Ground segment

A simplified spacecraft system. Orange arrows denote radio links; black arrows denote ground network links.

A ground segment consists of all the ground-based control elements of a spacecraft system, as opposed to the space segment and user segment. It serves to enable control of a spacecraft, and distribution of payload data and telemetry among interested parties on the ground.

Elements

Ground segment elements may be located together or separately, and may be operated by different parties. Individual ground segment elements may support multiple missions simultaneously. Primary elements of a ground segment include:

• Mission (or flight) control (or operations) centers (generally with backup sites), which process and distribute telemetry and issue commands, data uploads, and software updates to the spacecraft
• Ground (or earth) stations (generally with backups), which provide radio interfaces to the spacecraft for telemetry, tracking, and command (TT&C) and payload data transmission
• Remote terminals, which may be accessed by flight controllers, support personnel, and stakeholders
• Integration and test (I&T) facilities, where vehicles and their interfaces are assembled and tested prior to launch
• Launch facilities, from which vehicles are launched into space (The launch vehicle itself is sometimes said to constitute a "transfer segment", distinct from the space and ground segments.)
• Ground networks, which provide for communication and data transfer between ground segment elements

Mission control centers

See also: List of mission control centers

Control center at NASA's Jet Propulsion Laboratory

Staffing

Control centers may be continuously or periodically monitored, or (increasingly commonly) they may be set up for "lights-out", or automated, operation as a means of controlling costs. Monitoring is typically greatest during the early phases of a mission, and during critical procedures and periods. 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.

Telemetry processing

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 initial processing of received telemetry, including:

1. Separation and distribution of virtual channels
2. Time-ordering and gap-checking of received frames (gaps may be filled by commanding a re-transmission)
3. Decommutation of parameter values, and association with mnemonics
4. Conversion of raw data to calibrated (engineering) values
5. Limit and constraint checking
6. Data archival, typically for the full lifetime of the spacecraft

Information on the position and frequency of parameters within frames, and their associated mnemonics, calibrations, and limits, are contained within a spacecraft database, which may be updated to maintain consistency with flight software and operating procedures that can change during the life of a mission.

Commanding

Spacecraft commands are formatted and validated according to the spacecraft database before being transmitted. Commands may be issued manually in real time, or they may be part of automated or semi-automated procedures. 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 may be encrypted to prevent unauthorized access.

Analysis

Mission control centers may rely on offline data processing subsystems to handle analytical tasks such as:

• Orbit determination and maneuver planning
• Collision avoidance
• Conjunction assessment
• On-board memory management
• Mission planning and scheduling
• Path planning, in the case of planetary rovers
• Long-term trend analysis

Dedicated spaces may be provided for certain mission support roles, such as flight dynamics and network control.

As on-board computing power and software complexity have increased, there is a trend toward more automated data processing taking place on board the spacecraft.

Ground stations

See also: Ground station

Radio dishes at an Embratel earth station in Tanguá, Brazil

Passes

The location of ground stations, along with the characteristics of the spacecraft orbit, determines when passes will occur. Tracking networks, such as NASA's Near Earth Network and Space Network (the latter of which uses relay satellites) may handle communications with multiple spacecraft through time-sharing.

Transmission and reception

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 appropriate radio frequency (RF) band. The RF signal is then amplified to high power and carried via waveguide to an antenna for transmission.

Received ("downlinked") signals are passed through a low-noise amplifier (often located in the antenna hub) before being down-converted to IF; these 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 may also perform automated ranging; ranging signals may be multiplexed with command and telemetry signals.

Monitoring and control

Ground stations must track spacecraft in order to point their antennas properly, and must account for Doppler shifting due to the motion of the spacecraft. In colder climates, electric heaters or hot air blowers may be necessary to prevent ice buildup on the parabolic dish. Ground station equipment may be monitored and controlled remotely, often via serial or IP interfaces.

Remote terminals

Remote terminals are interfaces on the ground network which may be accessed by support personnel such as system administrators and software development teams, and stakeholders, including science teams. They may be receive-only, or they may transmit data to the ground network. Satellite terminals used by customers, including ISPs and end users, are collectively called the "user segment", as distinguished from the ground segment.

Ground networks

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 are often encrypted to provide information security, with network security provided by firewalls. Intrusion detection systems may provide an added layer of security.

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. Redundancy, as with other aspects of the spacecraft system, is the primary means of achieving the required reliability.

Costs

Costs associated with the establishment and operation of a ground segment are highly variable. According to a study by Delft University of Technology (based on a model described in Space Mission Analysis and Design, third edition, by James W. Wertz and Wiley J. Larson), 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.

Images

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  Space Telescope Operations Control Center at Goddard Space Flight Center, during servicing of the Hubble Space Telescope

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  Antenna belonging to the Deep Space Network

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  Integration of flight hardware at a JAXA facility in Tsukuba, Japan

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  Decommissioned launch site at the Guiana Space Centre

See also

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