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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.