Transcript Document
Wednesday: 16:00-17:30
Rocco Malservisi:
e-mail [email protected]
phone 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
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.
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
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.
Frequenc Waveleng
y (MHz)
th(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.
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 receiver’s generated
code enables a receiver to estimate its Range to a satellite.
11000111101000 11
Transmitted
code
Received
11000111101000 11 code
11000111101000 11
dt
D T=(Tr-Ts)
Receiver
generated
Range (receiver-satellite) = DT x c + errors
Pseudorange = DT x c
main error source - receiver clock (d t)
THE BASIC IDEA
FIND YOUR TIME
Perfect clock
Using an extra satellite
Slow clock
HOW TO COMPUTE DISTANCE FROM SV
CODE 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*
• 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.
Ionosphe
Troposph
re
ere
* For more See Leick (1995)
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). Ionospheric Range Correction [m]
Frequency
TEC=1016
[el/m2]
TEC=1018
[el/m2]
100 MHz
40.
4000
400 MHz
2.5
250
L2
0.26
26
L1
0.16
16
2 GHz
0.1
10
10 GHz
0.004
0.4
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.
NOISES
MULTIPATH
NOISES
SATELLITE CONFIGURATION/GEOMETRY
GDOP Geometric Dilution of Precision
HOW TO COMPUTE DISTANCE FROM SV
PHASE PSEUDORANGE
Precise relative positioning
Single Difference
GPS
Single Difference phase
observable cancels most
common SV errors, such as SV
clock error.
dts
rB
rA
Other errors decrease as the
length of the Baseline is shorter.
DrAB =rA- rB
A
dtA
Baseline
dtB B
Illustration: IGS/JPL/NASA
Precise relative positioning
Double Difference
Uses the L1 and L2 Carrier frequencies
(wavelength ~ 19-24 cm) to calculate precise
positioning between 2 GPS stations.
GPSi
GPSj
Double differencing received
signals at both stations cancels
r
out most systematic errors
i
B
rBj
rBj
rAi
(station and satellite clock offsets).
A
Baseline
B
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 basestation to various users.
• Positioning accuracy is 1-2 m (Pseudorange
wavelength ~ 300 m).
Illustration: garmin.com
GPS METHODS COMPARISON
Lecture 3 May 10th 2005
Permanent sites examples
www.pbo.unavco.org
Lecture 3 May 10th 2005
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
Coseismic Offset
Eruption
Co-Seismic Offsets (Model
from InSAR & local GPS)
[Pedersen et al., 2003]
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
Hekla Deformation
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
Co-Seismic Corrected
Velocity Field Relative to Stable
North America