A ground segment consists of all the ground-based elements of a space system used by operators and support personnel, as opposed to the space segment and user segment. The ground segment enables management of spacecraft, and distribution of payload data and telemetry among 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 Remote terminals, used by support personnel Spacecraft integration and test facilities Launch facilities Ground networks, which allow for communication between the other ground elements 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. Some elements may support multiple spacecraft simultaneously.
Elements
Ground stations
Ground stations provide radio interfaces between the space and ground segments for telemetry, tracking, and command (TT&C), as well as payload data transmission and reception. Tracking networks, such as NASA's Near Earth Network and Space Network, handle communications with multiple spacecraft through time-sharing. Ground station equipment may be monitored 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 a Continuity of Operations plan.
Transmission and reception Signals to be uplinked to a spacecraft must first be extracted from ground network packets, encoded to baseband, and modulated, typically onto an intermediate frequency (IF) carrier, before being up-converted to the assigned radio frequency (RF) band. The RF signal is then amplified to high power and carried via waveguide to an antenna for transmission. In colder climates, electric heaters or hot air blowers may be necessary to prevent ice or snow buildup on the 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 demodulated, and the data stream extracted via bit and frame synchronization and decoding. Data errors, such as those caused by signal degradation, are identified and corrected where possible. The extracted data stream is then packetized or saved to files for transmission on ground networks. Ground stations may temporarily 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 streams, depending on bandwidth and other requirements.
Passes The timing of passes, when a line of sight exists to the spacecraft, is determined by the location of ground stations, and by the characteristics of the spacecraft orbit or trajectory. The Space Network uses geostationary relay satellites to extend pass opportunities over the horizon.
Tracking and ranging Ground stations must track spacecraft in order to point their antennas properly, and must account for Doppler shifting of RF frequencies due to the motion of the spacecraft. Ground stations may also perform automated ranging; ranging tones may be 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, which in turn may guide antenna pointing.
Mission control centers
Mission control centers process, analyze, and distribute spacecraft telemetry, and issue commands, data uploads, and 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 archival. 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 separate virtual channels. Flight control software performs the initial processing of received telemetry, including:
Separation and distribution of virtual channels, if not handled by ground station Time-ordering and gap-checking of received frames (gaps may be filled by commanding a retransmission) Decommutation of parameter values, and association of these values with parameter names called mnemonics Conversion of raw data to calibrated (engineering) values, and calculation of derived parameters Limit and constraint checking (which may generate alert notifications) Generation of telemetry displays, which may be take the form of tables, plots of parameters against each other or over time, or synoptic displays (sometimes called mimics) – essentially flow diagrams that present component or subsystem interfaces and their state 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. 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 upgrades, hardware degradation in the space environment, and changes to mission parameters.
… excerpt ends here. Continue reading the full article.
