Wednesday: 16:00-17:30

Rocco Malservisi: e-mail rocco@geophysik.lmu.dephone 21804202

Class Web page: www.geophysik.lmu.de/~malservisi/TectGPS.html

COMPARISON OF DIFFERENT TECHNIQUES

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

• 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

HOW GPS WORKS

GPS is based on a 3 segment system:

SATELLITE VEICLES

CONTROL SEGMENT

USERS SEGMENT (Receivers, data analysis)

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) (11:58 hr orbits).

• The satellites travel at speed of 3 km/s, and are built to last 10 years.

• 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) (11:58 hr orbits).

• The satellites travel at speed of 3 km/s, and are built to last 10 years.

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

• 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

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.

• 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.

Frequency (MHz)

Wavelength(m)

C/A code 1.023 293

P-code 10.23 29.3

L1 1574.42 0.19

L2 1227.6 0.24

data 30 sec

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.

• 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.

HOW GPS WORKSUSERS SEGMENT (Receivers, data analysis)

• Receivers generate the same code as transmitted by satellites.

• The time delay (t) between a received signal and the receiver’s generated code enables a receiver to estimate its Range to a satellite.

• Receivers generate the same code as transmitted by satellites.

• The time delay (t) between a received signal and the receiver’s generated code enables a receiver to estimate its Range to a satellite.

Range (receiver-satellite) = T x c + errorsPseudorange = T x cRange (receiver-satellite) = T x c + errorsPseudorange = T x c

110 1111 1 1100 0 000

110 1111 1 1100 0 000

110 1111 1 1100 0 000

T=(Tr-Ts)

Received codeReceived code

Receiver generatedReceiver generated

Transmitted codeTransmitted code

t

main error source - receiver clock ( t)main error source - receiver clock ( t)

THE BASIC IDEA

FIND YOUR TIMEPerfect clock Slow clock

Using an extra satellite

HOW TO COMPUTE DISTANCE FROM SVCODE PSEUDORANGE

NOISES

IONOSPHERE

TROPOSPHERE

MULTIPATH

SATELLITE CONFIGURATION/GEOMETRY (DOP)

CLOCKS

MONUMENTS

ORBITS

ANTI SPOOFING (AS)

SELECTIVE AVAILABILITY (S/A)

NOISES

IONOSPHERE and TROPOSPHERE

NOISES

Ionospheric & Tropospheric Effects*

Ionosphere

IonosphereTroposphere

Troposphere

• 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.

• 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)

Atmospheric EffectsSolutions 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).

• 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).

4000

250

26

16

10

0.4

40.

2.5

0.26

0.16

0.1

0.004

100 MHz

400 MHz

L2

L1

2 GHz

10 GHz

TEC=1018 [el/m2]

TEC=1016 [el/m2]Frequency

Ionospheric Range Correction [m]Ionospheric Range Correction [m]

Atmospheric Effects

• 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.

• 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.

Solutions for Tropospheric Effect

NOISES

MULTIPATH

NOISES

SATELLITE CONFIGURATION/GEOMETRYGDOP Geometric Dilution of Precision

HOW TO COMPUTE DISTANCE FROM SVPHASE PSEUDORANGE

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.

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.

Single Difference

GPS

A

B

A

B

AB =A- B

ts

tA

tBBaselineBaseline

Precise relative positioning

Illustration: IGS/JPL/NASA

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).

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). Ai

Bi

AB

GPSi

GPSj

Bj

Bj

BaselineBaseline

Double Difference

Precise relative positioning

Illustration: IGS/JPL/NASA

Relative positioning (DGPS)

• For precise positioning we use a GPS

receiver at known location.

• Since we know this receiver’s 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).

• For precise positioning we use a GPS

receiver at known location.

• Since we know this receiver’s 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

GPS METHODS COMPARISON

Lecture 3 May 10th 2005

www.pbo.unavco.org

Lecture 3 May 10th 2005

Permanent sites examples

GPS Data AnalysisGPS 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

Coseismic Offset

Eruption

Co-Seismic Offsets (Model Co-Seismic Offsets (Model from InSAR & local GPS)from InSAR & local GPS)

[Pedersen et al., 2003]

Co-Seismic CorrectedCo-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

Hekla DeformationHekla Deformation

Co-Seismic CorrectedCo-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

Co-Seismic CorrectedCo-Seismic Corrected

Velocity Field Relative to Stable Velocity Field Relative to Stable North AmericaNorth America