Contributions of Geodesy to Oceanography B. Tapley and J. Ries Center for Space Research, The University of Texas at Austin Dynamic Planet 2005 Cairns, Australia August 22-26, 2005 August 22-26, 2005 Dynam ic Planet 2005 1
Sustained Ocean Observing System for Climate Initial System Design -- Now 48% complete. Sea Surface Temperature, Sea Surface Height, Surface Vector Wind, Sea Ice, and Ocean Color from Space Tide Gauge Network 58 % complete 3 x3 Argo Profiling Float Array 35% complete 5 x5 Surface Drifting Buoy Array 45 % complete Moored Buoy Existing Planned Ocean Reference Station Existing Planned High Resolution XBT and Flux Line Existing Planned Frequently Repeated XBT Line Existing Planned Carbon Inv entory & Deep Ocean Line Global Surv ey @ 10 years
Geodesy and Satellite Altimetry A Global Geodetic Observing System is essential to the use of oceanographic measurements Three pillars of Geodesy Terrestrial Reference Frame (esp., origin and scale) Earth Orientation (EOP) Mean and Time-variable Gravity Field Important examples are mean sea level and absolute surface currents Geodesy impacts observation, determination and attribution August 22-26, 2005 Dynam ic Planet 2005 3
Mean Sea Level Determination 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 August 22-26, 2005 Dynam ic Planet 2005 4
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) August 22-26, 2005 Dynam ic Planet 2005 5
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 Could the true 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 ITRF2000 scale (ppb) August 22-26, 2005 Dynam ic Planet 2005 6
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 August 22-26, 2005 Dynam ic Planet 2005 7
Reference Frame Drift 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) August 22-26, 2005 Dynam ic Planet 2005 8
Geodesy and GIA Space geodesy helps discriminate between various models for Glacial Isostatic Adjustment Accurately modeling the vertical motion at the tide gauge sites using GIA models (where there are no geodetic observations of the height change) allows for more accurate altimeter calibration and mean sea level determination Confidence in the models for GIA also aids interpretation of geological sea level signals August 22-26, 2005 Dynam ic Planet 2005 9
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 points 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 multiple techniques Techniques must be well distributed with good ties to sort out 14 degrees of freedom (origin, scale, rotation and rates) August 22-26, 2005 Dynam ic Planet 2005 10
Why do we have three techniques? High precision geodesy is very challenging Accuracy of 1 part per billion Fundamentally different observations with unique capabilities Technique Signal Source Obs. Type Celestial Frame UT1 Polar Motion Scale VLBI Microwave Quasars Time difference SLR Optical Satellite Two-way absolute range No GPS Microwave Satellites Range change No Together, they provide cross validation and increased accuracy Geocenter Geographic Density No No No Seasonal variations To realize the advantages of each technique, good distribution and accurate ties are required Real-time Decadal Stability August 22-26, 2005 Dynam ic Planet 2005 11
Geodetic Networks: GPS Site Map Targets: GPS Spacecraft August 22-26, 2005 Dynam ic Planet 2005 12
Geodetic Networks: VLBI Site Map Targets: Quasars August 22-26, 2005 Dynam ic Planet 2005 13
Geodetic Networks: SLR Site Map Targets: LAGEOS-1, LAGEOS-2 August 22-26, 2005 Dynam ic Planet 2005 14
Geodetic Networks: SLR Site Map Targets: LAGEOS-1, LAGEOS-2 August 22-26, 2005 Dynam ic Planet 2005 15
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 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) August 22-26, 2005 Dynam ic Planet 2005 16
Geocenter Motion and Oceanography Degree 1 variations (i.e. geocenter motion) from SLR data analysis represent an important component of the seasonal mass redistribution within the Earth system Seasonal Exchange Between Oceans and Land Agreement in the global ocean mass variation computed from seasonal altimetry and steric GMSL (blue) and GRACE (red) is improved when geocenter motion is included (in mm of water) (Chambers, Wahr & Nerem, GRL, 2004) August 22-26, 2005 Dynam ic Planet 2005 17
Gravity and Satellite Altimetry Gravity Model Impact IMPROVED ORBIT Gravity + Altimetry Absolute surface currents Changes in deep ocean currents and mass transport Improved altimeter satellite orbits Seasonal and longterm ocean mass change IMPROVED GEOID IMPROVED OBSERVATION OF MASS CHANGES August 22-26, 2005 Dynam ic Planet 2005 18
Dynamic Ocean Topography The dynamic ocean topography is the difference between the mean sea surface (observed from altimeter data) and the geoid. This difference is caused by the ocean currents. With no currents, the ocean surface would coincide with the geoid. Gradients of DOT provides ocean current estimates August 22-26, 2005 Dynam ic Planet 2005 19
Gravity and Ocean Currents Using GRACE gravity model Zonal ocean currents from altimetry and GRACE Using EGM96 Using WOA01 Hydrography August 22-26, 2005 Dynam ic Planet 2005 20
POD for Altimeter Missions Mean difference between Jason-1 orbits using JGM-3 and GGM01 Using a GRACE gravity model essentially eliminates geographically correlated orbit errors, removing a particularly insidious source of altimeter error From N. Zelensky, GSFC Removing geographically correlated error also results in better geodetic results such as station positioning Better positioning leads to better gravity determination Count EGM96 (sub-set) 14 12 10 8 6 4 2 DORIS-GPS local tie comparison EGM96 (1993-2003 DORIS data) sub-set of stations SYOB FAIA HBKA+HBKB+HBLA KRUB 0 0 0.05 0.1 0.15 0.2 Range PDMB SANA Count GGM01 25 20 15 10 5 DORIS-GPS local tie comparison GGM01 (2002-2003 DORIS data) AREB RIOA-RIOB PDMB 0 0 0.05 0.1 0.15 0.2 Range From P. Willis, JPL/IGN August 22-26, 2005 Dynam ic Planet 2005 21
Conclusions The TRF/EOP provide the stable coordinate system that allows us to connect 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 and services provide the structure and the observations that supports high precision orbits The global gravity field model is also a critical component for POD, ocean circulation determination and mass transport within the Earth system These requirements place the establishment and maintenance of a global geodetic observing systems as a central requirement for satellite oceanography studies August 22-26, 2005 Dynam ic Planet 2005 22
"In all things it is a good idea to hang a question mark now and then on the things we have taken for granted. VLBI Bertrand Russell GPS August 22-26, 2005 Dynam ic Planet 2005 23