Editing Earth observation satellite

{{Short description|Satellite specifically designed to observe Earth from orbit}}
[[File:A-Train w-Time2013 Web.jpg|thumb|upright=1.40|Six Earth observation satellites comprising the [[A-train (satellite constellation)|A-train]] satellite constellation as of 2014.]]
{{Spaceflight sidebar}}

An '''Earth observation satellite''' or '''Earth remote sensing satellite''' is a [[satellite]] used or designed for [[Earth observation]] (EO) from [[orbit]], including [[spy satellite]]s and similar ones intended for non-military uses such as [[environmental monitoring]], [[meteorology]], [[cartography]] and others. The most common type are '''Earth imaging satellites''', that take [[satellite image]]s, analogous to [[aerial photograph]]s; some EO satellites may perform [[remote sensing]] without forming pictures, such as in [[GNSS radio occultation]].

The first occurrence of satellite remote sensing can be dated to the launch of the first artificial satellite, [[Sputnik 1]], by the Soviet Union on October 4, 1957.<ref name="Tatem"/> Sputnik 1 sent back radio signals, which scientists used to study the [[ionosphere]].<ref name="Kuznetsov">{{cite journal|last1=Kuznetsov|first1=V.D.|last2=Sinelnikov|first2=V.M.|last3=Alpert|first3=S.N.|title=Yakov Alpert: Sputnik-1 and the first satellite ionospheric experiment|journal=Advances in Space Research|date=June 2015|volume=55|issue=12|pages=2833–2839|doi=10.1016/j.asr.2015.02.033|bibcode=2015AdSpR..55.2833K}}</ref>
The United States Army Ballistic Missile Agency launched the first American satellite, [[Explorer 1]], for NASA's Jet Propulsion Laboratory on January 31, 1958. The information sent back from its radiation detector led to the discovery of the Earth's [[Van Allen radiation belt]]s.<ref name="Allen">{{cite web|title=James A. Van Allen|url=http://www.nmspacemuseum.org/halloffame/detail.php?id=86|website=nmspacemuseum.org|publisher=New Mexico Museum of Space History|access-date=14 May 2018}}</ref> The [[TIROS-1]] spacecraft, launched on April 1, 1960, as part of NASA's [[Television Infrared Observation Satellite]] (TIROS) program, sent back the first television footage of weather patterns to be taken from space.<ref name="Tatem"/>

In 2008, more than 150 Earth observation satellites were in orbit, recording data with both passive and active sensors and acquiring more than 10 terabits of data daily.<ref name="Tatem">{{cite journal |last1=Tatem|first1=Andrew J.|last2=Goetz|first2=Scott J.|last3=Hay|first3=Simon I.|title=Fifty Years of Earth-observation Satellites|journal=American Scientist|date=2008|volume=96|issue=5|pages=390–398 |doi=10.1511/2008.74.390|pmid=19498953|pmc=2690060}}</ref> By 2021, that total had grown to over 950, with the largest number of satellites operated by US-based company [[Planet Labs]].<ref>{{cite web|title=How many Earth observation satellites are orbiting the planet in 2021?|date=18 August 2021 |url=https://www.pixalytics.com/eo-sats-2021/}}</ref>

Most [[Earth]] observation satellites carry instruments that should be operated at a relatively low altitude. Most orbit at altitudes above {{convert|500|to|600|km|mi|sp=us}}. Lower orbits have significant [[Drag (physics)|air-drag]], which makes frequent orbit [[reboost]] maneuvers necessary. The Earth observation satellites [[European Remote-Sensing Satellite|ERS-1, ERS-2]] and [[Envisat]] of [[European Space Agency]] as well as the [[MetOp]] spacecraft of [[EUMETSAT]] are all operated at altitudes of about {{cvt|800|km|mi}}. The [[PROBA|Proba-1]], [[Proba-2]] and [[Soil Moisture and Ocean Salinity satellite|SMOS]] spacecraft of European Space Agency are observing the Earth from an altitude of about {{cvt|700|km|mi}}. The Earth observation satellites of UAE, [[DubaiSat-1]] & [[DubaiSat-2]] are also placed in [[Low Earth orbit]]s (LEO) orbits and providing [[satellite imagery]] of various parts of the Earth.<ref>{{cite web|title=DubaiSat-2, Earth Observation Satellite of UAE|url=http://mbrsc.ae/en/page/dubai-sat-2|publisher=Mohammed Bin Rashid Space Centre|access-date=2016-07-04|archive-date=2019-01-17|archive-url=https://web.archive.org/web/20190117074703/https://mbrsc.ae/en/page/dubai-sat-2|url-status=dead}}</ref><ref>{{cite web|title=DubaiSat-1, Earth Observation Satellite of UAE|url=http://mbrsc.ae/en/page/dubai-sat-1|publisher=Mohammed Bin Rashid Space Centre|access-date=2016-07-04|archive-date=2016-03-04|archive-url=https://web.archive.org/web/20160304080451/http://mbrsc.ae/en/page/dubai-sat-1|url-status=dead}}</ref>

