Ground segment

A simplified spacecraft system. Dotted orange arrows denote radio links; solid black arrows denote ground network links. (Customer terminals typically rely on only one of the indicated paths for access to space-segment resources.)

Select ground segment facilities worldwide

Aground segmentconsists of all the ground-based elements of aspacesystemused by operators and support personnel, as opposed to thespace segmentand user segment.The ground segment enables management of a spacecraft, and distribution ofpayload dataandtelemetryamong interested parties on the ground. The primary elements of a ground segment are:

• Ground (or Earth) stations,which provide radio interfaces with spacecraft
• Mission control (or operations) centers,from which spacecraft are managed
• Remoteterminals,used by support personnel
• Spacecraftintegration and testfacilities
• Launch facilities
• Groundnetworks,which allow for communication between the other ground elements

These elements are present in nearly all space missions, whethercommercial,military,orscientific.They may be located together or separated geographically, and they may be operated by different parties.Some elements may support multiple spacecraft simultaneously.

Elements

Ground stations

Main article:Ground station

Radio dishes at anEmbratelearth station inTanguá,Brazil

Ground stations provide radiointerfacesbetween the space and ground segments for telemetry, tracking, and command (TT&C), as well as payload data transmission and reception.Tracking networks, such asNASA'sNear Earth NetworkandSpace Network,handle communications with multiple spacecraft throughtime-sharing.

Ground station equipment may bemonitored and controlled remotely.There are often 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 aContinuity of Operationsplan.

Transmission and reception

Signals to beuplinkedto a spacecraft must first be extracted from groundnetwork packets,encodedtobaseband,andmodulated,typically onto anintermediate frequency(IF) carrier, before beingup-convertedto the assignedradio frequency(RF) band. The RF signal is thenamplifiedto high power and carried viawaveguideto anantennafor transmission. In colder climates, electric heaters or hot air blowers may be necessary to prevent ice or snow buildup on theparabolic dish.

Received ( "downlinked" ) signals are passed through alow-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 alow-noise block downconverter.The IF signal is thendemodulated,and the data stream extracted viabitandframe synchronizationand decoding.Data errors, such as those caused by signaldegradation,areidentified and correctedwhere possible.The extracted data stream is thenpacketizedor saved to files for transmission on ground networks. Ground stations may temporarilystorereceived 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 supportdelay-tolerant networking.

A single spacecraft may make use of multiple RF bands for different telemetry, command, and payload datastreams,depending on bandwidth and other requirements.

Passes

The timing ofpasses,when a line of sight exists to the spacecraft, is determined by the location of ground stations, and by the characteristics of the spacecraftorbitortrajectory.The Space Network usesgeostationaryrelay satellitesto extend pass opportunities over the horizon.

Tracking and ranging

Ground stations musttrackspacecraft in order topoint their antennasproperly, and must account forDoppler shiftingof RF frequencies due to the motion of the spacecraft. Ground stations may also perform automatedranging;ranging tones may bemultiplexedwith 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 inorbit determination.

Mission control centers

See also:List of mission control centers

Control center at NASA'sJet Propulsion Laboratory

Mission control centers process, analyze, and distribute spacecrafttelemetry,and issuecommands,datauploads,andsoftware updatesto spacecraft. For crewed spacecraft, mission control manages voice and video communications with the crew. Control centers may also be responsible forconfiguration managementand dataarchival.As with ground stations, there are often 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.Housekeeping, diagnostic, science, and other types of telemetry may be carried on separatevirtual channels.Flight control software performs the initial processing of received telemetry, including:

1. Separation and distribution of virtual channels
2. Time-ordering and gap-checkingof receivedframes(gaps may be filled by commanding a retransmission)
3. Decommutationof parameter values,and association of these values with parameter names calledmnemonics
4. Conversion of raw data tocalibrated(engineering) values, and calculation of derived parameters
5. Limit and constraint checking (which may generate alert notifications)
6. Generation of telemetry displays, which may be take the form of tables,plotsof parameters against each other or over time, or synoptic displays (sometimes called mimics) – essentiallyflow diagramsthat present component or subsystem interfaces and their state

A spacecraftdatabaseprovided 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.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 toupgrades,hardware degradation in thespace environment,and changes to mission parameters.

