Satellite geodesy
Geodesy |
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Satellite geodesy is
The main goals of satellite geodesy are:
- Determination of the figure of the Earth, positioning, and navigation (geometric satellite geodesy)[1]: 3
- Determination of geoid, Earth's gravity field and its temporal variations (dynamical satellite geodesy[2] or satellite physical geodesy)
- Measurement of geodynamical phenomena, such as crustal dynamics and polar motion[1]: 4 [1]: 1
Satellite geodetic data and methods can be applied to diverse fields such as navigation, hydrography, oceanography and geophysics. Satellite geodesy relies heavily on orbital mechanics.
History
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First steps (1957-1970)
Satellite geodesy began shortly after the launch of
Soviet military satellites undertook geodesic missions to assist in
Toward the World Geodetic System (1970-1990)
The
Modern Era (1990-present)
The 1990s were focused on the development of permanent geodetic networks and reference frames.
Measurement techniques
Techniques of satellite geodesy may be classified by instrument platform: A satellite may
- be observed with ground-based instruments (Earth-to-space-methods),
- carry an instrument or sensor as part of its payload to observe the Earth (space-to-Earth methods),
- or use its instruments to track or be tracked by another satellite (space-to-space methods).[1]: 6
Earth-to-space methods (satellite tracking)
Radio techniques
Global navigation satellite systems are dedicated radio positioning services, which can locate a receiver to within a few meters. The most prominent system,
In geodesy, GNSS is used as an economical tool for
- Examples: Galileo
Doppler techniques
- Examples: DORIS, Argos
Optical triangulation
In optical triangulation, the satellite can be used as a very high target for triangulation and can be used to ascertain the geometric relationship between multiple observing stations. Optical triangulation with the BC-4, PC-1000, MOTS, or Baker Nunn cameras consisted of photographic observations of a satellite, or flashing light on the satellite, against a background of stars. The stars, whose positions were accurately determined, provided a framework on the photographic plate or film for a determination of precise directions from camera station to satellite. Geodetic positioning work with cameras was usually performed with one camera observing simultaneously with one or more other cameras. Camera systems are weather dependent and that is one major reason why they fell out of use by the 1980s.[3]: 51
- Examples: PAGEOS, Project Echo, ANNA 1B
Laser ranging
In satellite laser ranging (SLR) a global network of observation stations measure the round trip time of flight of ultrashort pulses of
- Example: LAGEOS
Space-to-Earth methods
Altimetry
Satellites such as
Spaceborne radar altimeters have proven to be superb tools for mapping
Laser altimetry
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A
Radar altimetry
A radar altimeter uses the round-trip flight-time of a microwave pulse between the satellite and the Earth's surface to determine the distance between the spacecraft and the surface. From this distance or height, the local surface effects such as tides, winds and currents are removed to obtain the satellite height above the geoid. With a precise ephemeris available for the satellite, the geocentric position and ellipsoidal height of the satellite are available for any given observation time. It is then possible to compute the geoid height by subtracting the measured altitude from the ellipsoidal height. This allows direct measurement of the geoid, since the ocean surface closely follows the geoid.[3]: 64 The difference between the ocean surface and the actual geoid gives ocean surface topography.
- Examples: SWOT (satellite)
Interferometric synthetic aperture radar (InSAR)
Interferometric synthetic aperture radar (InSAR) is a
The technique can potentially measure centimetre-scale changes in deformation over timespans of days to years. It has applications for geophysical monitoring of natural hazards, for example earthquakes, volcanoes and landslides, and also in structural engineering, in particular monitoring of subsidence and structural stability.- Example: Seasat, TerraSAR-X
Space-to-space methods
Gravity gradiometry
A gravity gradiometer can independently determine the components of the gravity vector on a real-time basis. A gravity gradient is simply the spatial derivative of the gravity vector. The gradient can be thought of as the rate of change of a component of the gravity vector as measured over a small distance. Hence, the gradient can be measured by determining the difference in gravity at two close but distinct points. This principle is embodied in several recent moving-base instruments. The gravity gradient at a point is a tensor, since it is the derivative of each component of the gravity vector taken in each sensitive axis. Thus, the value of any component of the gravity vector can be known all along the path of the vehicle if gravity gradiometers are included in the system and their outputs are integrated by the system computer. An accurate gravity model will be computed in real-time and a continuous map of normal gravity, elevation, and anomalous gravity will be available.[3]: 71
- Example: GOCE
Satellite-to-satellite tracking
This technique uses satellites to track other satellites. There are a number of variations which may be used for specific purposes such as gravity field investigations and orbit improvement.
- A high altitude satellite may act as a relay from ground tracking stations to a low altitude satellite. In this way, low altitude satellites may be observed when they are not accessible to ground stations. In this type of tracking, a signal generated by a tracking station is received by the relay satellite and then retransmitted to a lower altitude satellite. This signal is then returned to the ground station by the same path.
- Two low altitude satellites can track one another observing mutual orbital variations caused by gravity field irregularities. A prime example of this is GRACE.
- Several high altitude satellites with accurately known orbits, such as GPSsatellites, may be used to fix the position of a low altitude satellite.
These examples present a few of the possibilities for the application of satellite-to-satellite tracking. Satellite-to-satellite tracking data was first collected and analyzed in a high-low configuration between
- Example: GRACE
GNSS tracking
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- Examples: GOCE
List of geodetic satellites
- ANNA-1B
- Beidou
- BLITS
- CHAMP
- Diadème
- Echo
- Envisat
- ERS-1
- ERS-2
- Etalon
- Experimental Geodetic Payload"Ajisai"
- Explorer program
- Galileo
- Geo-IK-2
- GEOS-3
- Geosat
- Geosat Follow-On
- GFZ-1[9]
- GLONASS
- GRACE
- GOCE
- GPS
- ICESat-1
- ICESat-2
- LAGEOS
- LARES
- Larets
- H-IIA-LRE[10]
- PAGEOS
- Seasat
- Starlette and Stella
- TOPEX/Poseidon
- TRANSIT
- WESTPAC[11]
See also
- Geodetic astronomy
- Satellite gravimetry
References
- ^ ISBN 978-3-11-017549-3.
- ISBN 978-8393889808.
- ^ a b c d e Defense Mapping Agency (1983). Geodesy for the Layman (PDF) (Report). United States Air Force.
- ^ ISBN 978-3-030-91821-7.
- ISBN 978-8393889808.
- S2CID 24519422
- ISBN 9780792369455
- ^ "International Laser Ranging Service". Ilrs.gsfc.nasa.gov. 2012-09-17. Retrieved 2022-08-20.
- ^ H2A-LRE
- ^ "International Laser Ranging Service". Ilrs.gsfc.nasa.gov. 2012-09-17. Retrieved 2022-08-20.
Attribution
This article incorporates text from this source, which is in the public domain: Defense Mapping Agency (1983). Geodesy for the Layman (PDF) (Report). United States Air Force. Archived from the original (PDF) on 2017-05-13. Retrieved 2021-02-19.
Further reading
- François Barlier; Michel Lefebvre (2001), A new look at planet Earth: Satellite geodesy and geosciences (PDF), Kluwer Academic Publishers
- Smith, David E. and Turcotte, Donald L. (eds.) (1993). Contributions of Space Geodesy to Geodynamics: Crustal Dynamics Vol. 23, Earth Dynamics Vol. 24, Technology Vol. 25, American Geophysical Union Geodynamics Series ISSN 0277-6669.