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.[1][2]:1 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.[3][4]:25 Individual ground segment elements may support multiple missions simultaneously. Primary elements of a ground segment include:

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.[9] Monitoring is typically greatest during the early phases of a mission,[5]:21 and during critical procedures and periods.[9] 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.[9]

Telemetry processing

Control centers use received telemetry to determine the status of a spacecraft and its systems.[5]:485 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[5]:393
  2. Time-ordering and gap-checking of received frames (gaps may be filled by commanding a re-transmission)
  3. Decommutation of parameter values,[10] and association with mnemonics
  4. Conversion of raw data to calibrated (engineering) values
  5. Limit and constraint checking[5]:479
  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[5]:21 such as:

Dedicated spaces may be provided for certain mission support roles, such as flight dynamics and network control.[5]:475

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.[9]:2–3

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.[13] 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.[5]:22

Transmission and reception

Baseband signals to be uplinked to a spacecraft must first be extracted from ground network packets, encoded, and modulated,[10] 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.[10] Data errors, such as those caused by signal degradation, are identified and corrected where possible.[10] 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.[14] 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.[15] 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%.[16]:10

Overall ground segment cost drivers include requirements placed on facilities, hardware, software, network connectivity, security, and staffing.[17] The cost of ground stations in particular depends on such variables as RF band(s), transmission power, and the suitability of preexisting facilities.[14]:703 Control centers may be highly automated as a means of controlling staffing costs.[9]

Images

See also

References

  1. ↑ "Ground Segment". JSAT International. Retrieved 5 November 2015.
  2. 1 2 3 4 Elbert, Bruce (2014). The Satellite Communication Ground Segment and Earth Station Handbook (2nd ed.). Artech House. ISBN 978-1-60807-673-4. Retrieved 5 November 2015.
  3. ↑ "Ground Segment Overview". European Space Agency. Retrieved 5 November 2015.
  4. 1 2 Reiniger, Klaus; Diedrich, Erhard; Mikusch, Eberhard (August 2006). "Aspects of Ground Segment Design for Earth observation missions" (PDF). Alpbach Summer School.
  5. 1 2 3 4 5 6 7 8 9 10 Ley, Wilfried; Wittmann, Klaus; Hallmann, Willi, eds. (2008). Handbook of Space Technology. Wiley. ISBN 0470742410. Retrieved 30 December 2015.
  6. ↑ "Radio Frequency Components". JSAT International. Retrieved 5 November 2015.
  7. ↑ "Earth Stations/Teleports - Hub.". JSAT International. Retrieved 5 November 2015.
  8. ↑ "ERS Ground Segment". European Space Agency. Retrieved 5 November 2015.
  9. 1 2 3 4 5 "Operations Staffing". Satellite Operations Best Practice Documents. Space Operations and Support Technical Committee, American Institute of Aeronautics and Astronautics. Retrieved 28 December 2015.
  10. 1 2 3 4 "Chapter 10: Telecommunications". Basics of Spaceflight. NASA Jet Propulsion Laboratory. Retrieved 28 December 2015.
  11. ↑ "Chapter 13: Spacecraft Navigation". Basics of Spaceflight. NASA Jet Propulsion Laboratory. Retrieved 28 December 2015.
  12. ↑ Uhlig, Thomas; Sellmaier, Florian; Schmidhuber, Michael, eds. (2014). Spacecraft Operations. Springer-Verlag. ISBN 978-3-7091-1802-3. Retrieved 28 December 2015.
  13. ↑ Wood, Lloyd (July 2006). Introduction to satellite constellations: Orbital types, uses and related facts (PDF). ISU Summer Session. Retrieved 17 November 2015.
  14. 1 2 Tirró, Sebastiano, ed. (1993). Satellite Communication Systems Design. Springer Science+Business Media. ISBN 1461530067. Retrieved 8 January 2016.
  15. ↑ Zandbergen, B.T.C. "Cost Estimation for Space System Elements, v.1.02" (Excel spreadsheet). Retrieved 8 January 2016.
  16. ↑ de Weck, Olivier; de Neufville, Richard; Chang, Darren; Chaize, Mathieu. "Technical Success and Economic Failure". Communications Satellite Constellations (PDF). Massachusetts Institute of Technology.
  17. ↑ Matthews, Anthony J. (February 25, 1996). "A ground cost model (G-COST) for military systems". AIAA International Communications Satellite Systems Conference (American Institute of Aeronautics and Astronautics): 1416–1421. doi:10.2514/6.1996-1111. Retrieved 8 January 2016.
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