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Applications of the Global Positioning System
Prof. Thomas Herring
Department of Earth, Atmosphere and Planetary Sciences
12.080 Seminar Fall 2004.
http://geoweb.mit.edu/~tah
10/01/2004 12.080 GPS 2
Overview
• Briefly review history of GPS: original aims of few-meter positioning
• Examine some MIT projects where GPS is used to make sub-millimeter position measurements and study deformation processes.– New Zealand– Oil field deformation– North America Plate deformation
10/01/2004 12.080 GPS 3
GPS Original Design (circa 1970)
• Started development in the late 1960s as NAVY/USAF project to replace Doppler positioning system
• Aim: Real-time positioning to < 10 meters, capable of being used on fast moving vehicles.
• Limit civilian (“non-authorized”) users to 100 meter positioning.
10/01/2004 12.080 GPS 4
GPS Design• Innovations:
– Use multiple satellites (originally 21, now ~28)– All satellites transmit at same frequency– Signals encoded with unique “bi-phase,
quadrature code” generated by pseudo-random sequence (designated by PRN, PR number): Spread-spectrum transmission.
– Dual frequency band transmission (allows propagation delay due to the ionosphere to be removed):• L1 ~1.5 GHz, L2 ~1.25 GHz
– Use of phase measurements allows millimeter level position determinations
10/01/2004 12.080 GPS 5
Latest Block IIR satellite(1,100 kg)
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• Total system cost is over $10B • Average cost per satellite is
~$60M• None of these costs are passed
directly to users.• Satellites transmit signals that
any one with the correct receivers can use (no use tax on receivers)
10/01/2004 12.080 GPS 6
Measurements• Measurements:
– Time difference between signal transmission from satellite and its arrival at ground station (called “pseudo-range”, precise to 0.1–10 m)
– Carrier phase difference between transmitter and receiver (precise to a few millimeters)
– Doppler shift of received signal• All measurements relative to “clocks” in ground
receiver and satellites (but use of multiple satellites and receivers allow this problem to be removed).
• High precision, dual frequency receivers now $4000-$6000
10/01/2004 12.080 GPS 7
Satellite constellation
• Since multiple satellites need to be seen at same time (four or more):– Many satellites (original 21 but now 28)– High altitude so that large portion of Earth
can be seen (20,000 km altitude —MEO)
10/01/2004 12.080 GPS 8
Current constellation
• Relative sizes correct (inertial space view)
• “Fuzzy” lines not due to orbit perturbations, but due to satellites being in 6-planes at 55o inclination.
10/01/2004 12.080 GPS 9
Some MIT projects using GPS
• The MIT Geodesy and Geodynamics group (http://geoweb.mit.edu) is involved in many projects around the world with using GPS.
• Three projects to discuss (all involving measurement of height changes)– Uplift in New Zealand Southern Alps– Subsidence in oil fields– Uplift and subsidence across North America (response to
last ice-age: Glacial Isostatic Adjustment GIA)• All these projects involve collaborations with other groups.
