# Latest Advances in the Global Navigation Satellite System--Reflectometry (GNSS-R) Field
## 8 Conclusions  
This review article has covered the latest advances in GNSS-R up to Q4-2022. A total of 43 out of 147 of the articles referenced here have been extracted from the Special Issue \"Applications of GNSS Reflectometry for Earth Observation I, II and III\", strengthening the highly relevant contributions of these two Special Issues from MDPI's _Remote Sensing_ to the GNSS-R community. Moreover, out of the 176 articles published in MDPI with the topic \"GNSS-R\", \"Reflectometry\", or \"GNSS-IR\", 42 of them (23.8%) originate from the Special Issue \"Applications of GNSS Reflectometry for Earth Observation I and II\". This review article focuses on all the key areas of interest of GNSS-R. We presented the technological chronology of GNSS-R, from the first concept and reflection captured, to the latest advances in ground-based, airborne, and space-borne GNSS-R instruments. Summaries of the technological features and achievements by different instruments are presented, with a focus on power consumption and mass/volume, and included in this analysis are capabilities, such as multi-band or polarimetry uses. Thereupon, the most relevant O=ocean applications of GNSS-R are presented. We have covered several novel studies on retrievals of wind speed over the ocean, wind direction, swell estimation, and ocean altimetry. In terms of land applications, this review article has covered all key applications of GNSS-R over land developed up to date. First, the specular reflection point location based on different DEM models is covered. Achieving a highly accurate specular point location is crucial for applications that require very high spatial resolutions, which might be the case for higher bandwidth signals. Second, several articles are presented where the amount of coherent and incoherent components of the reflected signal over land are analyzed. Third, different soil moisture applications are presented from ground-based, airborne, and space-borne datasets. Fourth, the ability of GNSS-R to detect inland water bodies is also presented as a key application that can enable flood monitoring and other crucial applications. Cryosphere applications are also reviewed in this manuscript. The potential of GNSS-R to estimate open water (diffuse reflection) or ice (specular reflection) can be key to enhancing already existing radar and microwave radiometry products that contour the sea-ice. Additionally, polarimetric studies are also presented as being key to the extraction of more information about the ice shelf, such as young ice, multi-year ice, or snow on top the sea-ice. Other selected applications are also briefed. Those most relevant are mesoscale ocean eddies, target detection over the ocean surface, ionospheric scintillation, river flow, phytoplankton bloom monitoring, microplastics, and desert roughness. GNSS-R is able to cover three crucial hydrological variables in order to model the Earth's climate in the following areas: water in soil, ocean winds, and ice. However, GNSS-R applications are not limited to these three uses, but extend to other applications as well. Future technological developments point towards the use of higher bandwidth signals and polarimetric retrievals, potentially providing enhanced accuracy and resolution to already existing retrievals. All authors contributed in the preparation of this review article. Conceptualization, N.R.-A., J.F.M.-M. and M.M. ; methodology, N.R.-A., J.F.M.-M. and M.M. ; investigation, N.R.-A., J.F.M.-M. and M.M. ; writing--original draft preparation, N.R.-A., J.F.M.-M. and M.M. ; writing--review and editing, N.R.-A., J.F.M.-M. and M.M. ; visualization, N.R.-A. and J.F.M.-M. All authors have read and agreed to the published version of the manuscript. This research received no funding. Certain data included herein are derived from Clarivate Web of Science. Copyright Clarivate 2023. All rights reserved.