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Global Navigation Satellite Systems
(GNSS) for Earth Sciences
Prof. Thomas Herring,
Massachusetts Institute of Technology
Cambridge, MA USA
[email protected] http://www-gpsg.mit.edu/~tah
July 17, 2002
Zambia GNSS Earth Science 2002
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Introduction
• Earth Science applications of global navigation
satellite systems (GNSS) place some the most
stringent requirements on 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 GNSS satellites.
– Studies of atmospheric effects: Analysis of propagation delays
of signals
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Zambia GNSS Earth Science 2002
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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
Positioning System (GPS)
• Examples of atmospheric delay studies
• Contributions to the global applications
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Zambia GNSS Earth Science 2002
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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 the
Eurasian Plate at ~45 mm/yr
– To the north east the Arabian plate is converging on Eurasia at
~25 mm/yr
– The eastern part of Africa is being rifted by the East African
Rift.
• Consequences of these motions are earthquakes and
volcanoes. 10 mm/yr=1 meter of motion in 100 years
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Zambia GNSS Earth Science 2002
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Global Plate motions
-Convergence
of Africa and
Europe
-Proposed
Somalia Plate
-Spreading of
mid-Atlantic
Ridge
-Features of
plate tectonics
evident is
earthquake
pattern
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Zambia GNSS Earth Science 2002
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Earthquakes
1977-1997
-North African
events are collision
events
-Events in East
Africa are
associated with
rifting
-Southern
boundary of rift
system not distinct
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Zambia GNSS Earth Science 2002
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Locations of
earthquakes
since 1900
• Largest
events in
Africa
marked.
Catalog Source
National Earthquake
Information System
http://neic.usgs.gov/
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Zambia GNSS Earth Science 2002
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Major African Volcanic Features
Red triangles are
volcanoes
Dashed lines
mark the East
African Rift Zone
Volcanic activity
associated with
rift zone and
motion of Arabian
Plate
Oldoinyo Lengai
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Zambia GNSS Earth Science 2002
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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 and material properties associates with
earthquakes.
• Occurrence of some earthquakes, affect where future
events where future events will occur (stress
transients)
• Volcanic systems often have precursory signals as
pressure builds in magma chamber
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Zambia GNSS Earth Science 2002
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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 data
transmission to meet the IGS data processing
deadlines.
• One new system installed in Lusaka in March 2002
and became 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-located sites
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Zambia GNSS Earth Science 2002
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Example of VLBI/GPS system
Hartebeesthoek Radio Astronomy
Observatory
VLBI System
GPS Antenna
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Zambia GNSS Earth Science 2002
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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 which
the stations are moving).
• Since all the tectonic plates move relative to each
other, when the results are plotted we show them
relative to a fixed plate. For African results we
choose either a Eurasia-fixed or 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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Zambia GNSS Earth Science 2002
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Motion of Africa
relative to
Eurasia
Northward motion
of Africa
Rapid motions in
parts of the
convergence
zone
95% confidence error
ellipses
July 17, 2002
Zambia GNSS Earth Science 2002
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Motion relative to
Geologic Africa
Notice in “geologic”
frame sites move
south, indicating
geologic rate too
fast, partly due
Somalia plate not
modeled well in
geology
Motion of Africa
needed for
geophysical
modeling
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Zambia GNSS Earth Science 2002
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GPS Defined
African Plate
Within the current
uncertainties of the
measurements, plate
is reasonably stable
but some sites have
only been operating
for ~1 year
Extension between
Kenya and Cabon
suggested but longer
time series needed
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Zambia GNSS Earth Science 2002
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Some details of Northern Collision
Measured GPS
Motions in
Turkey and
Greece
Continuously
operating GPS
systems allow
these types of
dense networks
Note difference in scale of
velocity vectors from
previous plots
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Zambia GNSS Earth Science 2002
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Meteorological Applications
• GPS measurements are not only sensitive to the
positions of the GPS antenna but also the medium
through which the GPS signals propagate
• Three main contributions:
– Charged particle layer called ionosphere; variations effect
radio communications and power grids. GPS networks can be
used monitor variations and warn of on coming ionospheric
storms (dual frequency measurements)
– Neutral Atmosphere (Oxygen/Nitrogen mainly). Delays well
modeled by surface pressure measurements
– Water vapor delay: GPS very sensitive and water vapor most
uncertain meteorological forecast models. Still being
evaluated by GPS helps in predicting severe storms.
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Zambia GNSS Earth Science 2002
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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 of
millimeter when water is withdrawn.
– Antenna needs a clear view of the sky. Vegetation growth can
affect the 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 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 know
political boundaries: Earth science applications of GNSS help
everyone in a region.
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