View source for Earth:GNSS reflectometry

{{short description|Earth observation technology}}
[[File:GNSS-R system diagram.svg|thumb|300px|GNSS-R system diagram]]
'''GNSS reflectometry''' (or GNSS-R) involves making measurements from the reflections from the Earth of navigation signals from Global Navigation Satellite Systems such as [[Engineering:GPS|GPS]]. The idea of using reflected GNSS signal for earth observation became more and more popular in the mid-1990s 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 GNSS-R are found 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|journal=Radio Science|language=en|volume=48|issue=4|pages=427–440|doi=10.1002/rds.20049|url=http://gfzpublic.gfz-potsdam.de/pubman/item/escidoc:321321|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 |doi=10.3390/rs9070742 |bibcode=2017RemS....9..742R |issn=2072-4292|doi-access=free |hdl=2117/114540 |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.|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|s2cid=12668682|issn=0196-2892}}</ref>
* 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 |url=https://ieeexplore.ieee.org/document/5475216 |journal=IEEE Transactions on Geoscience and Remote Sensing |volume=49 |issue=1 |pages=71–84 |doi=10.1109/TGRS.2010.2049023 |bibcode=2011ITGRS..49...71R |s2cid=27516781 |issn=0196-2892}}</ref>

GNSS reflectometry is [[Engineering:Passive radar|passive sensing]] that takes advantage of and relies on separate active sources - the satellites generating the navigation signals. For this, the GNSS receiver measures the signal delay from the satellite (the [[Earth:Pseudorange|pseudorange]] measurement) and the rate of change of the range between satellite and observer (the [[Physics:Doppler effect|Doppler]] measurement). The surface area of the reflected GNSS signal also provides the two parameters time delay and [[Physics:Frequency|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 [[Physics:Dielectric|dielectric properties]] and its [[Physics:Surface roughness|roughness state]]. Further derivation of geophysical information rely on these measurements.

GNSS reflectometry works as a [[Physics:Bistatic radar|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 [[Physics:Multistatic radar|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 [[Engineering:Disaster Monitoring Constellation|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 [[Astronomy:Low Earth orbit|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 }}, [[Earth:Geophysical Research Letters|Geophysical Research Letters]], 29 January 2009.</ref>

== See also ==
* Cyclone Global Navigation Satellite System (CYGNSS)

== References ==
<References/>

== 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. |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, 20th 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.

[[Category:Satellite navigation]]
[[Category:Remote sensing]]

{{Sourceattribution|GNSS reflectometry}}