To get global coverage with a low orbit, a [[polar orbit]] is used. A low orbit will have an orbital period of roughly 100 minutes and the Earth will rotate around its polar axis about 25° between successive orbits. The [[ground track]] moves towards the west 25° each orbit, allowing a different section of the globe to be scanned with each orbit. Most are in [[Sun-synchronous orbit]]s.

A [[geostationary orbit]], at {{cvt|36000|km|mi}}, allows a satellite to hover over a constant spot on the earth since the orbital period at this altitude is 24 hours. This allows uninterrupted coverage of more than 1/3 of the Earth per satellite, so three satellites, spaced 120° apart, can cover the whole Earth. This type of orbit is mainly used for [[Weather satellite|meteorological satellites]].

== History ==
{{See also|Remote sensing#History}}
[[File:Lednickie Lake (Poland) seen by the American reconnaissance satellite Corona 98 (KH-4A 1023) (1965-08-23).png|thumb|Lednickie Lake (Poland) seen by the American reconnaissance satellite [[CORONA (satellite)|CORONA]] 98, 1965]]
[[Herman Potočnik]] explored the idea of using orbiting spacecraft for detailed peaceful and military observation of the ground in his 1928 book, ''The Problem of Space Travel''. He described how the special conditions of space could be useful for scientific experiments. The book described [[geostationary]] satellites (first put forward by [[Konstantin Tsiolkovsky]]) and discussed communication between them and the ground using radio, but fell short of the idea of using satellites for mass broadcasting and as telecommunications relays.<ref>{{Cite web|url=http://www.sasmac.cn/portal/html/fc4f335929b0df0d0129b0e348f90003/_content/10_07/09/1278668367217.html|title=Introduction to satellite|website=www.sasmac.cn|date=2 September 2016|access-date=19 April 2017|archive-date=16 September 2016|archive-url=https://web.archive.org/web/20160916064537/http://www.sasmac.cn/portal/html/fc4f335929b0df0d0129b0e348f90003/_content/10_07/09/1278668367217.html|url-status=dead}}</ref>

The onset of the [[Cold War]] prompted the rapid development of [[Launch vehicle|Satellite launch systems]] and camera technology capable of sufficient Earth observation to garner intelligence on enemy military infrastructure and evaluate nuclear posture.<ref>{{Cite web |title=Cold War in Space: Top Secret Reconnaissance Satellites Revealed |url=https://www.nationalmuseum.af.mil/Visit/Museum-Exhibits/Fact-Sheets/Display/Article/195923/cold-war-in-space-top-secret-reconnaissance-satellites-revealed/ |website=nationalmuseum.af.mil}}</ref> Following the U-2 incident in 1960, which highlighted the risks of aerial spying, the U.S. accelerated surveillance satellite programs like [[CORONA (satellite)|CORONA]]. Satellites largely replaced aircraft overflights for surveillance after 1960.<ref>{{Cite journal |last=Muszyński-Sulima |first=Wawrzyniec |date=2023-06-30 |title=Cold War in Space: Reconnaissance Satellites and US-Soviet Security Competition |url=https://journals.openedition.org/ejas/20427 |journal=European Journal of American Studies |language=en |volume=18 |issue=2 |doi=10.4000/ejas.20427 |issn=1991-9336|doi-access=free }}</ref>

== Applications ==
=== Weather ===
{{Main|Weather satellite}}
{{See also|Satellite temperature measurements}}
[[File:GOES 8 Spac0255.jpg|thumb|300px|[[GOES-8]], a [[United States]] weather satellite.]]

