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

July 17, 2002 Zambia GNSS Earth Science 2002 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

July 17, 2002 Zambia GNSS Earth Science 2002 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

July 17, 2002 Zambia GNSS Earth Science 2002 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/yr=1 meter of motion in 100 years

July 17, 2002 Zambia GNSS Earth Science 2002 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

July 17, 2002 Zambia GNSS Earth Science 2002 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

July 17, 2002 Zambia GNSS Earth Science 2002 7

• Largest events in Africa marked.

Catalog Source

National Earthquake Information System

http://neic.usgs.gov/

Locations of earthquakes since 1900

July 17, 2002 Zambia GNSS Earth Science 2002 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

July 17, 2002 Zambia GNSS Earth Science 2002 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

July 17, 2002 Zambia GNSS Earth Science 2002 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

July 17, 2002 Zambia GNSS Earth Science 2002 11

Example of VLBI/GPS systemHartebeesthoek Radio Astronomy Observatory

VLBI System

GPS Antenna

July 17, 2002 Zambia GNSS Earth Science 2002 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.

July 17, 2002 Zambia GNSS Earth Science 2002 13

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 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

July 17, 2002 Zambia GNSS Earth Science 2002 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

July 17, 2002 Zambia GNSS Earth Science 2002 16

Some details of Northern CollisionMeasured GPS Motions in Turkey and Greece

Continuously operating GPS systems allow these types of dense networksNote difference in scale of velocity vectors from previous plots

July 17, 2002 Zambia GNSS Earth Science 2002 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.

July 17, 2002 Zambia GNSS Earth Science 2002 18

Example of real-time 2-hr water vapor measurements

Available from http://www.suominet.ucar.edu/

July 17, 2002 Zambia GNSS Earth Science 2002 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)

July 17, 2002 Zambia GNSS Earth Science 2002 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.