Ground segment: Difference between revisions

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{{shortShort description|groundGround-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. (UserCustomer terminals typically rely on only one of the indicated paths for access to space-segment resources.)]]

[[File:Ground systems.png|thumb|Worldwideupright=1.8|Select ground segment facilities worldwide]]

A '''ground segment''' consists of all the ground-based elements of a [[spacecraftspaceflight|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:

* [[Ground station|Ground (or Earth) station]]s, which provide radio interfaces with spacecraft<ref name=elbert/>{{rp|142}}

* [[Mission control center|Mission control (or operations) center]]s, from which spacecraft are managed<ref name=ley/>{{rp|20}}

* Ground [[Telecommunications network|network]]s, which connect the other ground elements to one another<ref name=elbert/>{{rp|142}}<ref name=esa_ers/>

* Remote [[terminal (telecommunication)|terminal]]s, used by support personnel<ref name=elbert/>{{rp|142}}

* Spacecraft [[System integration testing|integration and test]] facilities

* [[Spaceport|Launch facilities]]<ref name=ley/>{{rp|21}}

* Ground [[Telecommunications network|network]]s, which connectallow for communication between the other ground elements to one another<ref name=elbert/>{{rp|142}}<ref name=esa_ers/>

These elements are present in nearly all space missions, whether commercial, military, or scientific. They may be located together or separated geographically, and they may be operated by different parties.<ref name=esa_copernicus/><ref name=reiniger/>{{rp|25}} Some elements may support multiple spacecraft simultaneously.{{r|Fortescue-GS}}{{rp|480,481}}

These elements are present in nearly all space missions, whether [[Commercial use of space|commercial]], [[Militarisation of space|military]], or [[Outline of space science|scientific]]. They may be located together or separated geographically, and they may be operated by different parties.<ref name=esa_copernicus/><ref name=reiniger/>{{rp|25}} Some elements may support multiple spacecraft simultaneously.{{r|Fortescue-GS}}{{rp|480,481}}

==Elements==

===Ground stations===

{{see alsomain|Ground station}}

[[File:Rio antennas.jpg|thumb|upright=1.20|Radio dishes at an [[Embratel]] earth station in [[Tanguá]], Brazil]]

Ground stations provide radio [[telecommunications link|interfaces]] between the space and ground segments for telemetry, tracking, and command (TT&C), as well as payload data transmission and reception.<ref name=reiniger/>{{rp|4}}<ref name=jsat_RF/><ref name=jsat_teleport/> Tracking networks, such as [[NASA]]'s [[Near Earth Network]] and [[Space Network]], handle communications with multiple spacecraft through [[time-sharing]].<ref name=ley/>{{rp|22}}

Ground station equipment may be [[remote monitoring and control|monitored and controlled remotely]], often via [[Serial communication|serial]] and/or [[Internet Protocol|IP]] interfaces. There are typicallyoften backup stations from which radio contact can be maintained if there is a problem at the primary ground station which renders it unable to operate, such as a natural disaster. Such contingencies are considered in a [[United States federal government continuity of operations|Continuity of Operations]] plan.

====Transmission and reception====

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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====Tracking and ranging====

Ground stations must [[radio direction finder|track]] spacecraft in order to [[main lobe|point their antennas]] properly, and must account for [[Doppler effect|Doppler shifting]] of RF frequencies due to the motion of the spacecraft. Ground stations may also perform automated [[ranging]]; ranging tones may be [[multiplexing|multiplexed]] with command and telemetry signals. Ground station tracking and ranging data are passed to the control center along with spacecraft telemetry, where they are often used in [[orbit determination]].

===Mission control centers===

{{see also|List of mission control centers}}

[[File:JPLControlRoom.jpg|thumb|upright=1.53|Control center at NASA's [[Jet Propulsion Laboratory]]]]

Mission control centers process, analyze, and distribute spacecraft [[telemetry]], and issue [[telecommand|command]]s, data [[upload]]s, and [[Software maintenance|software updates]] to spacecraft. For crewed spacecraft, mission control manages voice and video communications with the crew. Control centers may also be responsible for [[configuration management]] and data [[backup|archival]].{{r|Fortescue-GS}}{{rp|483}} As with ground stations, there are typicallyoften backup control facilities available to support continuity of operations.

