1. 29 August 2005Geosciences Australia1 Space Geodesy, SLR and Global Sea Level Change John Ries Canberra, Australia August 29,,2005

2. 29 August 2005Geosciences Australia2 Why is SLR Important? “SLR has developed into the most important geodetic instrument for the establishment of an accurate global geodetic infrastructure and earth monitoring science. SLR contributes to: –The definition of the International Terrestrial Reference Frame (ITRF) by being the only space geodetic technique which defines the earth's centre of mass. In addition, provides scale and the core network for the ITRF. –Monitoring Earth Rotation and Polar Motion to provide the relationship with the Celestial Reference Frame (CRF). –Modelling the temporal and spatial variation of the Earth's gravity field. –Determination of ocean and earth tides. –Monitoring tectonic plates and horizontal and vertical crustal deformation. –Orbit determination for spaceborne altimeters and radar measurements for studies in global ocean circulation and changes in ice masses. “ from: http://www.ga.gov.au/geodesy/slr/slroverview.jsp

3. 29 August 2005Geosciences Australia3 Global mean sea level determination… how is it done? An example: “…the geocentric rate of global mean sea level rise over the last decade (1993–2003) is now known [very accurately], +2.8±0.4 mm/yr, as determined from TOPEX/Poseidon and Jason altimeter measurements…” Cazenave & Nerem, Present-day sea level change: Observations and causes, Reviews Of Geophysics, 42, RG3001, 2004. What do the authors assume to be available that is implicitly or explicitly required for their analysis? What infrastructure allows them to make this observation ?

4. 29 August 2005Geosciences Australia4 Role of the TRF/EOP The Terrestrial Reference Frame (TRF) and the associated Earth Orientation Parameters (EOP) underpin geocentric mean sea level determination through: –The TRF provides the background against which position and velocity have meaning –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

5. 29 August 2005Geosciences Australia5 Can Systematic TRF Errors Hurt? Consider an erroneous drift of the TRF along the Z-axis –The computed orbit and the observed sea surface height follow this drift –Computed global mean sea level trend is then biased by 10% of the Z-drift and regional sea level trends up to 40%-50% (Nerem et al., 1998) –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 ITRF2000 origin (mm)

6. 29 August 2005Geosciences Australia6 Reference Frame Scale and Altimeter Calibration (1) Altimeter-based sea level changes are meaningless without reliable 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 Current scale drift rate might be ~0.03 ppb/yr, or ~0.2 mm/yr sea level equivalent ITRF2000 scale (ppb)

7. 29 August 2005Geosciences Australia7 Reference Frame Scale and Altimeter Calibration (2) To be confident that the VLBI/SLR scale is globally applicable, we would need VLBI/SLR everywhere There is probably no tide gauge where VLBI is, and few by SLR….mostly GPS. How then to control GPS scale by VLBI and SLR? Dan MacMillan (Fall AGU 2004) compared GPS and VLBI baseline length time series and found a scale rate difference GPS-VLBI of 0.25 ppb/yr (1.8 mm/yr!) –Average vertical rate difference for co-located stations was 1.5 mm/yr If tide gauge vertical rates are tied to GPS, this is a problem –Problem is same for geophysical studies –GPS provides great precision; absolute velocity, in the context of the terrestrial reference frame, is another matter

8. 29 August 2005Geosciences Australia8 Is a Local Analysis Truly Local? An example: “…sea level at Hilo…has risen an average of 1.8±0.4 mm/yr faster than at Honolulu…However, GPS measurements indicate that Hilo is sinking relative to Honolulu at a rate of -0.4±0.5 mm/yr…” Caccamise II et al., Sea level rise at Honolulu and Hilo, Hawaii: GPS estimates of differential land motion, Geo. Res. Lett., 32, L03607, 2005. Such analyses of the relative motion of two close points, regardless of the measurement system, are not ‘free-standing’ They assume an accurate TRF (for the fixed sites), an accurate Earth orientation time series, and accurate satellite orbits (in this case, IGS orbits for the GPS spacecraft) Earth orientation, in turn, requires all techniques Techniques must be well distributed with good ties to sort out 14 degrees of freedom (bias and rates for rotation, origin and scale)

9. 29 August 2005Geosciences Australia9 TRF/EOP…the Critical Infrastructure The TRF provides the essential stable coordinate system that allows us to link measurements over space and time –Absolute position/velocity only have meaning in the context of the TRF The TRF must be sufficiently redundant to remain stable over decades, even through the evolution of both the ground-based network and the space-based segment –Sites with long-term occupations, like Yaragadee, are particularly valuable for the accuracy and long-term stability of the TRF The space geodetic networks provide the critical infrastructure necessary to develop and maintain the TRF, and SLR plays a unique and crucial role in that infrastructure

10. 29 August 2005Geosciences Australia10 Why Do We Have Three Techniques? 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 Technique Signal Source Obs. Type VLBI Microwave Quasars Time difference SLR Optical Satellite Two-way absolute range GPS Microwave Satellites Range change Celestial Frame UT1 Yes No Polar Motion Yes Scale Yes GeocenterNo Yes Seasonal variations Geographic Density No Yes Real-timeYes Decadal Stability Yes

11. 29 August 2005Geosciences Australia11 Geodetic Networks: GPS Site Map Targets: GPS Spacecraft

12. 29 August 2005Geosciences Australia12 Geodetic Networks: VLBI Site Map Targets: Quasars

13. 29 August 2005Geosciences Australia13 Geodetic Networks: SLR Site Map Targets: LAGEOS-1, LAGEOS-2

14. 29 August 2005Geosciences Australia14 Geodetic Networks: SLR Site Map

15. 29 August 2005Geosciences Australia15 TRF/EOP…the Critical Infrastructure The TRF and EOP provide the stable coordinate system that allows us to link measurements over space and time –They provide the background against which position and velocity have meaning –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 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) – They provide Earth system change observations themselves

16. 29 August 2005Geosciences Australia16

17. 29 August 2005Geosciences Australia17 Reference Frame Drifts and Altimeter Calibration Translational drift along X and/or Y axes adds 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)

18. 29 August 2005Geosciences Australia18 SLR Contributions and Future Developments Current and Future Science Results from SLR –Maintaining and improving terrestrial frame (esp. translation and its impact on sea level monitoring) –Monitor geocenter motion (important component of mass transport) –Improved GM and terrestrial scale –Improved results for secular and non-tidal gravity changes –Tidal dissipation –Tie new and old altimeter missions together, absolute verification of satellite height –Accurate link between low orbit and lunar orbit –Tracking support for science missions (esp. those with failed on-board systems) –S/C force model studies (e.g., Yarkovsky-types) Required Developments –Stable (long-term commitment), well-distributed, high precision sites, with ties to other techniques –Continue efforts to reduce biases to mm level and improve troposphere modeling –New optimized satellite targets (properly designed reflectors in more useful orbits)

19. 29 August 2005Geosciences Australia19 Geodetic Laser-ranged Satellites LAGEOS Etalon

20. 29 August 2005Geosciences Australia20 Importance of Yaragadee to the TRF Drift change in TRF when Yaragadee data not included: Tx = ~ 0.6 mm/yr Ty = ~ 1.0 mm/yr Tz = ~ 1.0 mm/yr Scale = ~ 0.3 mm/yr This level of impact is similar in size as the uncertainties in past TRF solutions; it is not the direction we want to be heading as we try to improve the TRF to support future science requirements

21. 29 August 2005Geosciences Australia21 What Else is Implicitly Assumed? “…if the effects of postglacial rebound are removed “ 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 Other areas of geophysics rely on the TRF as well (studies of plate motion, plate deformation....)