The primary elements of a ground segment, discussed in detail below, include:Mission (or flight) control (or operations) centers
Ground (or Earth) stations
Integration and test facilities
These ground segment elements are present in nearly all space missions, whether commercial, military, or scientific. They may be located together or separated geographically, and may be operated by different parties. Certain elements may support multiple spacecraft simultaneously.
Mission control centers process, distribute, and perform some analysis of telemetry; they also issue commands, data uploads, and software updates to spacecraft. For manned spacecraft, mission control manages voice and video communications with the crew. Control centers may also be responsible for configuration management and data archival.
There are typically backup sites from which mission control can be maintained if there is a problem at the primary control center which renders it unable to operate, such as a natural disaster; such contingencies are considered in a Continuity of Operations plan.
Control centers may be continuously or regularly staffed by flight controllers. Staffing is typically greatest during the early phases of a mission, and during critical procedures and periods. Increasingly commonly, control centers for unmanned spacecraft may be set up for "lights-out" (or automated) operation, as a means of controlling costs. Flight control software will typically generate alerts regarding significant events – both planned and unplanned – in the ground or space segment that may require operator action.
Control centers use received 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
- Time-ordering and gap-checking of received frames (gaps may be filled by commanding a re-transmission)
- Decommutation of parameter values, and association of these values with mnemonics
- Conversion of raw data to calibrated (engineering) values, and calculation of derived parameters
- Limit and constraint checking, which may generate alert notifications
A spacecraft database 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. This database—especially telemetry calibrations and limits—may be updated to maintain consistency with flight 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.
Spacecraft commands are formatted according to the spacecraft database, and are validated against the database before being transmitted. Commands may be issued manually in real time, or they may be part of automated or semi-automated procedures. Typically, received commands are acknowledged in telemetry. In certain cases, closed-loop control may be performed. Commanded activities may pertain directly to mission objectives, or they may be part of housekeeping. Commands (and telemetry) may be encrypted to prevent unauthorized access to the spacecraft or its data.
Spacecraft procedures are often developed and tested against a spacecraft simulator prior to use with the actual spacecraft.
Mission control centers may rely on "offline" (i.e., non-real-time) data processing subsystems to handle analytical tasks such as:Orbit determination and maneuver planning
On-board memory management
Mission planning and scheduling
Path planning, in the case of planetary rovers
Short- and long-term trend analysis
Dedicated physical spaces may be provided in the control center for certain mission support roles, such as flight dynamics and network control, or these roles may be handled via remote terminals outside the control center. As on-board computing power and flight software complexity have increased, there is a trend toward performing more automated data processing on board the spacecraft.
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. As with control centers, there are typically backup ground stations available to support continuity of operations. Tracking networks, such as NASA's Near Earth Network and Space Network, may handle communications with multiple spacecraft through time-sharing.
Ground station equipment may be monitored and controlled remotely, often via serial and/or IP interfaces.
The timing of passes is determined by the location of ground stations, and by the characteristics of the spacecraft orbit. The Space Network uses geostationary relay satellites to extend pass opportunities over the horizon.
Baseband signals to be uplinked to a spacecraft must first be extracted from ground network packets, encoded, 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 decoded data stream is then packetized or saved to files for transmission on the ground network. Ground stations may temporarily store received telemetry for later playback to control centers.
A single spacecraft may make use of multiple RF bands for different telemetry, command, and payload data streams, depending on bandwidth and other requirements.
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.
Ground networks handle data transfer and voice communication between different elements of the ground segment. These networks often combine LAN and WAN elements, for which different parties may be responsible. Geographically separated elements may be connected via leased lines or virtual private networks.
WAN links often incorporate encryption protocols and firewalls to provide information and network security. Antivirus software and intrusion detection systems provide additional security at network endpoints.
The design of ground networks is driven by requirements on security, reliability, and bandwidth. Reliability is a particularly important consideration for critical systems, with uptime and mean time to recovery as paramount concerns. As with other aspects of the spacecraft system, redundancy of network components is the primary means of achieving the required system reliability.
Remote terminals are interfaces on the ground network separate from the mission control center, which may be accessed by payload controllers, telemetry analysts, instrument and science teams, and support personnel, such as system administrators and software development teams. They may be receive-only, or they may transmit data to the ground network.
Satellite terminals used by service customers, including ISPs and end users, are collectively called the "user segment", as distinguished from the ground segment.
Space vehicles and their interfaces are assembled and tested at integration and test (I&T) facilities. I&T provides an opportunity to fully test communications with the spacecraft prior to launch.
Vehicles are delivered to space via launch facilities, which handle the logistics of rocket launches. The launch vehicle itself is sometimes said to constitute a "transfer segment", which may be considered distinct from both the space and ground segments.
Costs associated with the establishment and operation of a ground segment are highly variable, and depend on accounting methods. According to a study by Delft University of Technology, the ground segment contributes approximately 5% to the total cost of a space system. According to a report by the RAND Corporation on NASA small spacecraft missions, operation costs 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%.
Overall ground segment cost drivers include requirements placed on facilities, hardware, software, network connectivity, security, and staffing. The cost of ground stations in particular depends on such variables as RF band(s), transmission power, and the suitability of preexisting facilities. Control centers may be highly automated as a means of controlling staffing costs.