# Latest Advances in the Global Navigation Satellite System--Reflectometry (GNSS-R) Field
## 2 Technology Advancements
### Airborne Receivers  
Airborne GNSS-R receivers have been used as technology demonstrators for novel techniques and technologies. As previously stated, the first GNSS-R signal acquired by [PERSON] and [PERSON] was retrieved from an airborne platform [4]. The number of GNSS-R instruments that have flown in airborne campaigns is large, which was especially true before the deployment of the CYGNSS constellation. In this section, we will cover those instruments that, in recent years, have provided large technological development with respect to previous versions or actual satellite-based versions.  
In chronological order, the GOLD-R instrument (2010-2014), developed by the Institut de Ciencies de l'Espai/Institut Estudis Espaials de Catalunya (ICE-CSIC/IEEC) with the collaboration of UPC and ESA, was a pioneer instrument capable of working at the GPSL1 band at multiple polarizations [15]. The basic design works by sampling up to 20 MHz of bandwidth at the GPS L1 band using a single-LHCP patch antenna. The instrument is able to work in different modes thanks to the collection of the IF samples (e.g., I/Q down-sampled data) of both the direct and the reflected signals. It was developed to show the differences between iGNSS-R and cGNSS-R processing modes [16].  
The second airborne instrument covered in this review is the global navigation satellite system reflectometry instrument (GLORI) [17]. Designed in 2015, this airborne instrument was a novel approach and was used to perform polarimetric studies overland. It includes a dual polarization RHCP/LHCP antenna system with 7 dBic of peak gain, capable of receiving GPS L1 and L2 signals. The receiver is based on an SDR working at 4 MHz, thus providing a delay resolution of 1/4 chips at GPS L1 CA. The instrument has demonstrated the differences in RHCP and LHCP signals overland in previous work [18], being of interest for future spaceborne missions, such as HydroGNSS [16].  
The third airborne instrument with notable technological advancements is the UPC microwave interferometric reflectometer (MIR) [19]. Designed in 2015-2018, it conducted its maiden flight in 2018. This instrument is the first GNSS-R instrument capable of providing collocated GNSS (GPS and Galileo) L1 and L5 measurements using a 21/18 dB (L1/L5) directive antenna with beamforming. The instrument had different modes of operation, including cGNSS-R and iGNSS-R, and it aimed at proving the capabilities of high-directive antennas to remove iGNSS-R cross-talk [20]. Additionally, the instrument demonstrated, in cGNSS-R mode, the enhanced capabilities of the GPS/Galileo L5/E5 waveforms for sea state, soil moisture, vegetation, and monitoring sea altimetry [21; 22; 23; 24].  
The last airborne instrument covered in this review is the next-generation GNSS-R instrument proposed by [PERSON] et al. [25]. The instrument was conceived after CYGNSS success from a set of scientists directly involved in the CYGNSS mission. The proposed instrument was designed for operation in airborne platforms. It is able to work in L1/L5 bands in both GPS and Galileo constellations. Moreover, its down-looking antenna was designed to receive both RHCP and LCHP signals. Thus, it combined characteristics from the previously presented instruments, GLORI and MIR. The instrument was deployed in mid-2021 for continuous operation in domestic aircrafts by Air New Zealand [26].  
Thanks to electronics miniaturization, low-cost GNSS-R receivers are becoming popular among the community. Studies have shown the Earth observation potential of incorporating GNSS-R receivers into commercial aircrafts [27]. Furthermore, UAV-based GNSS-R receivers are being developed by different research centers for low-cost and high-resolution Earth monitoring [28; 29; 30].