observation of mesoscale sea surface height using GNSS-R interferometric altimetry [42]. In 2016, [PERSON] et al. introduced a concept called \"Cookie\", a satellite particularly suitable for dense spatial sampling in future GNSS remote sensing constellations [43]. In 2017, a team of 33 multidisciplinary scientists proposed the GNSS Transpolar Earth Reflectometry exploriNg (G-TERN) project for the ESA's Earth Explorer 9 mission, focusing mainly on polar sea ice observation [33]. However, none of the proposed missions, including PARIS IOD, GEROS-ISS, Cookie, and G-TERN, have been launched into orbit. Since 2018, the National Space Science Center (NSSC) of the Chinese Academy of Sciences (CAS) has developed a novel triple-system (BeiDou Navigation Satellite System (BDS), Global Positioning System (GPS), and Galileo Satellite Navigation System (GAL))-compatible interferometric GNSS-R altimeter, hereinafter referred to as the NSSC iGNSS-R altimeter or the iGNSS-R altimeter. After extensive shore-based testing and improvements [44], the instrument was successfully launched aboard China's altimetry satellite in 2023, marking the first in-orbit validation of satellite-based iGNSS-R altimetry technology [45]. This paper presents the first in-orbit validation experiment for iGNSS-R SSH altimetry, along with its preliminary SSH results.