2-3 November 2009 NASA Sea Level Workshop 1

The Terrestrial Reference Frame and itsImpact on Sea Level Change Studies

GPS

VLBI

John Ries

Center for Space ResearchThe University of Texas at Austin

Errors, especially drifts, in the Terrestrial Reference Frame (TRF) biases sea level change estimates and interpretation in several systematic ways

Directly, through erroneous calibration of altimeter drift from tide gauges

Scale drift (global vertical rate errors) directly affects calibration

X and Y drift (impacts calibration through non-uniform distribution of tide gauges)

Even if TRF origin and scale from SLR and VLBI is well-determined, access to the TRF is typically through GPS; is the GPS-determined frame accurately tied to SLR/VLBI frame?

Indirectly, through erroneous drifts in computed orbit as it ‘tries’ to follow the TRF origin rather than the true mass center

2-3 November 2009 NASA Sea Level Workshop 2

Role of TRF/EOP

• The Terrestrial Reference Frame (TRF) and the associated Earth Orientation Parameters (EOP) underpin geocentric mean sea level determination through:

– Calculation and verification of precise (cm-level) orbits for altimeter satellites

– Calibration of altimeter systems using tide gauges or altimeter calibration sites

– Connecting sea level change across different missions

2-3 November 2009 NASA Sea Level Workshop 3

Determination of TRF and EOP

• High precision geodesy is very challenging– Accuracy of 1 part per billion

• Fundamentally different observations with unique capabilities

• Together, they provide cross validation and increased accuracy

• To realize the advantages of each technique, good distribution and accurate ties are required

TechniqueSignalSourceObs. Type

VLBI MicrowaveQuasarsTime difference

SLROpticalSatelliteTwo-way absolute range

GNSSMicrowaveSatellitesRange change

Celestial FrameUT1

Yes No No

Polar Motion Yes Yes Yes

Scale Yes Yes Yes

Geocenter No Yes Yes

Geographic Density

No No Yes

Real-time Yes Yes Yes

Decadal Stability

Yes Yes Yes

2-3 November 2009 NASA Sea Level Workshop 4

Geodetic Networks: SLR Site Map

Targets: LAGEOS-1, LAGEOS-2 (LEO and GNSS in the future)

2-3 November 2009 NASA Sea Level Workshop 5

Geodetic Networks: VLBI Site Map

Targets: Quasars

2-3 November 2009 NASA Sea Level Workshop 6

How large might the TRF errors be?

0.5 mm/y

2-3 November 2009 NASA Sea Level Workshop 7

Can systematic TRF errors hurt?

• Consider an offset or drift of the TRF along the Z-axis

– The computed orbit and the observed sea surface height follow this drift almost one-for-one

– Computed global mean sea level trend is then biased by ~10% of the Z-drift and regional sea level trends up to 40%-50%

– Assuming a possible TRF Z-drift of up to 1.8 mm/yr, this leads to ~0.2 mm/yr in global mean sea level and up to 0.9 mm/yr in some regions

2-3 November 2009 NASA Sea Level Workshop 8

Relative Origin of ITRF2005 vs ITRF2000

Mean X, Y and Z difference between Jason-1 SLR/DORIS orbits computed with ITRF2000 and ITRF2005

Changes observed global MSL rise by ~0.15 mm/y

ITRF2005 - ITRF2000(70 core stations)

2-3 November 2009 NASA Sea Level Workshop 9

TOPEX Radial Orbit Difference TrendITRF2005 – GDR (CSR95)

2-3 November 2009 NASA Sea Level Workshop 10

Reference frame scale andaltimeter calibration

• The calibration of the altimeter drift is based on comparisons with tide gauges

• A drift in the scale of the reference frame leads to a uniform error in all vertical rates including at tide gauges

• Even if current scale drift rate is only ~0.04 ppb/yr, it would still be >0.2 mm/yr sea level equivalent

• Could the scale drift rate (based on VLBI/SLR) be larger than this?

– The apparent internal consistency of the VLBI solutions may reflect more a strong commonality in processing than true accuracy

– To be confident that the VLBI/SLR scale is globally applicable, we would need VLBI/SLR all over the world

– These stations are generally not located at tide gauges; positioning typically from GNSS

– Is the scale of the GNSS reference frame the same as SLR/VLBI

ITRF2000 scale (ppb)

2-3 November 2009 NASA Sea Level Workshop 11

Reference frame driftsand altimeter calibration

• Translational drift along X and/or Y axes could add additional error

– Distribution of tide gauges not well balanced along either axis

– Internal consistency of SLR results suggests good reliability but it is the only technique that provides a strong tie to the origin (no redundancy)

(from Cazanave & Nerem, 2004) ITRF2000 origin (mm)

2-3 November 2009 NASA Sea Level Workshop 12

Post Glacial Rebound

Space geodesy helps discriminate between various models for PGR (or GIA), which has two impacts:

Accurately modeling the vertical motion at the tide gauge sites using GIA models (where there are no other geodetic observations of the height change) allows for more precise altimeter calibration

Confidence in the models for GIA also aids interpretation of geological sea level signals

2-3 November 2009 NASA Sea Level Workshop 13

TRF/EOP…the critical infrastructure

• TRF/EOP provides the stable coordinate system that allows us to link measurements over space and time

– Errors in the TRF/EOP can have important impacts on sea level observation accuracy

– The geodetic networks provide the structure and observations that supports high precision orbit determination

– They provide Earth system change observations themselves

• Gravity changes from SLR showing long wavelength water redistribution

• Loading signals from GPS

• Earth rotation variations due to changing mass distribution

Reported by Cox and Chao, (SCIENCE, 2002); Cheng and Tapley (JGR, 2004)

2-3 November 2009 NASA Sea Level Workshop 14

Space geodesy is a key observation systemfor identifying Earth System variations

“Enhanced and interconnected” GPS, SLR and VLBI networks are a required future capability for the Earth Surface and Interior focus area within the NASA ESE Strategy (Oct. 2003)

“ITRF and EOP, hence the networks [VLBI, SLR, GPS], should continue to be maintained and improved and their data routinely acquired at the best possible accuracy and temporal resolution.” (SESWG Report 2002)

(from the Strategic Plan for the U.S. Integrated Earth Observing Program)