# Latest Advances in the Global Navigation Satellite System--Reflectometry (GNSS-R) Field  
[PERSON]  
1 Planetary Radar and Radio Science Group, Jet Propulsion LaboratoryGNSS-R publications are distributed as follows: 260 publications in IEEE journals, 160 publications in MDPI, out of which 43 were published in the MDPI Special Issue series \"Applications of GNSS Reflectometry for Earth Observation I, II and III\" (e.g., 27% of all MDPI publications), 43 publications by Elsevier, 30 publications by Springer Nature, and 28 publications by American Geophysical Union Publications. The data were collected from the Clarivate Web of Science Core Collection using the keywords \"remote sensing and (reflectometry GNSS or GNSS-R or GNSS-IR)\". Figure 2 shows the distribution of GNSS-R-related publications per major region or continent, applying the Clarivate Web of Science Core Collection \"core country/region metric\". The large majority of publications for GNSS-R have been written by researchers based in Europe, Asia and North America, with 49.71%, 27.61% and 19.88% of all publications, respectively. Fewer publications have been produced by those based in South America, Africa and Oceania which account for the remaining 2.8%. GNSS-R is a bistatic radar technique based on signals of opportunity. The transmitted signal is sent by a GNSS satellite; then, the signal scatters off the Earth's surface and is received by a GNSS receiver. The GNSS receiver can be placed on a ground-based instrument, on an aircraft, on a satellite or on a constellation of satellites. Each platform provides different capabilities in terms of spatial and temporal resolution. Generally speaking, a ground-based platform can offer high-resolution data, but its coverage is limited to a very small area within the static antenna beam. An airborne platform provides a medium spatial resolution in the order of 300 m to 1 km, depending on the flight height, and potentially provides regional spatial coverage in the order of 100's of kilometers. A  
Figure 1: Progression of GNSS-R publications per year from 2008 to 2022. Left \(y\) axis describes the number of publications in the general remote sensing field (blue) and the number of publications specific to the GNSS-R field (orange). Right \(y\) axis describes the number of publications in GNSS-R over the number of publications in remote sensing, showing the growth rate. On 2022, the total number of remote sensing publications was 16,155, from which GNSS-R contributed 137 publications, representing 0.85% of the total. Data from the Clarivate Web of Science Core Collection. Remote sensing papers correspond to the search “remote sensing” in any field, and to “gnss-r” from the combination of “remote sensing and (reflectometry GNSS or GNSS-R or GNSS-IR). Data retrieved in January 2023”. satellite platform provide global coverage, which depends on the selected orbit, and the spatial resolution ranges from 1 km to hundreds of kilometers, being set primarily by the scattering properties of the surface under observation. For example, sea ice will be observed at 1 km spatial resolutions, while ocean surface will be observed at > 25 km spatial resolutions. Figure 3 provides an illustration of the remote sensing platforms used for GNSS-R instruments. As the GNSS signal is scattered from the Earth's surface, it is affected by its characteristics. For example, over the ocean, the contribution of an area of scattering increases or decreases as the ocean is rougher or calmer. Over the land, there are many geophysical parameters that can affect signals. The reflected GNSS signal is affected by the surface roughness of the scene as well as the water content of the soil since the moisture content of the soil changes the dielectric properties of the surface. Although the GNSS-L-band signal  
Figure 3: Platforms used for GNSS-R: ground-based, UAV-based, aircraft-based, and satellite-based. Figure 2: Distribution by researcher affiliation, including first author and co-authors, among all GNSS-R publications.