10/01/2004 12.080 GPS 10
A Direct Geodetic Measurement of the Uplift Rate of the Southern Alps
John Beavan1
Mikael Denham2
Paul Denys2
Brad Hager3
Tom Herring3
Chuck Kurnik4
Dion Matheson1
Peter Molnar5
Chris Pearson2
1 GNS2 Otago University3 MIT4 UNAVCO5 Univ. Colorado
10/01/2004 12.080 GPS 11
New ZealandTectonic and bathymetric
setting
Image from NIWANational Institute of Water and Atmospheric Research Ltd
Central South Island experiences oblique continental collision at about 40 mm/yr
Shortening component normal to Alpine fault is about 10 mm/yr
10/01/2004 12.080 GPS 12
SAGENZ Profile: Karangarua to Lake Tekapo
MTJO
QUAR
Christchurch
Southern Alps Geodetic Experiment - New Zealand
Pacific Plate
Australian Plate
Image courtesy of Earth Sciences and Image Analysis Laboratory, NASA Johnson Space Center
40 mm
/yr
10/01/2004 12.080 GPS 13
OCCUPATION STRATEGY for CONTINUOUS and SEMI-CONTINUOUS GPS STATIONS
• Permanent ground marks at all sites, with force-centered antenna mounts
• Five sites occupied continuously, with data transmitted by radio modem and dial-up internet
• Six other sites occupied semi-continuously by rotating 3 receivers between the sites every 3 months
• 60-second sampling used to enable 3 months of data to be stored in internal memory
• Same antennas used at each site during each occupation
10/01/2004 12.080 GPS 14A Southern Alps semi-continuous GPS station
10/01/2004 12.080 GPS 15
Sometimes we have to find the sites
10/01/2004 12.080 GPS 16
And test snow integrity
10/01/2004 12.080 GPS 17
MTJO
CNCL
KARA
QUARNETT
MAKA
VEXA
WAKA
LEOC
PILK
REDD
Continuous station
Semi-continuous
SAGENZ GPS STATIONSA
lpin
e
Fau
ltNW SE
Image courtesy of Earth Sciences and Image Analysis Laboratory, NASA Johnson Space Center
10/01/2004 12.080 GPS 18
Vertical position time series after
filtering to remove annual signals and
common-mode noise
• RMS about a straight line fit is typically 2.5 - 4 mm. Worst case (NETT) is 6.5 mm.
10/01/2004 12.080 GPS 19Measured uplift rates and 95% confidence
10/01/2004 12.080 GPS 20
For a complete change in climate:Oil field monitoring in Oman.Summer daytime temperatures of 50C (120F)
10/01/2004 12.080 GPS 21
Basic GPS setupSite at the suspected center of the subsidence
10/01/2004 12.080 GPS 22
Continuous GPS site (5 of these)
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GPS Monument and antenna
Passive cooled equipment box Telemetry antenna
Solar panelQuickTime™ and a
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Drilled-braced monument
10/01/2004 12.080 GPS 23
Rover GPS sites (45 of these)
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355 mm
Pillar is ~3 meters deep
10/01/2004 12.080 GPS 24
Time series of height estimates in the center of the field
Receiver failure and repair
Error bars scaled based on “global” GPS analysis
Two analysesshown that treatatmospheric delaydifferently
10/01/2004 12.080 GPS 25
Horizontalmotion at REM3
Note: if the sitewere truly at the center of the subsidence, no horizontal motion would be expected
Some non-steady motion can be seen
10/01/2004 12.080 GPS 26
Deformation from 9-months GPS
10/01/2004 12.080 GPS 27
Postglacial rebound in North America
• Here is combine temporal changes in position estimates from ~300 sites across North America.
• These results are generated by many different groups and need to be carefully evaluated.
• The results from the best sites are used to infer which GIA models best match the measurements. (Depends on structure of the Earth)
10/01/2004 12.080 GPS 28
Fit of COD to GIA: 9/1995-3/2004: 17 sites
This minimummoves with increasing LT
Lithosphere Thickness (LT) 71 kmDetails here depend geographic sites distribution
10/01/2004 12.080 GPS 29
Comparison of PUR solution (red, 50% confidence ellipses) with GIA model 71 km LT, UM 1, LM 2x1021 Pa-s
Fit: 26-sitesN 0.6 mm/yrE 0.3 mm/yrU 1.9 mm/yr
10/01/2004 12.080 GPS 30
Time series of Site GDAC for North and Height
10/01/2004 12.080 GPS 31
Conclusions
• GPS dual-use technology: Applications in civilian world widespread– Geophysical studies (mm accuracy)– Engineering positioning (<cm in real-time)– Commercial positioning: cars, aircraft, boats (cm
to m level in real-time)• MIT has projects using GPS in many parts of the
world studying a variety of problems• Plate Boundary Observatory will be a major project
over the next 10 years in the US