## 2 A Review of Spaceborne GNSS-R Missions  
The evolution of these spaceborne GNSS-R missions are presented in Figure 1, with each of the mission summarized below. **UK-DMC:** In December 2003, the UK-DMC satellite was launched into a sun-synchronous orbit with an orbit altitude of about 680 km. With the aim of exploration of GNSS-R technique from space, SSTL built an experimental GPS reflectometry receiver into the UK-DMC satellite [33]. The experimental receiver consists of a navigational receiver, with modifications to allow recording raw IF signal for short periods of time (\(\sim\)20 s). More than 100 sets of raw intermediate frequency (IF) signals had been collected by the UK-DMC satellite [34], from which reflected GPS signals have been detected over the ocean, ice and land [13]. The UK-DMC experiment and the data collections played an important role in optimizing the design of SSTL's new version GNSS-R instrument [35], which have been used in the UK TDS-1 and CYGNSS missions. **UK TDS-1:** Following the successful detection of reflected signals by UK-DMC, SSTL launched the UK TDS-1 on 8 July 2014. The satellite was placed into a Low Earth Orbit with a nominal altitude of 635 km and an inclination of 98.4\({}^{\circ}\)[14]. The TDS-1 satellite carries a more capable GNSS-R receiver known as the Space GNSS Receiver Remote Sensing  
Figure 1: The evolution of spaceborne GNSS-R missions. Note: Spire grazing angle GNSS-R measurements have been acquired by upgrading the GNSS receivers’ firmware since early 2019, which were launched nominally to collect GNSS radio occultation measurements. DoT-1 satellite started to collect GNSS-R measurement by upgrading its payload firmware on September 2020, i.e., more one year after its launch. UK DMC, UK TDS-1 and UK DOT-1 image credits: SSTL. CYGNSS image credit: University of Michigan. Buffeng-1 image credit: China Academy of Space Technology. Spire satellites image credit: Spire Global, Inc. FY-3E image credit: China Meteorological Administration. Instrument (SGR-ReSI), which can process the reflected GPS signal in real-time to generate the Delay-Doppler Map (DDM) of GPS scattered power [34]. As a technique demonstration satellite, the SGR-ReSI operations alternated between the other different payloads, which only operated two days out of every eight days in orbit. The TDS-1 GNSS-R measurement was operational since September 2014, and delivered a unique dataset of globally distributed spaceborne GNSS-R data, spanning a period of four years. Although mainly used for ocean surface wind measurements [35], the TDS-1 DDM products have been also analyzed for some other remote sensing applications, such as ocean altimetry [36], soil moisture [37; 38], sea ice detection and classification [39; 40; 41; 42], ice sheet sensing [43; 44; 45] and freeze-thaw detection [46; 47]. In addition to the standard DDM products, TDS-1 mission also collected the raw IF samples of the direct and reflected signals occasionally, from which new GNSS-R applications have been demonstrated [24; 32; 48]. **CYGNSS:** As a NASA's Earth Venture Program mission, CYGNSS mission consists of a constellation of eight microsatellites for innovative research and applications using GNSS-R technique [15]. The CYGNSS satellites were launched on 15 December 2016, and deployed in a common circular low Earth orbit at 35\({}^{\circ}\) inclination and 520 km altitude. An evolutionary version of the SGR-ReSI receiver is embarked on each CYGNSS satellite. The primary objective is the measurement of sea surface wind speed in and near the inner core of tropical cyclones [15]. Similar to TDS-1, the main data products of CYGNSS are the delay-Doppler maps (DDMs) of the forward scattered power and the bistatic radar cross sections, which are stored in the Level 1 data product together with the corresponding metadata.