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July 17, 2002 Zambia GNSS Earth Science 2002 1
Global Navigation Satellite Systems
(GNSS) for Earth Sciences
Prof. Thomas Herring,Massachusetts Institute of Technology
Cambridge, MA [email protected] http://www-gpsg.mit.edu/~tah
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Introduction
Earth Science applications of global navigation satellitesystems (GNSS) place some the most stringent requirementson the accuracy of these systems.
Application areas: Studies of Earth deformation: millimeter accuracy positioning required Support for global Earth science applications: Global distribution of
tracking networks needed to determine accurate orbits for GNSSsatellites.
Studies of atmospheric effects: Analysis of propagation delays ofsignals
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Topics to be addressed
Tectonics of the African region
Global setting: Northern motion toward Eurasia
East Africa rift system: Volcanism
Convergence in Northern Africa
Examples of deformation studies with the Global PositioningSystem (GPS)
Examples of atmospheric delay studies
Contributions to the global applications
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Global tectonic setting
Major tectonic elements: Africa moves north relative to Eurasia (name of the combined Europe
and Asian tectonic plates) at ~10 mm/yr
To the west the mid-Atlantic ridge is opening at rate of ~20 mm/yr
To the east the rapidly move Indian Plate is converging on theEurasian Plate at ~45 mm/yr
To the north east the Arabian plate is converging on Eurasia at ~25mm/yr
The eastern part of Africa is being rifted by the East African Rift.
Consequences of these motions are earthquakes andvolcanoes. 10 mm/yr=1 meter of motion in 100 years
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Earthquakes
1977-1997-North African eventsare collision events
-Events in East Africaare associated withrifting-Southern boundaryof rift system not
distinct
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Largest eventsin Africa
marked.
Catalog Source
National EarthquakeInformation System
http://neic.usgs.gov/
Locations ofearthquakes
since 1 00
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Role of GNSS Modern GNSS (particularly GPS) allow the measurement of
strain accumulation that can lead to earthquakes.Particularly areas outside of obvious deformation zones(intraplate earthquakes)
Analysis of GNSS series of measurements after earthquakes(post seismic motion) reveals information about forces andmaterial properties associates with earthquakes.
Occurrence of some earthquakes, affect where future eventswhere future events will occur (stress transients)
Volcanic systems often have precursory signals as pressurebuilds in magma chamber
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GNSS and geodetic systems in Africa
African plate region has 5 GPS systems that regularly supplydata to the International GPS service (IGS)
There are 5 other systems that occasionally supply data butthese systems are to irregular in data transmission to meetthe IGS data processing deadlines.
One new system installed in Lusaka in March 2002 andbecame operational in June 2002.
One system in South Africa has a very long baseline system
(VLBI) as well. One of limited number of global co-locatedsites
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Example of VLBI/GPS system
Hartebeesthoek Radio Astronomy Observatory
VLBI System
GPS Antenna
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Results from African GPS sites
Following figures give results from the African GPS sitesexpressed as velocity vectors (the rates at which the stationsare moving).
Since all the tectonic plates move relative to each other,when the results are plotted we show them relative to a fixedplate. For African results we choose either a Eurasia-fixedor African fixed frame.
We can also compare the measured results with geologic
estimates (last 1Myr). For Africa-Eurasian collision, convergence rate from geology
differs from geodesy.
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Motion of Africa
relative to Eurasia
Northward motion ofAfrica
Rapid motions inparts of theconvergence zone
95% confidence errorellipses
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Motion relative toGeologic Africa
Notice in geologicframe sites movesouth, indicatinggeologic rate too fast,
partly due Somaliaplate not modeled wellin geology
Motion of Africa
needed forgeophysical modeling
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GPS DefinedAfrican Plate
Within the currentuncertainties of themeasurements, plate is
reasonably stable butsome sites have onlybeen operating for ~1year
Extension betweenKenya and Cabonsuggested but longertime series needed
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Some details of Northern Collision
Measured GPSMotions in Turkeyand Greece
Continuouslyoperating GPSsystems allowthese types ofdense networks
Note difference in scale ofvelocity vectors from previousplots
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Meteorological Applications
GPS measurements are not only sensitive to the positions ofthe GPS antenna but also the medium through which the GPSsignals propagate
Three main contributions:
Charged particle layer called ionosphere; variations effect radiocommunications and power grids. GPS networks can be used monitorvariations and warn of on coming ionospheric storms (dual frequencymeasurements)
Neutral Atmosphere (Oxygen/Nitrogen mainly). Delays well modeledby surface pressure measurements
Water vapor delay: GPS very sensitive and water vapor most uncertainmeteorological forecast models. Still being evaluated by GPS helps inpredicting severe storms.
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Example of real-time 2-hr water vapor measurements
Available from http://www.suominet.ucar.edu/
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Requirements for GPS network
GPS equipment costs about $10,000US but continuedoperation is most costly aspect
Continuously operating sites need: Power (modern receivers need 2-8 Watts at 12-volts)
Communications (about 1Mbyte per day for 30-sec sampling) Security (site needs protection from theft and damage (sometimes
natural)
Antenna must be securely connected to the Earth. Major problems inareas of no bedrock. Sediments move by tens of millimeter whenwater is withdrawn.
Antenna needs a clear view of the sky. Vegetation growth can affectthe accuracy of measurements (again tens of millimeters)
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Conclusions
Primary application GNSS in Earth Science is deformation measurement.
Continuously operating networks supply direct measurements ofdeformation but also:
Support densification of networks using occasional occupations
Contribute to GNSS orbit determination which improves accuracy in regionswith continuous stations
Can be used to support other GNSS applications with real-time telemetry ofdata.
For near-time systems support meteorological applications.
Earthquakes, volcanoes and weather systems do not know politicalboundaries: Earth science applications of GNSS help everyone in aregion.