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{{Short description|Ground-based elements of a spacecraft system}}
[[File:Ground segment.png|thumb|upright=1.8|A simplified spacecraft system. Dotted orange arrows denote radio links; solid black arrows denote ground network links. (
[[File:Ground systems.png|thumb|upright=1.8|Select ground segment facilities worldwide]]
A '''ground segment''' consists of all the ground-based elements of a [[spaceflight|space]] [[system]] used by operators and support personnel, as opposed to the [[Satellite space segment|space segment]] and user segment.<ref name=jsat_intro/><ref name=elbert/>{{rp|1}} The ground segment enables management of a spacecraft, and distribution of [[Payload (computing)|payload data]] and [[telemetry]] among interested parties on the ground. The primary elements of a ground segment are:
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Signals to be [[Transmission (telecommunications)|uplink]]ed to a spacecraft must first be extracted from ground [[network packet]]s, [[Line code|encoded]] to [[baseband]], and [[modulation|modulated]],<ref name=JPL_10/> typically onto an [[intermediate frequency]] (IF) carrier, before being [[Frequency mixer|up-converted]] to the assigned [[radio frequency]] (RF) band. The RF signal is then [[RF power amplifier|amplified]] to high power and carried via [[waveguide (electromagnetism)|waveguide]] to an [[Parabolic antenna|antenna]] for transmission. In colder climates, electric heaters or hot air blowers may be necessary to prevent ice or snow buildup on the [[parabolic reflector|parabolic dish]].
Received ("downlinked") signals are passed through a [[low-noise amplifier]] (often located in the antenna hub to minimize the distance the signal must travel) before being down-converted to IF; these two functions may be combined in a [[low-noise block downconverter]]. The IF signal is then [[demodulation|demodulated]], and the data stream extracted via [[Self-clocking signal|bit]] and [[frame synchronization]] and decoding.<ref name=JPL_10/> Data errors, such as those caused by signal [[Degradation (telecommunications)|degradation]], are [[forward error correction|identified and corrected]] where possible.<ref name=JPL_10/> The extracted data stream is then [[network packet|packet]]ized or saved to files for transmission on ground networks. Ground stations may temporarily [[computer data storage|store]] received telemetry for later playback to control centers, often when ground network bandwidth is not sufficient to allow real-time transmission of all received telemetry. They may support [[delay-tolerant networking]].
A single spacecraft may make use of multiple RF bands for different telemetry, command, and payload data [[stream (computing)|streams]], depending on bandwidth and other requirements.
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# Conversion of raw data to [[calibration|calibrated]] (engineering) values, and calculation of derived parameters{{r|Fortescue-GS}}{{rp|483}}
# Limit and constraint checking (which may generate alert notifications)<ref name=ley/>{{rp|479}}{{r|Fortescue-GS}}{{rp|484}}
# Generation of telemetry displays, which may be take the form of tables, [[Plot (graphics)|plots]] of parameters against each other or over time, or synoptic displays (sometimes called mimics)
A spacecraft [[database]] provided by the spacecraft manufacturer is called on to provide information on telemetry frame formatting, the positions and frequencies of parameters within frames, and their associated mnemonics, calibrations, and soft and hard limits.{{r|Fortescue-GS}}{{rp|486}} The contents of this database—especially calibrations and limits—may be updated periodically to maintain consistency with onboard software and operating procedures; these can change during the life of a mission in response to [[upgrade]]s, hardware degradation in the [[space environment]], and changes to mission parameters.{{r|Fortescue-Telecom}}{{rp|399}}
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Mission control centers may rely on "offline" (i.e., non-[[real-time computing|real-time]]) [[data processing]] subsystems to handle analytical tasks<ref name=ley/>{{rp|21}}{{r|Fortescue-GS}}{{rp|487}} such as:
* [[Orbit determination]] and [[Orbital maneuver|maneuver]] planning<ref name=JPL_13/>
* [[Conjunction (astronomy)|Conjunction]] assessment and [[Collision avoidance (spacecraft)|collision avoidance]] planning{{r|Fortescue-GS}}{{rp|
* Mission planning and scheduling{{r|Fortescue-GS}}{{rp|
* On-board [[memory management]]<ref name=uhlig/>{{rp|247–249}}
* Short- and long-term [[trend analysis]]<ref name=ley/>{{rp|21}}
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Remote terminals are interfaces on ground networks, separate from the mission control center, which may be accessed by [[payload]] controllers, telemetry analysts, [[scientific instrument|instrument]] and [[outline of space science|science]] teams, and [[technical support|support]] personnel, such as [[system administrator]]s and [[software development]] teams. They may be receive-only, or they may transmit data to the ground network.
Terminals used by [[telecommunications service|service]] customers, including [[Internet service provider|ISPs]] and [[Direct-broadcast satellite|end users]], are collectively called the "user segment", and are typically distinguished from the ground segment. User terminals including [[satellite television]] systems and [[satellite phone]]s communicate directly with spacecraft, while other types of user terminals rely on the ground segment for data receipt, transmission, and processing.
