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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.^[1] It was also investigated by researchers at NASA Langley Research Center^[2] and is also known as GPS reflectometry.

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.^[3]^[4]

Research applications of space-based GNSS-R are focused in the following areas:

Altimetry ^[5]^[6]
Oceanography (Wave Height and Wind Speed)^[3]
Cryosphere monitoring^[2]^[7]
Soil moisture monitoring^[8]

Data from the Cyclone Global Navigation Satellite System (CYGNSS) mission have been used to investigate many of these areas.

GNSS 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 Dopper 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 ^[9]and the height of snow.^[10]

References

^ Martín-Neira, M. "A passive reflectometry and interferometry system (PARIS): Application to ocean altimetry". ESA Journal. 17 (4): 331–355.
^ ^a ^b Komjathy, A.; Maslanik, J.; Zavorotny, V.U.; Axelrad, P.; Katzberg, S.J. (2000). "Sea ice remote sensing using surface reflected GPS signals". 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). Vol. 7. Honolulu, HI, USA: IEEE. pp. 2855–2857. doi:10.1109/IGARSS.2000.860270. hdl:2060/20020004347. ISBN 978-0-7803-6359-5. S2CID 62042731.
^ ^a ^b Gleason, S.; Hodgart, S.; Yiping Sun; Gommenginger, C.; MacKin, S.; Adjrad, M.; Unwin, M. (2005). "Detection and Processing of bistatically reflected GPS signals from low Earth orbit for the purpose of ocean remote sensing". IEEE Transactions on Geoscience and Remote Sensing. 43 (6): 1229–1241. Bibcode:2005ITGRS..43.1229G. doi:10.1109/TGRS.2005.845643. S2CID 6851145.
^ M. P. Clarizia et al., Analysis of GNSS-R delay-Doppler maps from the UK-DMC satellite over the ocean Archived 2011-06-06 at the Wayback Machine, Geophysical Research Letters, 29 January 2009.
^ Semmling, A. M.; Wickert, J.; Schön, S.; Stosius, R.; Markgraf, M.; Gerber, T.; Ge, M.; Beyerle, G. (2013-07-15). "A zeppelin experiment to study airborne altimetry using specular Global Navigation Satellite System reflections: A ZEPPELIN EXPERIMENT TO STUDY AIRBORNE ALTIMETRY". Radio Science. 48 (4): 427–440. doi:10.1002/rds.20049.
^ Rius, Antonio; Cardellach, Estel; Fabra, Fran; Li, Weiqiang; Ribó, Serni; Hernández-Pajares, Manuel (2017). "Feasibility of GNSS-R Ice Sheet Altimetry in Greenland Using TDS-1". Remote Sensing. 9 (7): 742. Bibcode:2017RemS....9..742R. doi:10.3390/rs9070742. hdl:2117/114540. ISSN 2072-4292.
^ Rivas, M.B.; Maslanik, J.A.; Axelrad, P. (2009-09-22). "Bistatic Scattering of GPS Signals Off Arctic Sea Ice". IEEE Transactions on Geoscience and Remote Sensing. 48 (3): 1548–1553. doi:10.1109/tgrs.2009.2029342. ISSN 0196-2892. S2CID 12668682.
^ Rodriguez-Alvarez, Nereida; Camps, Adriano; Vall-llossera, Mercè; Bosch-Lluis, Xavier; Monerris, Alessandra; Ramos-Perez, Isaac; Valencia, Enric; Marchan-Hernandez, Juan Fernando; Martinez-Fernandez, Jose; Baroncini-Turricchia, Guido; Perez-Gutierrez, Carlos (2011). "Land Geophysical Parameters Retrieval Using the Interference Pattern GNSS-R Technique". IEEE Transactions on Geoscience and Remote Sensing. 49 (1): 71–84. Bibcode:2011ITGRS..49...71R. doi:10.1109/TGRS.2010.2049023. ISSN 0196-2892. S2CID 27516781.
^ Larson, Kristine M.; Ray, Richard D.; Nievinski, Felipe; Freymueller, Jeff (September 2013). "The Accidental Tide Gauge" (PDF). IEEE Geoscience and Remote Sensing Letters. 10 (5): 1200. doi:10.1109/LGRS.2012.2236075. Retrieved 22 June 2024.
^ Larson, Kristine M.; Gutmann, Ethan; Zavorotny, Valery; Braun, John; Williams, Mark; Nievinski, Felipe (September 2009). "Can we measure snow depth with GPS receivers". Geophysical Research Letters. 36 (17). doi:10.1029/2009GL039430. Retrieved 23 June 2024.

External links

Reflecting on the future Archived 2007-05-14 at the Wayback Machine, The Engineer Online, 28 November 2006.
GNSS Applications and Methods, Artech House, September 2009.

[1] Martín-Neira, M. "A passive reflectometry and interferometry system (PARIS): Application to ocean altimetry". ESA Journal. 17 (4): 331–355.

[:0-2] Komjathy, A.; Maslanik, J.; Zavorotny, V.U.; Axelrad, P.; Katzberg, S.J. (2000). "Sea ice remote sensing using surface reflected GPS signals". 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). Vol. 7. Honolulu, HI, USA: IEEE. pp. 2855–2857. doi:10.1109/IGARSS.2000.860270. hdl:2060/20020004347. ISBN 978-0-7803-6359-5. S2CID 62042731.

[reflect-3] Gleason, S.; Hodgart, S.; Yiping Sun; Gommenginger, C.; MacKin, S.; Adjrad, M.; Unwin, M. (2005). "Detection and Processing of bistatically reflected GPS signals from low Earth orbit for the purpose of ocean remote sensing". IEEE Transactions on Geoscience and Remote Sensing. 43 (6): 1229–1241. Bibcode:2005ITGRS..43.1229G. doi:10.1109/TGRS.2005.845643. S2CID 6851145.

