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Real-Time GPS Processing with Carrier Phase
FILTER PARAMETER INFLUENCE ON GPS CARRIER PHASE REAL-TIME TIME AND
FREQUENCY TRANSFER FOR SHORT AND LONG BASELINES
Carsten Rieck, Per Jarlemark, Kenneth Jaldehag and Jan Johansson
SP Swedish National Testing and Research InstituteBox 857, S-501 15 Borås Sweden
36th PTTIDecember 7-9, 2004Washington DC
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Real-Time T&F GPS
1. Introduction
2. Real-time-GPS Filter
3. Station Position Correction Estimation
4. Satellite Orbits
5. Earth Tide Modeling
6. Long Baselines
7. Conclusions and Future Work
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Real-Time T&F GPS: Principle Operation
REC1
CLK1
REC2
CLK2
RECN
CLKN
.
.
.
Inte
rNet
wor
k T
CP
/IP
Cli
ent/
Ser
ver
Dat
a S
trea
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Rea
l-T
ime
Kal
man
Fil
ter
IGS-Orbits
Broadcast EPH
KF-States
CLK1-2, … CLK1-N
•TROP1, … TROPN
STAPOS1, … STAPOSN
•AMBIGUxy x – clockdiff y – satellite
Residuals: Receiver Pair/Sat
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Real-Time Kalman Filter
Raw Data
Modeling
RT Filter
CLKSTATROPSTAPOSAMBIGU
- common view differential phase observations, L3 ionospheric ”free” phase, P3 code support,
- satellite orbits (broadcast, IGS predicted,P3)- earth tides (simple elastic)- continental drift, rebound- ocean loading (optional)
- forward Kalman filter, Matlab/GNU Octave,
purely L3 based, RW noise model
non standard troposphere, RW noise model
station position corrections, constant
non-integer ambiguities, constant
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Method & Result Verification
• “real-time” streaming of RINEX observation and navigation files to the filter software
• RT results compared to solutions based on
• TWSTFT,• Circular-T,• GPS post-processed PPP, • GPS-P3• Real-Time
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Results: UTC(SP)-UTC(PTB) 623km
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Results: UTC(SP)-UTC(PTB) 623km
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Results: UTC(SP)-UTC(PTB) 623km
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Results: UTC(SP)-UTC(PTB) : RT-TWSTFT
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Station Position Offsets
•assume: perfect orbits & atmosphere modeling,
•max range error is initially absorbed by the respective (non-integer) satellite ambiguity parameter
•Several different filter state variables have to compensate for an absolute range error of 2:
• CLK
• STATROP • residuals
• SAT AMB
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West->East Dominance of Satellite Passes
A majority of all satellite passes have a west-east orientation and thus dominantly influence the initial satellite ambiguities in one direction.
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Station Position Offset Example SP-PTB
• East : 10 mm offset f 200 ps/day• North : 10 mm offset f 35 ps/day• Vertical : 10 mm offset f 2 ps/day
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Station Position Correction
• If station positions are not properly known they can be estimated as constant offsets from an approximate position.
• Offsets do not necessarily reflect real positions but correct for wrong baseline geometry (local).
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Orbit Position Modeling
• Broadcast Ephemerides have an uncertainty of about 2m (IGS).
• IGS ultra-rapid orbit predictions are available in real-time and can be used to correct broadcast orbits.
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Earth Tide Modeling , Simple Elastic (VLBI)
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Earth Tide Modeling – Complex & Ocean Loading
http://www.oso.chalmers.se/~loading
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Earth Tides – Differential Errors of Simple Model
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AMC2PTBB
MDVJ
ALGO
• support stations allow to estimate a better local troposphere precise station positions and good orbits needed !
>3x >3x
2361 km
786 km 622 km
1484 km
Transatlantic Baselines : UTC(SP)-UTC(USNO)
USNO
SP01
• 1 common sat observation versus 3 states (clk+2trop)
6184 km
>1x
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Transatlantic Baselines : UTC(SP)-UTC(USNO)
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Conclusions and Future Work
• In good agreement with GIPSY, TWSTFT and Circular-T.• UTC(SP) –UTC(PTB): 40ps/day frequency offset
between TWSTFT and RT.• RMS differences of 50ps between real-time processing
and GIPSY PPP post-processing are achievable.• To do … filter :
– combination with code, steer the long term frequency,– loading models, correct short term frequency,– short smoothing, – KF code migration to a ”clean” POSIX environment.
• To do … real-time data handling:– support for raw data streams, NTRIP,…– encryption, authentication,– multicast mode.
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Thanks and Contact
Thanks to:
IGSOpen Source CommunityLMV, National Land Survey of SwedenOSO, Onsala Space Observatory
Contact :Carsten Rieck carsten.rieck@sp.seSP / Mte http://tempus.sp.seBox 857 +46 33 165440Borås / Sweden
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