A weather satellite is a type of [[satellite]] that is primarily used to monitor the [[weather]] and [[climate]] of the [[Earth]].<ref name="NESDIS">NESDIS, [http://www.nesdis.noaa.gov/satellites.html Satellites.] {{Webarchive|url=https://web.archive.org/web/20080704195947/http://www.nesdis.noaa.gov/satellites.html |date=2008-07-04 }} Retrieved on 4 July 2008 {{PD-notice}}</ref> These meteorological satellites, however, see more than [[cloud]]s and cloud systems. City lights, [[fire]]s, effects of [[pollution]], [[auroral light|aurora]]s, [[Dust storm|sand and dust storms]], [[snow]] cover, [[ice]] mapping, boundaries of [[ocean current]]s, [[energy]] flows, etc., are other types of environmental information collected using weather satellites.

Weather satellite images helped in monitoring the volcanic ash cloud from [[Mount St. Helens]] and activity from other volcanoes such as [[Mount Etna]].<ref>NOAA, [http://www.spaceref.com/news/viewpr.html?pid=15216 NOAA Satellites, Scientists Monitor Mt. St. Helens for Possible Eruption.] {{Webarchive|url=https://archive.today/20120910225555/http://www.spaceref.com/news/viewpr.html?pid=15216 |date=2012-09-10 }} Retrieved on 4 July 2008 {{PD-notice}}</ref> Smoke from fires in the western United States such as [[Colorado]] and [[Utah]] have also been monitored.

=== Environmental monitoring ===
[[File:Blue Marble 2002.png|thumb|upright=1.40|left|Composite satellite image of the Earth, showing its entire surface in [[equirectangular projection]]]]

Other environmental satellites can assist [[environmental monitoring]] by detecting changes in the Earth's vegetation, atmospheric trace gas content, sea state, ocean color, and ice fields. By monitoring vegetation changes over time, droughts can be monitored by comparing the current vegetation state to its long term average.<ref>NASA, [http://earthobservatory.nasa.gov/Drought/ Drought.] {{webarchive |url=https://web.archive.org/web/20080819121047/http://earthobservatory.nasa.gov/Drought/|date=19 August 2008}} Retrieved on 4 July 2008 {{PD-notice}}</ref> For example, the 2002 oil spill off the northwest coast of [[Spain]] was watched carefully by the European [[Envisat|ENVISAT]], which, though not a weather satellite, flies an instrument (ASAR) which can see changes in the sea surface. Anthropogenic emissions can be monitored by evaluating data of tropospheric NO<sub>2</sub> and SO<sub>2</sub>.{{cn|date=March 2024}}

These types of satellites are almost always in [[Sun-synchronous orbit|Sun-synchronous]] and [[Frozen orbit|"frozen"]] orbits. A Sun-synchronous orbit passes over each spot on the ground at the same time of day, so that observations from each pass can be more easily compared, since the Sun is in the same spot in each observation. A [[Frozen orbit|"frozen"]] orbit is the closest possible orbit to a circular orbit that is undisturbed by the [[Geopotential model|oblateness of the Earth]], gravitational attraction from the Sun and Moon, [[solar radiation pressure]], and [[air drag]].{{cn|date=March 2024}}

=== Mapping ===
Terrain can be mapped from space with the use of satellites, such as [[Radarsat-1]]<ref>Grunsky, E.C. [https://ieeexplore.ieee.org/document/1024985/ The use of multi-beam Radarsat-1 satellite imagery for terrain mapping.] Retrieved on 4 July 2008</ref> and [[TerraSAR-X]].

==International regulations==
[[File:RapidEye Satellites Artist Impression.jpg|thumb|250px|[[RapidEye]] ''Earth exploration-satellite system'' in action around the Earth.]]

