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