[4] M. P. Clarizia et al., Analysis of GNSS-R delay-Doppler maps from the UK-DMC satellite over the ocean Archived 2011-06-06 at the Wayback Machine, Geophysical Research Letters, 29 January 2009.

[5] Semmling, A. M.; Wickert, J.; Schön, S.; Stosius, R.; Markgraf, M.; Gerber, T.; Ge, M.; Beyerle, G. (2013-07-15). "A zeppelin experiment to study airborne altimetry using specular Global Navigation Satellite System reflections: A ZEPPELIN EXPERIMENT TO STUDY AIRBORNE ALTIMETRY". Radio Science. 48 (4): 427–440. doi:10.1002/rds.20049.

[6] Rius, Antonio; Cardellach, Estel; Fabra, Fran; Li, Weiqiang; Ribó, Serni; Hernández-Pajares, Manuel (2017). "Feasibility of GNSS-R Ice Sheet Altimetry in Greenland Using TDS-1". Remote Sensing. 9 (7): 742. Bibcode:2017RemS....9..742R. doi:10.3390/rs9070742. hdl:2117/114540. ISSN 2072-4292.

[7] Rivas, M.B.; Maslanik, J.A.; Axelrad, P. (2009-09-22). "Bistatic Scattering of GPS Signals Off Arctic Sea Ice". IEEE Transactions on Geoscience and Remote Sensing. 48 (3): 1548–1553. doi:10.1109/tgrs.2009.2029342. ISSN 0196-2892. S2CID 12668682.

[8] Rodriguez-Alvarez, Nereida; Camps, Adriano; Vall-llossera, Mercè; Bosch-Lluis, Xavier; Monerris, Alessandra; Ramos-Perez, Isaac; Valencia, Enric; Marchan-Hernandez, Juan Fernando; Martinez-Fernandez, Jose; Baroncini-Turricchia, Guido; Perez-Gutierrez, Carlos (2011). "Land Geophysical Parameters Retrieval Using the Interference Pattern GNSS-R Technique". IEEE Transactions on Geoscience and Remote Sensing. 49 (1): 71–84. Bibcode:2011ITGRS..49...71R. doi:10.1109/TGRS.2010.2049023. ISSN 0196-2892. S2CID 27516781.

[9] Larson, Kristine M.; Ray, Richard D.; Nievinski, Felipe; Freymueller, Jeff (September 2013). "The Accidental Tide Gauge" (PDF). IEEE Geoscience and Remote Sensing Letters. 10 (5): 1200. doi:10.1109/LGRS.2012.2236075. Retrieved 22 June 2024.

[10] Larson, Kristine M.; Gutmann, Ethan; Zavorotny, Valery; Braun, John; Williams, Mark; Nievinski, Felipe (September 2009). "Can we measure snow depth with GPS receivers". Geophysical Research Letters. 36 (17). doi:10.1029/2009GL039430. Retrieved 23 June 2024.

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+{{short description|Earth observation technology}}
-'''GNSS reflectometry''' involves making measurements from the reflections from the Earth of navigation signals from [[GNSS|Global Navigation Satellite Systems]] such as [[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|NASA Langley research centre]]<ref name=":0">{{Cite journal|last=Komjathy|first=A.|last2=Maslanik|first2=J.|last3=Zavorotny|first3=V.U.|last4=Axelrad|first4=P.|last5=Katzberg|first5=S.J.|date=2000|title=Sea ice remote sensing using surface reflected GPS signals|journal=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)|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|hdl-access=free}}</ref> and is also known as ''GPS reflectometry''. Research applications of GNSS-R are found in
+[[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 [[GNSS|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''.
+GNSS reflectometry is [[Passive radar|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 effect|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|dielectric properties]] and its [[Surface roughness|roughness state]]. Further derivation of geophysical information rely on these measurements.
-* Altimetry <ref>{{Cite journal|last=Semmling|first=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}}</ref>
+GNSS reflectometry is a [[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 [[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|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>
+Research applications of space-based GNSS-R are focused in the following areas:
+* 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|last=Rivas|first=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|issn=0196-2892}}</ref>
+* 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|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>
-* Soil moisture monitoring
+Data from the [[Cyclone Global Navigation Satellite System (CYGNSS)]] mission have been used to investigate many of these areas.
+[[File:GPS reflections cartoon.png|thumb|300px|Geometry of GNSS-IR]]
+'''GNSS 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 Dopper 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 |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 |doi=10.1029/2009GL039430 |url=https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2009GL039430 |access-date=23 June 2024}}</ref>
-GNSS reflectometry is [[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 [[pseudorange]] measurement) and the rate of change of the range between satellite and observer (the [[Doppler effect|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|dielectric properties]] and its [[Surface roughness|roughness state]]. Further derivation of geophysical information rely on these measurements.
-GNSS-Reflectometry works as a [[Bistatic radar|bi-static radar]], where transmitter and receiver and separated by a significant distance. Since in GNSS-Reflectometry one receiver simultaneously can track multiple transmitters (i.e. GNSS satellites), the system also has nature of [[Multistatic radar|multi-static radar.]] The receiver of the reflected GNSS signal can be of different kind: Stationary stations, ship measurements, air planes or satellites, like the [[UK-DMC|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.}}</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], [[Geophysical Research Letters]], 29 January 2009.</ref>
 == References ==
@@ Line 15: / Line 26: @@
 == 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 |last=Larson |first=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}}
+* {{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].
+*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], The Engineer Online, 28 November 2006.
+* [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.