## 2 A Review of Spaceborne GNSS-R Missions
These high density and frequency GNSS-R measurements have made it possible to demonstrate different GNSS-R applications other than ocean surface wind, such as soil moisture measurement (e.g., in [49]), and inland water and flooding detection (e.g., in [50; 51]). In addition to standard science data, CYGNSS satellites have been also collected large amount of raw IF data for exploration of new GNSS-R applications (e.g., in [25; 30]). **BuFeng-1:** The first Chinese GNSS-R satellite mission, known as BuFeng-1 (BF-1) A/B twin satellites, were launched on 5 June 2019 [17]. Towards sea surface wind speed measurement during typhoons and hurricanes, the BF-1 satellites are also placed in a low inclination orbit (45\({}^{\circ}\)) with a nominal altitude of 579 km. BF-1's GNSS-R instrument is compatible with both reflected GPS signals at L1 band and BeIou (BDS) signal at B1I band (1561.098 MHz), which makes it possible to acquire more GNSS-R observations within the field-of-view of its science antennas. The raw IF data at GPS L1 and BDS B1I bands have been collected occasionally (\(\sim\)one raw IF dataset per week per BF-1 satellite) for diagnostic purpose. **Spire GNSS RO Satellites:** The Spire satellite constellation currently has over 100 3-Unit CubeSats located in low-Earth orbit between 450 and 600 km altitude. Most of these satellites are dedicated towards collecting GNSS RO measurements [52]. The instrumental and antenna configurations of the Spire GNSS RO satellites make it possible to receive earth reflected signals from GNSS satellites at grazing angle (GA). Since early 2019, the GNSS receivers onboard four Spire Lemur-2 satellites have been updated to be able to collect GA GNSS-R events. During the time period between 9 January 2019 and 11 April 2019, more than 2800 GA GNSS-R events over sea ice and open ocean had been collected. The GNSS-R measurements observed by the Spire satellites include both amplitude and phase information of the reflected signal. In addition, these measurements are collected at dual frequencies (GPS L1 and L2) and associated with precise orbit determination data. These features make the datasets unique for accurate and precise carrier phase altimetry [18]. To develop advanced signal processing methods for GA GNSS-R phase altimetry, a couple of dual frequencies raw IF datasets have been collected over inland water, open ocean and sea ice. Spire currently operates about 30 satellites with the ability to collect GA GNSS reflection data and plans to add more of them to ramp up data collection in the near future [18]. **Spire GNSS-R Satellites:** With the GNSS RO constellation in orbit, it is a natural progression for Spire to add GNSS-R scatterometer satellites to its constellation. On\(11\) December 2019, the first batch of two GNSS-R CubeSats were launched for early orbit experience [19]. Each Spire GNSS-R satellite carries an advanced GNSS-R receiver, which is capable of processing 16-24 simultaneous GNSS reflections from GPS, QZSS, Galileo and SBAS satellites. The second batch of Spire GNSS-R satellite were launch in 2021 to provide operational GNSS-R products for different applications, such as soil moisture, ocean wind/waves, sea ice and wetlands/flood inundation [19]. In addition to the operational datasets, a limited number of low-level, raw IF data have been also collected by the GNSS-R satellites. **FSSCat:** As the winner of the 2017 Copernicus Master ESA Sentinel Small Satellite Challenge, the FSSCat mission is an innovative concept consisting of two federated 6-Unit Cubesats, called 'Cat-5/A and 'Cat-5/B [16]. The main payload onboard is the Flexible Microwave Payload 2 (FMPL-2), which integrates two instruments in a single platform: a multi-constellation (GPS and Galileo) GNSS-R receiver, and a Total Power Radiometer (TPR) [53]. The FMPL-2 payload is able to produce scientific data over land and sea ice, such as sea ice detection and thickness monitoring, mapping water ponds over ice, and low resolution soil moisture measurement.