According to the [[International Telecommunication Union]] (ITU), '''Earth exploration-satellite service''' (also: '''Earth exploration-satellite radiocommunication service''') is – according to ''Article 1.51'' of the [[ITU Radio Regulations]] (RR)<ref>ITU Radio Regulations, Section IV. Radio Stations and Systems – Article 1.51, definition: ''earth exploration-satellite service / earth exploration-satellite radiocommunication service''</ref> – defined as:
<blockquote>A [[radiocommunication service]] between [[earth station]]s and one or more [[radio space station|space station]]s, which may include links between space stations, in which:
*information relating to the characteristics of the Earth and its natural phenomena, including data relating to the state of the environment, is obtained from passive or [[Radar|active sensors]] on [[satellites]]; 
*similar information is collected from airborne or Earth-based platforms;
*such information may be distributed to earth stations within the system concerned;
*platform interrogation may be included.
This service may also include feeder links necessary for its operation.</blockquote>

===Classification===
This ''radiocommunication service'' is classified in accordance with ''ITU Radio Regulations'' (article 1) as follows:{{cn|date=March 2024}} <br />
[[Fixed service]] (article 1.20)
*[[Fixed-satellite service]] (article 1.21)
*[[Inter-satellite service]] (article 1.22)
*<span style="color:#060;">'''Earth exploration-satellite service'''</span>
**[[Meteorological-satellite service]] (article 1.52)

===Frequency allocation===
The allocation of radio frequencies is provided according to ''Article 5'' of the ITU Radio Regulations (edition 2012).<ref>''ITU Radio Regulations, CHAPTER II – Frequencies, ARTICLE 5 Frequency allocations, Section IV – Table of Frequency Allocations''</ref>

In order to improve harmonisation in spectrum utilisation, the majority of service-allocations stipulated in this document were incorporated in national Tables of Frequency Allocations and Utilisations which is with-in the responsibility of the appropriate national administration. The allocation might be primary, secondary, exclusive, and shared.
*primary allocation:  is indicated by writing in capital letters (see example below)
*secondary allocation: is indicated by small letters
*exclusive or shared utilization: is within the responsibility of administrations 
However, military usage, in bands where there is civil usage, will be in accordance with the ITU Radio Regulations.

; Example of [[frequency allocation]]:
{| class=wikitable
|- bgcolor="#CCCCCC" align="center"
|align="center" colspan="3"| '''Allocation to services'''
|- align="center"
| [[International Telecommunication Union region|Region 1]] || Region 2 || Region 3
|-
|colspan="3"|401-402&nbsp;MHz &nbsp;&nbsp; &nbsp;&nbsp; METEOROLOGICAL AIDS<br />
::::: SPACE OPERATION (space-to-Earth) <br />EARTH EXPLORATION-SATELLITE (Earth-to-space) <br />METEOROLOGICAL-SATELLITE (Earth-to-space)<br />Fixed <br />Mobile except aeronautical mobile
|-
|colspan="3"|13.4-13.75&nbsp;GHz &nbsp;&nbsp;'''EARTH EXPLORATION-SATELLITE (active)'''<br />
::::: RADIOLOCATION<br />SPACE RESEARCH<br />Standard frequency and time signal-satellite (Earth-to-space) 
|-
|}

== See also ==
{{main cat|Earth observation satellites}}
* [[Committee on Earth Observation Satellites]]
* [[Data collection satellite]]
* [[Earth observation]]
* [[Earth observation satellites transmission frequencies]]
* [[Earth Observing System]] - a NASA program comprising a series of satellite missions
* [[First images of Earth from space]]
*[[ESA Φ-lab]]
* [[Satellite imagery#Imaging satellites|Imaging satellites]]
* [[List of Earth observation satellites]]
* [[Space telescope]]
* [[Satellite imagery]]
*[[GNSS radio occultation]]
*[[Microwave radiometer#Spaceborne]]
*[[Radar earth observation satellite]]
**[[Radar imaging]]
**[[Synthetic-aperture radar]]
***[[Interferometric synthetic-aperture radar]]
*[[Satellite altimetry]]

== References ==
{{Reflist}}

== External links ==
* [https://directory.eoportal.org/web/eoportal/satellite-missions EO Portal directory] {{Webarchive|url=https://web.archive.org/web/20130923090413/https://directory.eoportal.org/web/eoportal/satellite-missions |date=2013-09-23 }}
* [https://web.archive.org/web/20160406191132/http://www.campevans.org/_CE/html/tiros1-2.html The TIROS I and II Ground Control Station where the first Earth Observing Satellite (TIROS I) sent it first photos]

{{Spaceflight}}
{{Authority control}}

{{DEFAULTSORT:Earth Observation Satellite}}
[[Category:Earth observation satellites| ]]
[[Category:Satellites by type]]
[[Category:Satellite imagery]]
[[Category:Earth imaging satellites|*]]