===Integration and test facilities===
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===Ground networks===
Ground [[computer network|network]]s handle data transfer and voice communication between different elements of the ground segment.{{r|Fortescue-GS}}{{rp|
Reliability is a particularly important consideration for [[critical system]]s, with [[uptime]] and [[mean time to recovery]] being of paramount concern. As with other aspects of the spacecraft system, [[redundancy (engineering)|redundancy]] of network components is the primary means of achieving the required system reliability.
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{{reflist|2|refs=
<ref name=aiaa>{{cite web|title=Operations Staffing|url=https://info.aiaa.org/tac/SMG/SOSTC/Shared%20Documents/Operations_Staffing%2C_Rev._2.doc|website=Satellite Operations Best Practice Documents|publisher=Space Operations and Support Technical Committee, [[American Institute of Aeronautics and Astronautics]]|access-date=28 December 2015|archive-date=6 October 2016|archive-url=https://web.archive.org/web/20161006104457/https://info.aiaa.org/tac/SMG/SOSTC/Shared|url-status=dead}}</ref>
<ref name=elbert>{{cite book|last1=Elbert|first1=Bruce|title=The Satellite Communication Ground Segment and Earth Station Handbook|date=2014|publisher=Artech House|isbn=978-1-60807-673-4|edition=2nd|url=https://books.google.com/books?id=eepZBAAAQBAJ&pg=PA141|page=141}}</ref>
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<ref name=ley>{{cite book|editor1-last=Ley|editor1-first=Wilfried|editor2-last=Wittmann|editor2-first=Klaus|editor3-last=Hallmann|editor3-first=Willi|title=Handbook of Space Technology|date=2008|publisher=[[John Wiley & Sons|Wiley]]|isbn=978-0470742419|url=https://books.google.com/books?isbn=0470742410|access-date=30 December 2015}}</ref>
<ref name=matthews>{{cite journal|last1=Matthews|first1=Anthony J.|title=A ground cost model (G-COST) for military systems |journal=AIAA International Communications Satellite Systems Conference|date=February 25, 1996|pages=1416–1421|doi=10.2514/6.1996-1111|publisher=[[American Institute of Aeronautics and Astronautics]]}}</ref>
<ref name=reiniger>{{cite web|last1=Reiniger|first1=Klaus|last2=Diedrich|first2=Erhard|last3=Mikusch|first3=Eberhard|title=Aspects of Ground Segment Design for Earth observation missions|url=https://www.ffg.at/getdownload.php?id=230|publisher=Alpbach Summer School|format=PDF|date=August 2006|access-date=2015-11-06|archive-date=2020-07-09|archive-url=https://web.archive.org/web/20200709111558/https://www.ffg.at/getdownload.php%3Fid%3D230|url-status=dead}}</ref>
<ref name=tirro>{{cite book|editor1-last=Tirró|editor1-first=Sebastiano|title=Satellite Communication Systems Design|date=1993|publisher=[[Springer Science+Business Media]]|isbn=1461530067|url=https://books.google.com/books?isbn=1461530067|access-date=8 January 2016}}</ref>
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<ref name=deWeck>{{cite book|last1=de Weck|first1=Olivier|last2=de Neufville|first2=Richard|last3=Chang|first3=Darren|last4=Chaize|first4=Mathieu|title=Communications Satellite Constellations|publisher=[[Massachusetts Institute of Technology]]|url=http://ardent.mit.edu/real_options/de%20Weck%20System%20Study/unit1_summary.pdf|chapter=Technical Success and Economic Failure|access-date=2016-01-12|archive-url=https://web.archive.org/web/20050509094747/http://ardent.mit.edu/real_options/de%20Weck%20System%20Study/unit1_summary.pdf|archive-date=2005-05-09|url-status=dead}}</ref>
<ref name=wood>{{cite conference | url = http://personal.ee.surrey.ac.uk/Personal/L.Wood/publications/lloyd-wood-isu-summer-06-constellations-talk.pdf | title = Introduction to satellite constellations: Orbital types, uses and related facts | first = Lloyd | last = Wood | date = July 2006 | conference = [[International Space University|ISU]] Summer Session | access-date = 17 November 2015 | archive-date = 21 February 2019 | archive-url = https://web.archive.org/web/20190221003914/http://personal.ee.surrey.ac.uk/Personal/L.Wood/publications/lloyd-wood-isu-summer-06-constellations-talk.pdf | url-status = dead }}</ref>
<!--<ref name=zandbergenGS>{{cite|last1=Zandbergen|first1=B.T.C.|title=Cost Estimation for Space System Elements, v.1.02|section=Ground segm. & ops. cost |url=http://lr.tudelft.nl/fileadmin/Faculteit/LR/Organisatie/Afdelingen_en_Leerstoelen/Afdeling_SpE/Space_Systems_Eng./Space_Links/doc/Space_mission_cost_v1.02.xls|access-date=8 January 2016|format=Excel spreadsheet}}</ref>-->
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