Commanding

Commands sent to spacecraft are formatted according to the spacecraft database, and arevalidatedagainst the database before being transmitted via aground station.Commands may be issued manually in real time, or they may be part of automated or semi-automated procedures uploaded in their entirety.Typically, commands successfully received by the spacecraft are acknowledged in telemetry,and a command counter is maintained on the spacecraft and at the ground to ensure synchronization. In certain cases,closed-loop controlmay be performed. Commanded activities may pertain directly to mission objectives, or they may be part ofhousekeeping.Commands (and telemetry) may beencryptedto prevent unauthorized access to the spacecraft or its data.

Spacecraft procedures are generally developed and tested against a spacecraftsimulatorprior to use with the actual spacecraft.

Analysis and support

Mission control centers may rely on "offline" (i.e., non-real-time)data processingsubsystems to handle analytical taskssuch as:

• Orbit determinationandmaneuverplanning
• Conjunctionassessment andcollision avoidanceplanning
• Mission planning and scheduling
• On-boardmemory management
• Short- and long-termtrend analysis
• Path planning,in the case ofplanetary rovers

Dedicated physical spaces may be provided in the control center for certain mission support roles, such asflight dynamicsandnetworkcontrol,or these roles may be handled viaremote terminalsoutside the control center. As on-boardcomputing powerandflight softwarecomplexity have increased, there is a trend toward performing more automated data processingon board the spacecraft.

Staffing

Control centers may becontinuouslyorregularlystaffed byflight controllers.Staffing is typically greatest during theearly phasesof a mission,and duringcriticalprocedures and periods, such as when a spacecraft is ineclipseand unable to generate power.Increasingly commonly, control centers for uncrewed spacecraft may be set up for "lights-out" (orautomated) operation, as a means of controlling costs.Flightcontrolsoftware will typically generatenotificationsof significant events – both planned and unplanned – in the ground or space segment that may require operator intervention.

Remote terminals

Remote terminals are interfaces on ground networks, separate from the mission control center, which may be accessed bypayloadcontrollers, telemetry analysts,instrumentandscienceteams, andsupportpersonnel, such assystem administratorsandsoftware developmentteams. They may be receive-only, or they may transmit data to the ground network.

Terminals used byservicecustomers, includingISPsandend users,are collectively called the "user segment", and are typically distinguished from the ground segment. User terminals includingsatellite televisionsystems andsatellite phonescommunicate directly with spacecraft, while other types of user terminals rely on the ground segment for data receipt, transmission, and processing.

Integration and test facilities

Space vehicles and their interfaces are assembled and tested atintegration and test(I&T) facilities. Mission-specific I&T provides an opportunity to fully test communications between, and behavior of, both the spacecraft and the ground segment prior to launch.

Launch facilities

Vehicles are delivered to space vialaunch 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. Thelaunch vehicleitself is sometimes said to constitute a "transfer segment", which may be considered distinct from both the ground and space segments.

Ground networks

Groundnetworkshandle data transfer and voice communication between different elements of the ground segment.These networks often combineLANandWANelements, for which different parties may be responsible. Geographically separated elements may be connected vialeased linesorvirtual private networks.The design of ground networks is driven by requirements onreliability,bandwidth,andsecurity.Delay-tolerant networkingprotocols may be used.

Reliability is a particularly important consideration forcritical systems,withuptimeandmean time to recoverybeing of paramount concern. As with other aspects of the spacecraft system,redundancyof 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 incorporateencryptionprotocols andfirewallsto provideinformationandnetwork security.Antivirus softwareandintrusion detection systemsprovide additional security at network endpoints.

Costs

Costs associated with the establishment and operation of a ground segment are highly variable,and depend on accounting methods. According to a study byDelft University of Technology,the ground segment contributes approximately 5% to the total cost of a space system.According to a report by theRAND Corporationon NASA small spacecraft missions, operation costs alone contribute 8% to the lifetime cost of a typical mission, with integration and testing making up a further 3.2%, ground facilities 2.6%, and ground systems engineering 1.1%.

Ground segmentcost driversinclude requirements placed on facilities, hardware, software, network connectivity, security, and staffing.Ground station costs in particular depend largely on the required transmission power, RF band(s), and the suitability of preexisting facilities.Control centers may be highly automated as a means of controlling staffing costs.

Images

• 
  Antenna belonging to theDeep Space Network
• 
  Space Telescope Operations Control Center atGoddard Space Flight Center,during servicing of theHubble Space Telescope
• 
  Integration of flight hardware at aJAXAfacility inTsukuba,Japan
• 
  Decommissioned launch site at theGuiana Space Centre

See also

• Consultative Committee for Space Data Systems(CCSDS), which maintains standards for telemetry and command formatting
• Radiocommunication service,as defined byITU Radio Regulations
• On-board data handlingsubsystem