POD/Geoid Splinter Summary OSTS Meeting, Hobart 2007

POD report Revised orbit error budget Contributors to the orbit error budget have not changed since TOPEX/Poseidon –static gravity –tide models –variable gravity –solar radiation pressure –station / data errors –reference frame –geocenter motion –orbit determination noise but they are now lower and better characterized

Static Gravity Differences between newer GRACE models are small order 1 harmonics contribute largest GCEs

Tide models

Variable gravity

Time-Varying Atmospheric Gravity SLR residuals RMS globally reduced by ~1 mm when adding atmospheric gravity

Impact on GCEs Mean Radial Difference relative to JPL06b in mm (averaged over cycles 22 to 99 = Aug 02 to Sep 04) GDR-B SHDP AOD1B Atmospheric gravity reduces geographically correlated differences with JPL06b reduced dynamics orbits

Grace-derived time-variable gravity to 20x20 (annual terms only) Monthly solutions from KBRR data only Fit mean, trend, & annual harmonic 2 solutions: 1-yr & 3-yr data span Jason SLR/DORIS orbit summary (cycles 1-176) TVG model average RMS residuals DORIS (mm/s) SLR (cm) Crossovers (cm) atmosphere gravity + (no annual) atmosphere gravity + tvg1 (1-year annual) atmosphere gravity + tvg3 (3-year annual) Reference: ‘Monthly spherical harmonics gravity field solutions determined from GRACE inter-satellite data alone,’ Luthcke et al., Geophyscal Research Letters, 2006.

POD Results for JASON 54.7 mm 54.6 mm 54.3 mm.3219 mm/s.3185 mm/s.3181 mm/s 12.7 mm 12.2 mm 11.9 mm 11.5 mm RMS values of one arc per month of the obervations residuals after orbit adjustment

Solar radiation pressure There is evidence of small residual errors in both the scaled box-wing (GDR-B) and the UCL (JPL) model from L. Cerri

Solar Radiation Pressure SRP is largest surface force acting on T/P and Jason-1 (T/P area-to-mass ratio is half that of Jason-1) It is easily shown that a bias in the SRP model interacts with estimated empirical 1/rev parameters to create periodic Z-shift correlated with beta-prime Z variation due to 3% scale error in SRP model from J. Ries

Solar radiation pressure Jason SRP model performance cycles average RMS residuals DORIS (mm/s) SLR (cm) Crossovers (cm) nominal (C R =1.000) nominal (C R =0.914) UCL (untuned) *1.445*---- * fewer points used (dynamic editing) GSFC’s attempt to rescale model at GSFC leads to mixed results from F. Lemoine

Solar radiation pressure Z-shift induced by rescaling of SRP model not sufficient to explain Z offset between DORIS/SLR and GPS orbits

Station / Data errors

Jason DORIS SAA correction Jason SLR/DORIS ITRF2000 orbits cycles average RMS residuals DORIS (mm/s) SLR (cm) Crossov er (cm) GSFC GSFC w SAA corr

Jason DORIS SAA correction: Comparison of SLR/DORIS & SLR/Crossover Orbits ~ 0.9 mm oscillator jump at cycle 91

Jason DORIS SAA correction

Reference frame ITRF2005 –results indicate that ITRF2005 corrects a drift in the centering of ITRF2000 –Z drifts between the various orbits are significantly reduced with ITRF2005 –transition from ITRF2000 to ITRF2005 will induce a shift in Z

Evaluation of new Jason orbits based on ITRF2005 Jason Orbits SLR/DORIS Cycles DORIS RMS (mm/s) SLR RMS (cm) SLR mean (cm) Altimeter Crossover RMS (cm) ITRF ITRF ITRF2005 SLR-Rescaled w. scale-rate

Evaluation of new TOPEX orbits based on ITRF2005 TOPEX SLR/DORIS Orbits Cycles DORIS RMS (mm/s) SLR RMS (cm) SLR mean (cm) Altimeter Crossover RMS (cm) GDR ITRF ITRF ITRF2005 SLR- rescaled w. scale-rate Subset Analysis: 21 TOPEX Cycles ( ) ITRF ITRF ITRF2005 SLR- rescaled w. scale-rate

Annual signal amplitude (TVG annual – none) Geographically correlated error (GGM02C-JGM3) TOPEX Radial Orbit Difference Trends (ITRF2005 – GDR (CSR95))

Jason (and TOPEX) radial orbit differences are driven by the difference in Z Long-term Orbit Error and the Terrestrial Reference Frame (TRF)

TOPEX Radial Orbit Difference Trend ITRF2005 – GDR (CSR95) Consequent error in estimated regional and global MSL trends

Jason GSFC TRF orbit trends

Jason TRF orbit trends – JPL GPS

Geocenter motion