# Latest Advances in the Global Navigation Satellite System--Reflectometry (GNSS-R) Field
## 2 Technology Advancements
### Spaceborne Receivers  
The first GNSS-R signal received from space was captured during a calibration routine of the Space Shuttle radar imager in 2002 [31]. Later in 2003, the first GNSS-R receiver was launched into space as part of the UK Disaster Monitoring Constellation-1 (UK-DMC-1) mission from Surrey Satellite Technology Limited (SSTL), Guildford, UK. The mission included the first GNSS-R receiver built for that purpose as a technology demonstrator. The receiver had several configurable correlators implemented in a field-programmable gate array (FPGA) that were used to correlate the different GPS satellites in view. The antenna used was a four-element patch nadir-looking antenna array, working under left-hand circular polarization (LHCP) conditions with a directivity of \(\sim\)11 dB. The mission aimed to collect small portions of data to prove the feasibility of using GNSS-R to study the ocean. Following UK-DMC-1, in 2013, the UK-TDS-1 mission was conceived with an upgraded version of the SSTL GNSS-R receiver, called SGR-ReSi [32; 33], and with the same type of antennas as in UK-DMC-1. Both instruments were using a correlation approach called zoom transform correlator, detailed in [34], which has been the technique adopted for most GNSS-R receivers. SGR receivers were able to provide a 1/4 chip resolution for the GPS L1 CA signal, with a 500 Hz spectral resolution and a moderate power consumption below 100 W. The mission was designed with a polar orbit, and it collected data all around ocean, land, and ice. Three years later, in 2016, the National Aeronautics and Space Administration (NASA) Cyclone GNSS (CYGNSS) mission was launched, using a similar receiver configuration as the one used in the UK-TDS-1. A GNSS-R receiver, called a delay-Doppler map imager (DDMI), was conceived for the mission. It followed the same approach as SGR-ReSi in terms of correlation approach, but the mission was designed with two sets of 28\({}^{\circ}\)-looking LHCP antennas arrays placed on the sides of the satellite, thus ensuring a larger coverage. The DDMI provides the same resolution as the SGR-ReSi platform of 1/4 chip for the GPS L1 CA signal, with a 500 Hz spectral resolution and a moderate power consumption below 60 W. Moreover, the instrument is able to provide with an IF data mode, where the raw IQ counts can also be retrieved and downloaded down to Earth using the satellite's link. CYGNSS consists of eight microsatellites that cover mid-latitudes of \(\pm\)38\({}^{\circ}\), and it has demonstrated the capabilities of using moderate-cost micro-satellite constellations to perform Earth observation at better time resolutions and similar spatial and radiometric resolutions as compared with conventional satellite missions [35; 36]. The China Aerospace Science and Technology Corporation (CASC) has also deployed the first Chinese spaceborne GNSS-R instrument: the BuFeng-1 A/B constellation [37]. This was successfully launched into a polar orbit in 2019. The instrument for the BuFeng-1 constellation follows the same design as the SSTL SGR-ReSi and the CYGNSS DDMI and uses the same antenna configuration as the NASA CYGNSS mission, with two antennas tilted off the sides of the spacecraft. In this case, the looking angle with respect to the nadir in the BuFeng-1 case is 26\({}^{\circ}\), and it was also capable of receiving L1 BeiDou signals. The power consumption of the instrument is similar to the ones previously reported of ~55 W.  
More recently, the China Meteorological Administration (CMA) in 2021 launched the FY-3E mission [38]. The receiver included in the mission is capable of providing a larger time resolution from 1/4 of a GPS L1 chip to 1/8 of a chip. The missions' receiver has similar characteristics to the CYGNSS DDMI or the BuFeng-1 receiver, with a similar power consumption, being only able to fit into a micro-/ mini-satellite. The FY-3E mission provides near real-time data with a latency of 3 h with a polar orbit.