Aground segment consists of all the ground-based elements of aspacesystem used by operators and support personnel, as opposed to thespace segment and user segment.[1][2]: 1 The ground segment enables management of a spacecraft, and distribution ofpayload data andtelemetry among interested parties on the ground. The primary elements of a ground segment are:
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.[5][6]: 25 Some elements may support multiple spacecraft simultaneously.[7]: 480, 481
Ground stations provide radiointerfaces between the space and ground segments for telemetry, tracking, and command (TT&C), as well as payload data transmission and reception.[6]: 4 [8][9] Tracking networks, such asNASA'sNear Earth Network andSpace Network, handle communications with multiple spacecraft throughtime-sharing.[3]: 22
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 Operations plan.
Signals to beuplinked to a spacecraft must first be extracted from groundnetwork packets,encoded tobaseband, andmodulated,[10] typically onto anintermediate frequency (IF) carrier, before beingup-converted to the assignedradio frequency (RF) band. The RF signal is thenamplified to high power and carried viawaveguide to anantenna for 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 viabit andframe synchronization and decoding.[10] Data errors, such as those caused by signaldegradation, areidentified and corrected where possible.[10] The extracted data stream is thenpacketized or saved to files for transmission on ground networks. Ground stations may temporarilystore 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 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.
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 spacecraftorbit ortrajectory.[11] The Space Network usesgeostationaryrelay satellites to extend pass opportunities over the horizon.
Ground stations musttrack spacecraft in order topoint their antennas properly, and must account forDoppler shifting of RF frequencies due to the motion of the spacecraft. Ground stations may also perform automatedranging; ranging tones may bemultiplexed 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 inorbit determination.
Mission control centers process, analyze, and distribute spacecrafttelemetry, and issuecommands, datauploads, andsoftware updates to spacecraft. For crewed spacecraft, mission control manages voice and video communications with the crew. Control centers may also be responsible forconfiguration management and dataarchival.[7]: 483 As with ground stations, there are often backup control facilities available to support continuity of operations.
Control centers use telemetry to determine the status of a spacecraft and its systems.[3]: 485 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:
A spacecraftdatabase 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.[7]: 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 toupgrades, hardware degradation in thespace environment, and changes to mission parameters.[12]: 399
Commands sent to spacecraft are formatted according to the spacecraft database, and arevalidated against 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.[7]: 485 Typically, commands successfully received by the spacecraft are acknowledged in telemetry,[7]: 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 ofhousekeeping. Commands (and telemetry) may beencrypted to prevent unauthorized access to the spacecraft or its data.
Spacecraft procedures are generally developed and tested against a spacecraftsimulator prior to use with the actual spacecraft.[13]: 488
Mission control centers may rely on "offline" (i.e., non-real-time)data processing subsystems to handle analytical tasks[3]: 21 [7]: 487 such as:
Dedicated physical spaces may be provided in the control center for certain mission support roles, such asflight dynamics andnetwork control,[3]: 475 or these roles may be handled viaremote terminals outside the control center. As on-boardcomputing power andflight software complexity have increased, there is a trend toward performing more automated data processingon board the spacecraft.[16]: 2–3
Control centers may becontinuously orregularly staffed byflight controllers. Staffing is typically greatest during theearly phases of a mission,[3]: 21 and duringcritical procedures and periods, such as when a spacecraft is ineclipse and unable to generate power.[16] Increasingly commonly, control centers for uncrewed spacecraft may be set up for "lights-out" (orautomated) operation, as a means of controlling costs.[16] Flightcontrol software will typically generatenotifications of significant events – both planned and unplanned – in the ground or space segment that may require operator intervention.[16]
Remote terminals are interfaces on ground networks, separate from the mission control center, which may be accessed bypayload controllers, telemetry analysts,instrument andscience teams, andsupport personnel, such assystem administrators andsoftware development teams. They may be receive-only, or they may transmit data to the ground network.
Terminals used byservice customers, includingISPs andend users, are collectively called the "user segment", and are typically distinguished from the ground segment. User terminals includingsatellite television systems andsatellite phones communicate directly with spacecraft, while other types of user terminals rely on the ground segment for data receipt, transmission, and processing.
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.[7]: 480
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 vehicle itself is sometimes said to constitute a "transfer segment", which may be considered distinct from both the ground and space segments.[3]: 21
Groundnetworks handle data transfer and voice communication between different elements of the ground segment.[7]: 481–482 These networks often combineLAN andWAN elements, for which different parties may be responsible. Geographically separated elements may be connected vialeased lines orvirtual private networks.[7]: 481 The design of ground networks is driven by requirements onreliability,bandwidth, andsecurity.Delay-tolerant networking protocols may be used.
Reliability is a particularly important consideration forcritical systems, withuptime andmean time to recovery being of paramount concern. As with other aspects of the spacecraft system,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 incorporateencryption protocols andfirewalls to provideinformation andnetwork security.Antivirus software andintrusion detection systems provide additional security at network endpoints.
Costs associated with the establishment and operation of a ground segment are highly variable,[17] and depend on accounting methods. According to a study byDelft University of Technology,[Note 1] the ground segment contributes approximately 5% to the total cost of a space system.[18] According to a report by theRAND Corporation on 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%.[19]: 10
Ground segmentcost drivers include requirements placed on facilities, hardware, software, network connectivity, security, and staffing.[20] Ground station costs in particular depend largely on the required transmission power, RF band(s), and the suitability of preexisting facilities.[17]: 703 Control centers may be highly automated as a means of controlling staffing costs.[16]
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