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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 USAtah@mit.edu http://www-gpsg.mit.edu/~tah

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Introduction• Earth Science applications of global navigation

satellite systems (GNSS) place some the moststringent requirements on the accuracy of thesesystems.

• Application areas:– Studies of Earth deformation: millimeter accuracy positioning

required– Support for global Earth science applications: Global

distribution of tracking networks needed to determine accurateorbits for GNSS satellites.

– Studies of atmospheric effects: Analysis of propagation delaysof signals

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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 GlobalPositioning System (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 combinedEurope and Asian tectonic plates) at ~10 mm/yr

– To the west the mid-Atlantic ridge is opening at rate of ~20mm/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 Eurasiaat ~25 mm/yr

– The eastern part of Africa is being rifted by the East AfricanRift.

• Consequences of these motions are earthquakes andvolcanoes. 10 mm/yr=1 meter of motion in 100 years

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Global Plate motions-Convergenceof Africa andEurope-ProposedSomalia Plate-Spreading ofmid-AtlanticRidge-Features ofplate tectonicsevident isearthquakepattern

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Earthquakes1977-1997

-North Africanevents are collisionevents-Events in EastAfrica areassociated withrifting-Southernboundary of riftsystem not distinct

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• Largestevents inAfricamarked.

>8>7>6>5

1910 7.5

1990 7.4

1966 7.0

1992 7.0

1939 6.8

2000 6.7

1969 6.3

1983 6.4

1974 6.2

1945 6.2

Catalog SourceNational EarthquakeInformation Systemhttp://neic.usgs.gov/

Locations ofearthquakessince 1900

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Major African Volcanic Features

Red triangles arevolcanoesDashed linesmark the EastAfrican Rift ZoneVolcanic activityassociated withrift zone andmotion of ArabianPlate

Oldoinyo Lengai

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Role of GNSS• Modern GNSS (particularly GPS) allow the

measurement of strain accumulation that can lead toearthquakes. Particularly areas outside of obviousdeformation zones (intraplate earthquakes)

• Analysis of GNSS series of measurements afterearthquakes (post seismic motion) reveals informationabout forces and material properties associates withearthquakes.

• Occurrence of some earthquakes, affect where futureevents where future events will occur (stresstransients)

• Volcanic systems often have precursory signals aspressure builds in magma chamber

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GNSS and geodetic systems in Africa• African plate region has 5 GPS systems that regularly

supply data to the International GPS service (IGS)• There are 5 other systems that occasionally supply

data but these systems are to irregular in datatransmission to meet the IGS data processingdeadlines.

• One new system installed in Lusaka in March 2002and became operational in June 2002.

• One system in South Africa has a very long baselinesystem (VLBI) as well. One of limited number ofglobal co-located sites

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Example of VLBI/GPS systemHartebeesthoek Radio AstronomyObservatory

VLBI System

GPS Antenna

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Results from African GPS sites• Following figures give results from the African GPS

sites expressed as velocity vectors (the rates at whichthe stations are moving).

• Since all the tectonic plates move relative to eachother, when the results are plotted we show themrelative to a fixed plate. For African results wechoose either a Eurasia-fixed or African fixed frame.

• We can also compare the measured results withgeologic estimates (last 1Myr).

• For Africa-Eurasian collision, convergence rate fromgeology differs from geodesy.

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Motion of Africarelative toEurasia

Northward motionof AfricaRapid motions inparts of theconvergencezone95% confidence errorellipses

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Motion relative toGeologic Africa

Notice in “geologic”frame sites movesouth, indicatinggeologic rate toofast, partly dueSomalia plate notmodeled well ingeologyMotion of Africaneeded forgeophysicalmodeling

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GPS DefinedAfrican Plate

Within the currentuncertainties of themeasurements, plateis reasonably stablebut some sites haveonly been operatingfor ~1 yearExtension betweenKenya and Cabonsuggested but longertime series needed

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Some details of Northern CollisionMeasured GPSMotions inTurkey andGreeceContinuouslyoperating GPSsystems allowthese types ofdense networksNote difference in scale ofvelocity vectors fromprevious plots

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Meteorological Applications• GPS measurements are not only sensitive to the

positions of the GPS antenna but also the mediumthrough which the GPS signals propagate

• Three main contributions:– Charged particle layer called ionosphere; variations effect

radio communications and power grids. GPS networks can beused monitor variations and warn of on coming ionosphericstorms (dual frequency measurements)

– Neutral Atmosphere (Oxygen/Nitrogen mainly). Delays wellmodeled by surface pressure measurements

– Water vapor delay: GPS very sensitive and water vapor mostuncertain meteorological forecast models. Still beingevaluated by GPS helps in predicting 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 continued

operation 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 in areas of no bedrock. Sediments move by tens ofmillimeter when water is withdrawn.

– Antenna needs a clear view of the sky. Vegetation growth canaffect the accuracy of measurements (again tens ofmillimeters)

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Conclusions• Primary application GNSS in Earth Science is deformation

measurement.• Continuously operating networks supply direct measurements of

deformation but also:– Support densification of networks using occasional occupations– Contribute to GNSS orbit determination which improves accuracy in

regions with continuous stations– Can be used to support other GNSS applications with real-time

telemetry of data.– For near-time systems support meteorological applications.

• Earthquakes, volcanoes and weather systems do not knowpolitical boundaries: Earth science applications of GNSS helpeveryone in a region.