{{short description|Earth observation technology}} {{Use dmy dates|date=July 2024}} thumb|GNSS-R system diagram [[File:CYGNSS concept art.jpeg|thumb|CYGNSS concept art]] [[File:FutureEO ESA23861197 (HydroGNSS cropped).jpeg|thumb|HydroGNSS concept art]] '''GNSS reflectometry''' (or GNSS-R) involves making measurements from the reflections from the Earth of navigation signals from Global Navigation Satellite Systems such as GPS. The idea of using reflected GNSS signals for earth observation was first proposed in 1993 by Martin-Neira.<ref>{{cite journal |last1=Martín-Neira |first1=M |title=A passive reflectometry and interferometry system (PARIS): Application to ocean altimetry |journal=ESA Journal |volume=17 |issue=4|pages=331–355}}</ref> It was also investigated by researchers at NASA Langley Research Center<ref name=":0">{{Cite book|last1=Komjathy|first1=A.|last2=Maslanik|first2=J.|last3=Zavorotny|first3=V.U.|last4=Axelrad|first4=P.|author4-link= Penina Axelrad |last5=Katzberg|first5=S.J.|title=IGARSS 2000. IEEE 2000 International Geoscience and Remote Sensing Symposium. Taking the Pulse of the Planet: The Role of Remote Sensing in Managing the Environment. Proceedings (Cat. No.00CH37120) |chapter=Sea ice remote sensing using surface reflected GPS signals |date=2000|location=Honolulu, HI, USA|publisher=IEEE|volume=7|pages=2855–2857|doi=10.1109/IGARSS.2000.860270|isbn=978-0-7803-6359-5|hdl=2060/20020004347|s2cid=62042731|hdl-access=free}}</ref> and is also known as GPS reflectometry. Research applications of space-based GNSS-R are focused in altimetry,<ref>{{Cite journal |last1=Semmling |first1=A. M. |last2=Wickert |first2=J. |last3=Schön |first3=S. |last4=Stosius |first4=R. |last5=Markgraf |first5=M. |last6=Gerber |first6=T. |last7=Ge |first7=M. |last8=Beyerle |first8=G. |date=2013-07-15 |title=A zeppelin experiment to study airborne altimetry using specular Global Navigation Satellite System reflections: A ZEPPELIN EXPERIMENT TO STUDY AIRBORNE ALTIMETRY |url=http://gfzpublic.gfz-potsdam.de/pubman/item/escidoc:321321 |journal=Radio Science |language=en |volume=48 |issue=4 |pages=427–440 |doi=10.1002/rds.20049 |doi-access=free}}</ref><ref>{{Cite journal |last1=Rius |first1=Antonio |last2=Cardellach |first2=Estel |last3=Fabra |first3=Fran |last4=Li |first4=Weiqiang |last5=Ribó |first5=Serni |last6=Hernández-Pajares |first6=Manuel |date=2017 |title=Feasibility of GNSS-R Ice Sheet Altimetry in Greenland Using TDS-1 |journal=Remote Sensing |language=en |volume=9 |issue=7 |pages=742 |bibcode=2017RemS....9..742R |doi=10.3390/rs9070742 |hdl=2117/114540 |issn=2072-4292 |doi-access=free |hdl-access=free}}</ref> oceanography (wave height and wind speed),<ref name="reflect" /> cryosphere monitoring,<ref name=":0" /><ref>{{Cite journal |last1=Rivas |first1=M.B. |last2=Maslanik |first2=J.A. |last3=Axelrad |first3=P. |author3-link=Penina Axelrad |date=2009-09-22 |title=Bistatic Scattering of GPS Signals Off Arctic Sea Ice |journal=IEEE Transactions on Geoscience and Remote Sensing |volume=48 |issue=3 |pages=1548–1553 |doi=10.1109/tgrs.2009.2029342 |issn=0196-2892 |s2cid=12668682}}</ref> and soil moisture monitoring.<ref>{{Cite journal |last1=Rodriguez-Alvarez |first1=Nereida |last2=Camps |first2=Adriano |last3=Vall-llossera |first3=Mercè |last4=Bosch-Lluis |first4=Xavier |last5=Monerris |first5=Alessandra |last6=Ramos-Perez |first6=Isaac |last7=Valencia |first7=Enric |last8=Marchan-Hernandez |first8=Juan Fernando |last9=Martinez-Fernandez |first9=Jose |last10=Baroncini-Turricchia |first10=Guido |last11=Perez-Gutierrez |first11=Carlos |date=2011 |title=Land Geophysical Parameters Retrieval Using the Interference Pattern GNSS-R Technique |journal=IEEE Transactions on Geoscience and Remote Sensing |volume=49 |issue=1 |pages=71–84 |bibcode=2011ITGRS..49...71R |doi=10.1109/TGRS.2010.2049023 |issn=0196-2892 |s2cid=27516781}}</ref>
== Principles == GNSS reflectometry is passive sensing that takes advantage of and relies on multiple active sources - with the satellites generating the navigation signals. For this, the GNSS receiver measures the signal delay from the satellite (the pseudorange measurement) and the rate of change of the range between satellite and observer (the Doppler measurement). The surface area of the reflected GNSS signal also provides the two parameters time delay and frequency change. As a result, the Delay Doppler Map (DDM) can be obtained as GNSS-R observable. The shape and power distribution of the signal within the DDM is dictated by two reflecting surface conditions: its dielectric properties and its roughness state. Further derivation of geophysical information rely on these measurements.