====Telemetry processing====

Control centers use telemetry to determine the status of a spacecraft and its systems.<ref name=ley/>{{rp|485}} Housekeeping, diagnostic, science, and other types of telemetry may be carried on separate [[virtual circuit|virtual channel]]s. Flight control software performs the initial processing of received telemetry, including:

# Separation and distribution of [[virtual circuit|virtual channel]]schannels<ref name=ley/>{{rp|393}}

# [[Connection-oriented communication|Time-ordering and gap-checking]] of received [[Frame (networking)|frame]]s (gaps may be filled by commanding a re-transmissionretransmission)

# [[Commutation (telemetry)|Decommutation]] of parameter values,<ref name=JPL_10/> and association of these values with parameter names called [[mnemonic]]s

# 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 tabular,take graphicalthe form of tables, ([[Plot (graphics)|plots]] of parameters against each other or over time), or synoptic displays (interface-orientedsometimes graphicscalled mimics). – essentially [[flow diagram]]s that present component or subsystem interfaces and their state{{r|Fortescue-GS}}{{rp|484}}

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}}

====Commanding====

Commands sent to spacecraft are formatted according to the spacecraft database, and are [[data validation|validated]] against the database before being transmitted via a [[#Ground stations|ground station]]. Commands may be issued manually in real time, or they may be part of automated or semi-automated procedures uploaded in their entirety.{{r|Fortescue-GS}}{{rp|485}} Typically, commands successfully received by the spacecraft are acknowledged in telemetry,{{r|Fortescue-GS}}{{rp|485}} and a command counter is maintained on the spacecraft and at the ground to ensure synchronization. In certain cases, [[closed-loop control]] may be performed. Commanded activities may pertain directly to mission objectives, or they may be part of [[Maintenance, repair, and operations|housekeeping]]. Commands (and telemetry) may be [[encryption|encrypted]] to prevent unauthorized access to the spacecraft or its data.

Spacecraft procedures are generally developed and tested against a spacecraft [[simulator]] prior to use with the actual spacecraft.{{r|Fortescue-T&C}}{{rp|488}}

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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|478-479478–479}}

* Mission planning and scheduling{{r|Fortescue-GS}}{{rp|489-491489–491}}

* 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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====Staffing====

Control centers may be [[shift work|continuously]] or [[Business hours|regularly]] staffed by [[flight controller]]s. Staffing is typically greatest during the [[Launch and Early Orbit phase|early phases]] of a mission,<ref name=ley/>{{rp|21}} and during [[mission-critical|critical]] procedures and periods, such as when a spacecraft is in [[eclipse]] and unable to generate power.<ref name=aiaa/> Increasingly commonly, control centers for uncrewed spacecraft may be set up for "lights-out" (or [[automation|automated]]) operation, as a means of controlling costs.<ref name=aiaa/> Flight [[remote monitoring and control|control]] software will typically generate [[notification system|notifications]] of significant events – both planned and unplanned – in the ground or space segment that may require operator intervention.<ref name=aiaa/>

===Ground networks===

Ground [[computer network|network]]s handle data transfer and voice communication between different elements of the ground segment.{{r|Fortescue-GS}}{{rp|481-482}} These networks often combine [[local area network|LAN]] and [[wide area network|WAN]] elements, for which different parties may be responsible. Geographically separated elements may be connected via [[leased line]]s or [[virtual private network]]s.{{r|Fortescue-GS}}{{rp|481}} The design of ground networks is driven by requirements on [[reliability engineering|reliability]], [[bandwidth (computing)|bandwidth]], and [[computer security|security]].

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.

Security considerations are vital to protect space resources and sensitive data. WAN links often incorporate [[encryption]] protocols and [[Firewall (computing)|firewalls]] to provide [[information security|information]] and [[network security]]. [[Antivirus software]] and [[intrusion detection system]]s provide additional security at network endpoints.

===Remote terminals===

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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===Launch facilities===

Vehicles are delivered to space via [[Spaceport|launch facilities]], which handle the logistics of rocket launches. Launch facilities are typically connected to the ground network to relay telemetry prior to and during launch. The [[launch vehicle]] itself is sometimes said to constitute a "transfer segment", which may be considered distinct from both the spaceground and groundspace segments.<ref name=ley/>{{rp|21}}

===Ground networks===

Ground [[computer network|network]]s handle data transfer and voice communication between different elements of the ground segment.{{r|Fortescue-GS}}{{rp|481-482481–482}} These networks often combine [[local area network|LAN]] and [[wide area network|WAN]] elements, for which different parties may be responsible. Geographically separated elements may be connected via [[leased line]]s or [[virtual private network]]s.{{r|Fortescue-GS}}{{rp|481}} The design of ground networks is driven by requirements on [[reliability engineering|reliability]], [[bandwidth (computing)|bandwidth]], and [[computer security|security]]. [[Delay-tolerant networking]] protocols may be used.

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.

Security considerations are vital to protect space resources and sensitive data. WAN links often incorporate [[encryption]] protocols and [[Firewall (computing)|firewalls]] to provide [[information security|information]] and [[network security]]. [[Antivirus software]] and [[intrusion detection system]]s provide additional security at network endpoints.

==Costs==

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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>

<!--<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>-->