# Latest Advances in the Global Navigation Satellite System--Reflectometry (GNSS-R) Field
## 2 Technology Advancements
### Spaceborne Receivers
The Universitat Politecnica de Catalunya (UPC) NanoSat-Lab has been also contributing to the technological advances of spaceborne GNSS-R instruments. Its first CubeSats mission, 3 Cat-2 [39], launched in 2016, inaugurated the first ever spaceborne dual-band polarimetric GNSS-R instrument, which had previously been tested in a balloon experiment campaign [40]. The instrument inherited previous ground-based GNSS-R instruments developed by the university, and it comprised an open-loop and a closed-loop receiver to work in GNSS-R, iGNSS-R, and reconstructed GNSS-R (r-GNSS-R) modes [41]. In this case, the antenna used was different than the ones used in UK-TDS-1 or CYGNSS, and it was specifically designed for a 6U CubeSat structure. It consisted of a 6-element patch array, providing gains of up to 13 dB at L1, and up to 11.5 dB at the GPS L2 band. Moreover, as strategically designed for CubeSats, its volume, mass, and power consumption were of ~3U of a CubeSat (30 \(\times\) 30 \(\times\) 10 cm\({}^{3}\)) and less than 5 kg mass, with a power consumption below 15 W. However, spaceborne data could not be retrieved from the instrument due to a satellite bus malfunction. As an evolved version of 3 Cat-2, the UPC NanoSat-Lab leads the European Space Agency (ESA) FSSCat mission [42]. The mission was launched in September 2020, and included a combined GNSS-R receiver and an L-band radiometer using software-defined radio technology (SDR) [43], inheriting the road paved by the UPC passive advance units (PAU) concept [44]. The instrument called flexible microwave payload-2 (FMPL-2) includes both as GNSS-R and L-band radiometer in a single unit of a CubeSat (10 \(\times\) 10 \(\times\) 10 cm\({}^{3}\)), with a mass of ~1.3 kg and power consumption ~10 W. The system antenna arrangement is similar to that 3 Cat-2 of a 6U patch array, but for dual frequency (GPS L1 and the radiometry L-band at 1413.5 MHz). Another successful GNSS-R mission example, this time from a private entity source, is the Spire Global Inc. case. The company, known for its massive CubeSat GNSS radio occultation (GNSS-RO) constellation for ionospheric measurements, has recently launched several CubeSats to provide global GNSS-R measurements [45]. In 2019 and 2020, the company launched two batches of two 3U CubeSats with GNSS-R capabilities (two in 2019, two in 2020), providing up to 20 simultaneous correlation channels [46]. The miniaturized receiver, produced by Spire, is the outcome of years working with GNSS-RO receivers, and it provides a multi-constellation (GPS, QZSS, Galileo) capability. The antenna system is of \(3\times 1\) patch antennas working at LHCP with beamforming capabilities. The instrument is able to work continuously with deployable solar panels, weighing less than 5 kg [46]. Spire Global Inc. is currently retrieving LHCP measurements at near-nadir and RHCP measurements at grazing angles, combining their GNSS-R and GNSS-RO satellites. The next GNSS-R mission in the pipeline is the HydroGNSS mission [47]. The mission includes the SSTL newest version of the SGR-ReSi, the SGR-ReSi-Z. This new receiver is capable of providing multi-constellation (GPS/Galileo), multi-polarization (LHCP/RHCP), and multiband (L1 and L5) GNSS-R measurements. The receiver computes real-time L0b observations (e.g., delay-Doppler maps) at L1/E1, and the L5 data are downloaded and processed following the recommendations of [48]. In preparation for the launch of HydroGNSS, SSTL launched the DoT-1 mission in 2019 [47] as a tech demo with which to evaluate its new satellite bus. Last but not least, it is worth mentioning the contribution of the soil moisture active passive (SMAP) radar receiver tuned at the GPS L2C band, whose data have been termed SMAP-reflectometry [49]. The SMAP radar receiver collects IQ data in an open-loop mode. Data are downloaded and post-processed on Earth [49].