GNSS reflectometry is a bi-static radar, where transmitter and receiver are separated by a significant distance. Since in GNSS reflectometry one receiver simultaneously can track multiple transmitters (i.e. GNSS satellites), the system also has the nature of multi-static radar. The receiver of the reflected GNSS signal can be of different kinds: Ground stations, ship measurements, airplanes or satellites, like the UK-DMC satellite, part of the Disaster Monitoring Constellation built by Surrey Satellite Technology Ltd. It carried a secondary reflectometry payload that has demonstrated the feasibility of receiving and measuring GPS signals reflected from the surface of the Earth's oceans from its track in low Earth orbit to determine wave motion and windspeed.<ref name="reflect">{{Cite journal |doi = 10.1109/TGRS.2005.845643|bibcode = 2005ITGRS..43.1229G|title = Detection and Processing of bistatically reflected GPS signals from low Earth orbit for the purpose of ocean remote sensing|journal = IEEE Transactions on Geoscience and Remote Sensing|volume = 43|issue = 6|pages = 1229–1241|year = 2005|last1 = Gleason|first1 = S.|last2 = Hodgart|first2 = S.|last3 = Yiping Sun|last4 = Gommenginger|first4 = C.|last5 = MacKin|first5 = S.|last6 = Adjrad|first6 = M.|last7 = Unwin|first7 = M.|s2cid = 6851145}}</ref><ref>M. P. Clarizia ''et al.'', [http://www.agu.org/pubs/crossref/2009/2008GL036292.shtml Analysis of GNSS-R delay-Doppler maps from the UK-DMC satellite over the ocean] {{Webarchive|url=https://web.archive.org/web/20110606070637/http://www.agu.org/pubs/crossref/2009/2008GL036292.shtml |date=2011-06-06 }}, Geophysical Research Letters, 29 January 2009.</ref>
== Space missions ==
* '''CYGNSS''', satellite constellation by NASA using GNSS-R for improving hurricane forecasting, launched in 2016<ref>{{Cite journal |last=Ruf |first=Christopher |last2=Asharaf |first2=Shakeel |last3=Balasubramaniam |first3=Rajeswari |last4=Gleason |first4=Scott |last5=Lang |first5=Timothy |last6=McKague |first6=Darren |last7=Twigg |first7=Dorina |last8=Waliser |first8=Duane |date=2019 |title=In-Orbit Performance of the Constellation of CYGNSS Hurricane Satellites |url=https://journals.ametsoc.org/view/journals/bams/100/10/bams-d-18-0337.1.xml |journal=Bulletin of the American Meteorological Society |volume=100 |issue=10 |pages=2009–2023 |doi=10.1175/BAMS-D-18-0337.1 |issn=0003-0007|url-access=subscription }}</ref> * '''''TechDemoSat-1''''', technology demonstration small satellite by ESA, launched in 2019 * '''''PRETTY''''', technology demonstration CubeSat by ESA measuring sea state, sea ice, and ocean currents, launched in 2023 * '''HydroGNSS''''','' 2 identical small satellites by ESA for monitoring Essential Climate Variables related to the hydrological cycle, launched in November 2025<ref>{{Cite web |last=Gutierrez |first=Peter |date=2025-01-20 |title=GNSS Reflectometry Project HydroGNSS to Launch in 2025 |url=https://insidegnss.com/gnss-reflectometry-project-hydrognss-to-launch-in-2025/ |access-date=2025-09-24 |website=Inside GNSS - Global Navigation Satellite Systems Engineering, Policy, and Design |language=en-US}}</ref>
