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Vertical Reference Frames for Sea Level Monitoring

Thomas Herring

Department of Earth, Atmosphere and Planetary Sciences

http://www-gpsg.mit.edu/~tah

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Overview

• Examine scale variations in global reference (system scale directly effects heights)

• Effects of satellite phase center positions

• Systematic variations in scale and position

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Acknowledgements

• Scripps Orbit and Permanent Array Center (SOPAC) for making available re-processed permanent GPS array data. Available as full-covariance, loosely-constrained solutions

• Mike Heflin at JPL for reprocessed GIPSY analyses. Time series of results.

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Analysis from SOPAC reprocessed GPS data between 1992 and 2001

• GPS phase measurements at L1 and L2 from a global distribution of station used

• Analysis here is “un-constrained”– All site positions estimated– Atmospheric delay parameters estimated– “Real” bias parameters for each satellite global, integer

values for regional site combinations (<500 km)– Orbital parameters for all satellites estimated (1-day

orbits, 2-revolutions)• 6 Integration constants• 3 constant radiation parameters • 6 once-per-revolution radiation parameters

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Global GPS analysis

• Data used between 1992-2001• Full analysis has ~600 stations (analysis

here restricted to ~100 sites that have more than 5 years of data

• Large density of sites (~300) in California• Total data set has > 2 billion phase

measurements

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Specific sites analyzed

• Total of 100 sites analyzed of which 50 were used to realize coordinate system based on ITRF2000

• Since analysis has little constraint, it is:– Free to rotate

– Possibly free to translate (explicit estimation)

– Possibly free to change scale (explicit estimation)

• Latter two effects should not be present but these we will explore here.

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Network used in analysis

Black: Frame sites; Red other sites

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Results from analysis

• “Common Mode” errors seen in regional frames

• Scale variations: Comparison with ITRF-2000 and recent JPL analysis (Mike Heflin)

• Satellite phase center positions effects

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Scale estimates

-3

-2

-1

0

1

2

3

4

5

1994.0 1995.0 1996.0 1997.0 1998.0 1999.0 2000.0 2001.0 2002.0

GAMIT Δ ( )scale ppb ( )JPL Scale ppb

Year

2000 51- (0.9 / , 0.14 / )ITRF site average mm yr ppb yr

-0.24 /GAMIT Rate ppb yr -0.10 /JPL Rate ppb yr

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Estimated height rates as function of latitude

-15

-10

-5

0

5

10

15

-80.0 -60.0 -40.0 -20.0 0.0 20.0 40.0 60.0 80.0

GAMIT H rate (mm/yr)

H Rate (ITRF00)

H rate (IGSP51) Mean rate 0.9 mm/yr

Latitude (deg)

1 mm/yr error bars shown

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Effects of changes in the satellite phase center position

ψ Zenith Distance

γ

dhδργ

R

a

= dh cos ; sin = (R/a) sin ; (R/a) = 0.24

As the position of the phase center changes, an elevation angle dependent error is introduced.

dh is change in radial component

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Height changes per unit offset as function of minimum and maximum elevation angles used in data analysis

Polar sites will have limited max elevationTypical effect is 0.03 m/m

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Summary of effects

• GAMIT scale rate -0.24 ppb/yr; JPL scale rate -0.10 ppb/yr

• 51-GPS reference sites in ITRF2000 have average height rate of 0.9 mm/yr (0.14 ppb/yr)

• Satellite phase center: Block II 1.023 m or 0.953 m? expected scale change 0.35 ppb

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Conclusion• Both GAMIT and JPL analyses show negative

scale rate. Height rates 1.5-0.6 mm/yr.• Common annual signals in time series are likely

to be real (atmospheric/snow loading effect). ITRF may need non-secular components to site positions to be better define the terrestrial system.

• Evolution of GPS constellation with time might explain part of the scale change or Average height rates in ITRF2000 may be biased.