Tectonics through geodesy (GPS)

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Tectonics through geodesy (GPS)
Wednesday 1600-1730
Rocco Malservisi e-mail rocco_at_geophysik.lmu.d
e phone [phone]
Class Web page www.geophysik.lmu.de/malservisi/
TectGPS.html
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COMPARISON OF DIFFERENT TECHNIQUES
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The Global Positioning System

• The Global Positioning System (GPS) is a
  satellite-based navigation system.
• GPS was originally intended for military
  applications, but in the 1980s, the government
  made the system available for civilian use.
• GPS works in any weather conditions, anywhere in
  the world, 24 hours a day. There are no
  subscription fees or setup charges to use GPS
• Some civilian uses
• Navigation on land, sea, air
• and space
• Geophysics research
• Guidance systems
• Geodetic network densification
• Hydrographic surveys

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HOW GPS WORKS
GPS is based on a 3 segment system
SATELLITE VEICLES
CONTROL SEGMENT
USERS SEGMENT (Receivers, data analysis)
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HOW GPS WORKS
SATELLITE VEICLES (Space segment)

• The GPS satellite constellation includes 28
  satellites in 6 orbits (55 inclination).
• Satellite orbital path is near to circular, with
  a semi-major axis of about 26,600 km (20000 km
  hight) (1158 hr orbits).
• The satellites travel at speed of 3 km/s, and are
  built to last 10 years.

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HOW GPS WORKS
SATELLITE VEICLES (Space segment)

• Time kept by Cesium or Rubidium Clocks (3)
• SVs broadcast on 2 wavelenght L1 (1.5GHz, 19cm)
  L2 (1.2 GHz 24cm)
• Signals modulated by a code (discussed later)
• Message with satellite personal code,
  ephemerides and satellite health

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GPS SIGNAL

• Each satellite transmits low-power radio signals
  in 2 carrier frequencies
• L1 1575.42 MHz 154 time base oscillator
• L2 1227.6 MHz 120 time base oscillator
• The signal contains two complex patterns of
  digital signals Precise (P) code and
  Coarse/Acquisition (C/A) code
• A long period modulation broadcast
  data as SV or
  ephemerides.

Wavelength(m) Frequency (MHz)
293 1.023 C/A code
29.3 10.23 P-code
0.19 1574.42 L1
0.24 1227.6 L2
30 sec data
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HOW GPS WORKS
CONTROL SEGMENT

• ground-based facilities are used to monitor and
  control the satellites.
• Checking and reporting the satellites operational
  health.
• Checking their exact position in space.
• The master ground station transmits
• Corrections for the satellite's ephemeris
  constants.
• Clock offsets.
• The GPS signal is updated
  every 2 hours.
• The satellites can then incorporate
  these updates in
  the signals they
  send to GPS receivers.

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HOW GPS WORKS
USERS SEGMENT (Receivers, data analysis)

• Receivers generate the same code as transmitted
  by satellites.
• The time delay (Dt) between a received signal and
  the receivers generated code enables a receiver
  to estimate its Range to a satellite.

Range (receiver-satellite) DT x c
errors Pseudorange DT x c

• main error source - receiver clock (d t)

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THE BASIC IDEA
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FIND YOUR TIME
Using an extra satellite
Perfect clock
Slow clock
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HOW TO COMPUTE DISTANCE FROM SVCODE PSEUDORANGE
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NOISES

• IONOSPHERE
• TROPOSPHERE
• MULTIPATH
• SATELLITE CONFIGURATION/GEOMETRY (DOP)
• CLOCKS
• MONUMENTS
• ORBITS
• ANTI SPOOFING (AS)
• SELECTIVE AVAILABILITY (S/A)

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NOISES
IONOSPHERE and TROPOSPHERE
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NOISES
Ionospheric Tropospheric Effects

• Delay of GPS signal - code modulation and carrier
  phases
• Carrier phases are greatly effected by the free
  electrons in the Ionosphere.
• The Ionospheric effect increase as the Total
  Electron Content (TEC) increase.
• The Ionosphere is a dispersive medium its
  effect is frequency dependent.
• Troposphere is non-dispersive medium effecting
  both code modulation and carrier phases the same
  way.

For more See Leick (1995)
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Atmospheric Effects
Solutions for Ionospheric Effect

• The GPS message contains Ionospheric model
  data.This allow the computation of the
  approximate group delay.
• Dual-Frequency Ionospheric-free Solution by
  using dual-frequency (L1 L2) receivers
  (Expensive).

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Atmospheric Effects
Solutions for Tropospheric Effect

• The Tropospheric delay can vary from 2.0-2.5m in
  the zenith, to 20-28m at a 5o angle.
• The delay depends on the temperature, humidity
  and pressure
• The dry atmosphere can be accurately modeled to
  about 2-5 based on the laws of ideal gases
• The wet component is more difficult to quantify,
  but its contribution is only about 10 of the
  total effect
• The wet delay is about 5-30 cm. In continental
  midlatitudes.

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NOISES
MULTIPATH
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NOISES
SATELLITE CONFIGURATION/GEOMETRY GDOP Geometric
Dilution of Precision
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HOW TO COMPUTE DISTANCE FROM SVPHASE PSEUDORANGE
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Precise relative positioning
Single Difference

• Single Difference phase observable cancels most
  common SV errors, such as SV clock error.
• Other errors decrease as the length of the
  Baseline is shorter.

Illustration IGS/JPL/NASA
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Precise relative positioning
Double Difference

• Uses the L1 and L2 Carrier frequencies
  (wavelength 19-24 cm) to calculate precise
  positioning between 2 GPS stations.
• Double differencing received signals at both
  stations cancels out most systematic errors
  (station and satellite clock offsets).

Illustration IGS/JPL/NASA
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Relative positioning (DGPS)

• For precise positioning we use a GPS receiver at
  known location.
• Since we know this receivers exact location, we
  can determine the errors in the satellite
  signals.
• Corrections are transmitted from the base-station
  to various users.
• Positioning accuracy is 1-2 m (Pseudorange
  wavelength 300 m).

Illustration garmin.com
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GPS METHODS COMPARISON
Lecture 3 May 10th 2005
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Permanent sites examples
www.pbo.unavco.org
Lecture 3 May 10th 2005
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GPS Data Analysis

• GIPSY-OASIS 2.5 Zumberge et al. 1997
• JPL Precise Orbits
• ITRF-97
• Atmospheric ionospheric models
• Error Analysis Mao et al. 1999
• Position Uncertainties (mean) 3, 6 12 mm
• Rate Uncertainties (mean) 1.0, 1.3 2.5 mm/a

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Co-Seismic Offsets (Model from InSAR local GPS)
Pedersen et al., 2003
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Co-Seismic Corrected

• June 17 21, 2000 SISZ earthquakes
• Distributed slip model Pedersen et al., 2003
• Correct positions for offsets, recalculate time
  series
• Residual Feb. 28 March 6, 2000 Hekla eruption

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Hekla Deformation
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Co-Seismic Corrected

• June 17 21, 2000 SISZ earthquakes
• Distributed slip model Pedersen et al., 2003
• Correct positions for offsets, recalculate time
  series
• Residual Feb. 28 March 6, 2000 Hekla eruption

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Co-Seismic Corrected
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Velocity Field Relative to Stable North America