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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

min

g

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