Global Navigation Satellite Systems (GNSS) for Earth Sciences

1
Global Navigation Satellite Systems (GNSS) for
Earth Sciences

• Prof. Thomas Herring, Massachusetts Institute of
  Technology
• Cambridge, MA USAtah_at_mit.edu http//www-gpsg.mit.
  edu/tah

2
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

3
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

4
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/yr1 meter of motion in 100
  years

5
Global Plate motions
-Convergence of Africa and Europe-Proposed
Somalia Plate -Spreading of mid-Atlantic
Ridge -Features of plate tectonics evident is
earthquake pattern
6
Earthquakes 1977-1997
-North African events are collision
events-Events in East Africa are associated with
rifting-Southern boundary of rift system not
distinct
7
Locations of earthquakes since 1900

• Largest events in Africa marked.

Catalog Source National Earthquake Information
System http//neic.usgs.gov/
8
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
9
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

10
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

11
Example of VLBI/GPS system
Hartebeesthoek Radio Astronomy Observatory
VLBI System
GPS Antenna
12
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.

13
Motion of Africa relative to Eurasia
Northward motion of Africa Rapid motions in parts
of the convergence zone 95 confidence error
ellipses
14
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
15
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
16
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
17
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.

18
Example of real-time 2-hr water vapor measurements
Available from http//www.suominet.ucar.edu/
19
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)

20
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.