== GNSS Interferometric Reflectometry == thumb|Geometry of GNSS-IRGNSS Interferometric Reflectometry (or GNSS-IR) is a specialized case of GNSS-R. Here the receiving instrument is on the surface of the Earth. In this technique the interference of the direct and reflected signals is used rather than a Delay Doppler Map or measuring the two signals separately. In the example shown, a GNSS antenna is ~2.5 meters above a planar surface. Both direct (blue) and reflected (red) GNSS signals are shown. As a GNSS satellite rises or sets, the elevation angle changes; the direct and reflected signals will generate an interference pattern. The frequency of this interference pattern can be used to extract the height of the antenna above the planar surface, the reflector height. Changes in reflector height can be directly used to measure water surfaces <ref>{{cite journal |last1=Larson |first1=Kristine M. |last2=Ray|first2=Richard D. |last3=Nievinski|first3=Felipe|last4=Freymueller|first4=Jeff|title=The Accidental Tide Gauge |journal=IEEE Geoscience and Remote Sensing Letters |date=September 2013 |volume=10 |issue=5 |page=1200 |doi=10.1109/LGRS.2012.2236075 |hdl=2060/20140013285 |url=https://ntrs.nasa.gov/api/citations/20140013285/downloads/20140013285.pdf |access-date=22 June 2024}}</ref> and the height of snow.<ref>{{cite journal |last1=Larson |first1=Kristine M.|last2=Gutmann|first2=Ethan |last3=Zavorotny|first3=Valery|last4=Braun|first4=John|last5=Williams|first5=Mark|last6=Nievinski|first6=Felipe|title=Can we measure snow depth with GPS receivers |journal=Geophysical Research Letters |date=September 2009 |volume=36 |issue=17 |article-number=2009GL039430 |doi=10.1029/2009GL039430 |bibcode=2009GeoRL..3617502L |url=https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2009GL039430 |access-date=23 June 2024|url-access=subscription}}</ref>
== References == {{reflist}}
== Further reading == * {{cite magazine |last1=Zavorotny |first1=Valery U. |last2=Gleason |first2=Scott |last3=Cardellach |first3=Estel |last4=Camps |first4=Adriano |title=Tutorial on Remote Sensing Using GNSS Bistatic Radar of Opportunity |magazine=IEEE Geoscience and Remote Sensing Magazine |volume=2 |issue=4 |year=2014 |pages=8–45 |issn=2168-6831 |doi=10.1109/MGRS.2014.2374220}} * {{cite magazine |title=Environmental Sensing: A Revolution in GNSS Applications |journal=InsideGNSS |year=2014 |last1=Larson |first1=Kristine M. |authorlink1=Kristine M. Larson |last2=Small |first2=Eric E. |last3=Braun |first3=John |last4=Zavorotny |first4=Valery |volume=9 |number=4 |pages=36–46 |url=http://www.insidegnss.com/node/4094 |issn=1559-503X |access-date=2016-03-15 |archive-date=2016-03-15 |archive-url=https://web.archive.org/web/20160315120610/http://www.insidegnss.com/node/4094 |url-status=dead }} *Cardellach, Estel (2015): [http://www.e-gem.eu/file/uploads/eb5e98563d4c6c0e05d6f9cf77607faa.pdf E-GEM – GNSS-R Earth Monitoring; State of the Art Description Document] {{Webarchive|url=https://web.archive.org/web/20201128110014/http://www.e-gem.eu/file/uploads/eb5e98563d4c6c0e05d6f9cf77607faa.pdf |date=2020-11-28 }}. *Emery, William and Camps, Adriano (2017): Introduction to Satellite Remote Sensing 1st Edition Atmosphere, Ocean, Land and Cryosphere Applications, Chapter 6: Remote Sensing Using Global Navigation Satellite System Signals of Opportunity, Elsevier, 20 September 2017, Paperback {{ISBN|9780128092545}}, eBook {{ISBN|9780128092590}} * A complete list of references maintained by the GNSS-R Community can be found at: https://www.ice.csic.es/personal/rius/gnss_r_bibliography/index.html
== External links == * [http://www.e4engineering.com/Articles/297081/Reflecting+on+the+future.htm Reflecting on the future] {{Webarchive|url=https://web.archive.org/web/20070514191723/http://www.e4engineering.com/Articles/297081/Reflecting+on+the+future.htm |date=2007-05-14 }}, The Engineer Online, 28 November 2006. * [http://www.gnssapplications.org/chapter16.html GNSS Applications and Methods], Artech House, September 2009.
{{satellite navigation systems}}
Category:Satellite navigation Category:Remote sensing