1. 11-12 April 2005NASA Sea Level Workshop1 SPACE GEODETIC MEASUREMENT SYSTEM FOR GLOBAL SEA LEVEL CHANGE "In all things it is a good idea to hang a question mark now and then on the things we have taken for granted.” Bertrand Russell GPS VLBI John Ries Contributors: Zuheir Altamimi, Srinivas Bettadpur, Steve Klosko, Frank Lemoine, Chopo Ma, Angelyn Moore, Ruth Neilan, Carey Noll, Michael Pearlmann, Frank Webb, Pascal Willis, and Tom Yunck

2. 11-12 April 2005NASA Sea Level Workshop2 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, 3.1 mm/yr if the effects of postglacial rebound are removed.” 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 ?

3. 11-12 April 2005NASA Sea Level Workshop3 Role of the 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

4. 11-12 April 2005NASA Sea Level Workshop4 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)

5. 11-12 April 2005NASA Sea Level Workshop5 Reference frame scale and altimeter calibration Altimeter-based sea level changes are meaningless without reliable instrument 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 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 ITRF2000 scale (ppb)

6. 11-12 April 2005NASA Sea Level Workshop6 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)

7. 11-12 April 2005NASA Sea Level Workshop7 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

8. 11-12 April 2005NASA Sea Level Workshop8 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. 11-12 April 2005NASA Sea Level Workshop9 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 The space geodetic networks provide the critical infrastructure necessary to develop and maintain the TRF and the needed terrestrial and space-borne technology to support NASA’s missions and goals

10. 11-12 April 2005NASA Sea Level Workshop10 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 Geographic Density No Yes Real-timeYes Decadal Stability Yes

11. 11-12 April 2005NASA Sea Level Workshop11 Geodetic Networks: GPS Site Map Targets: GPS Spacecraft

12. 11-12 April 2005NASA Sea Level Workshop12 Geodetic Networks: VLBI Site Map Targets: Quasars

13. 11-12 April 2005NASA Sea Level Workshop13 Geodetic Networks: SLR Site Map Targets: LAGEOS-1, LAGEOS-2 (need better targets)

14. 11-12 April 2005NASA Sea Level Workshop14 Geodetic Networks: SLR Site Map Targets: LAGEOS-1, LAGEOS-2 (need better targets)

15. 11-12 April 2005NASA Sea Level Workshop15 Conclusions 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. –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)

16. 11-12 April 2005NASA Sea Level Workshop16 Recommendations Analogous to DSN, GNSS, TDRSS, the geodetic networks should be programmed/funded at the top or beyond the agency (e.g., NASA) level Science should not be competing with the very infrastructure that makes that science possible

17. 11-12 April 2005NASA Sea Level Workshop17 Space geodesy is a key observation system for 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)

18. 11-12 April 2005NASA Sea Level Workshop18 Backup material

19. 11-12 April 2005NASA Sea Level Workshop19 Space Geodetic Performance

20. 11-12 April 2005NASA Sea Level Workshop20 Monitoring Temporal Gravity Changes Using SLR Anomalistic behavior of J 2 time series Large-scale gravity changes reflect water being redistributed around the world Obvious important implications for sea level changes +0.6 correlation between S 2,2 time series and the SOI when S 2,2 is shifted forward in time by 12 months. Evidence of El Nino prediction? Reported by Cox and Chao, (SCIENCE, 2002); Cheng and Tapley (JGR, 2004) Reported by Cox, Chao et al. (AGU, 2003)

21. 11-12 April 2005NASA Sea Level Workshop21 What do the geodetic networks do? Provide the accurate TRF for the interpretation of satellite observations –Any description of position or velocity only has meaning within the context of the TRF and is limited by the accuracy of that TRF Provide the essential information to monitor Earth orientation –Accurate deep space navigation depends on accurate Earth orientation –All techniques contribute –Separation of TRF (translation and orientation) from Earth rotation is challenging and requires good geographic distribution of sites A cluster of geodetic observatories does not a reference frame make Directly measure Earth science parameters –e. g., volcanic and tectonic deformations, gravity field, Earth rotation Precise determination of the orbits of satellites –GRACE, TOPEX, Jason, LAGEOS, and many others

22. 11-12 April 2005NASA Sea Level Workshop22 What are the geodetic networks? GPS, SLR, and VLBI are the three principal technologies used in the geodetic networks. –DORIS, now with ever increasing time series lengths, can also contribute The Terrestrial Reference Frame (TRF) is an accurate, stable set of positions and velocities. The geodetic networks provide data for determination of the TRF as well as direct science observations.

23. 11-12 April 2005NASA Sea Level Workshop23 Who are the users of the data? NASA and non-NASA Flight Missions NSF Polar Programs USGS National Earthquake Hazards Reduction Program DoD Land Surveyors NOAA/NGS Practically everyone, directly or indirectly

24. 11-12 April 2005NASA Sea Level Workshop24 What are the geodetic services? Parts of the International Association of Geodesy (IAG) An example of Community Management Model –Develop standards –Self regulating –Performance monitoring –Define and deliver products 200+ Organizations in 80+ countries NASA actively participates in the services –International GPS Service (IGS) –International Laser Ranging Service (ILRS) –International VLBI Service (IVS) Services respond to NASA’s program needs

25. 11-12 April 2005NASA Sea Level Workshop25 Challenges and New Technologies Challenges: –Establishing the 1 mm reference frame stability –Rusty network and stations, declining resources –Losing capabilities –Need to maintain existing reference frame geodetic systems –New capabilities and technologies must be integrated New Technologies: –SLR improvements in automation and SLR 2000, better targets –VLBI smaller antennas, e-VLBI, higher recording rates –GPS - modernization, GNSS (GPS, Galileo, GLONASS) densification of reference frame

26. 11-12 April 2005NASA Sea Level Workshop26 Where are the geodetic networks going? Real-time access Higher temporal resolution Improved accuracy Improved global distribution Increased efficiency

27. 11-12 April 2005NASA Sea Level Workshop27 What are the products? TRF: 3-D station positions and temporal evolution Earth orientation Static and temporal variations in Earth’s gravity field Time transfer Raw data for other science users Precise orbits Atmospheric and ionospheric parameters

28. 11-12 April 2005NASA Sea Level Workshop28 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 gravity change –Low-degree non-tidal gravity variations from few week to ENSO time scales –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 –Optimized satellite targets (properly designed reflectors in more useful orbits)

29. 11-12 April 2005NASA Sea Level Workshop29 Geodetic Networks: VLBI Unique Capabilities Celestial Reference Frame using quasars Motion of axis in space Earth rotation rate Differential navigation for spacecraft