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On the use of temporal gravity field models derived from GRACE

On the use of temporal gravity field models derived from GRACE for altimeter satellite orbit determination F. Flechtner (1), R. Schmidt (1), Ch. Förste (1) R. Biancale (2), J.-M. Lemoine (2), S. Loyer (3), S. Bruinsma (2) GeoForschungsZentrum Potsdam CNES/GRGS Toulouse

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On the use of temporal gravity field models derived from GRACE

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  1. On the use of temporal gravity field models • derived from GRACE • for altimeter satellite orbit determination • F. Flechtner (1), R. Schmidt (1), Ch. Förste (1) • R. Biancale (2), J.-M. Lemoine (2), S. Loyer (3), S. Bruinsma (2) • GeoForschungsZentrum Potsdam • CNES/GRGS Toulouse • Noveltis Toulouse

  2. Introduction • GFZ has reprocessed a new time series of monthly GRACE-only gravity models (EIGEN-GRACE05S, RL04) for February 2003 till November 2006. • GRGS has prolonged its 10d times series EIGEN-GL04S to nearly 4 years. • Besides GFZ, also CSR and JPL as part of the GRACE Science Data System have recently reprocessed RL04 time series. • From these time series, dominant annual and (less dominant) semi-annual gravity field variations have been derived and compared with the hydrological model WGHM (WaterGAP Global Hydrological Model, Döll et al. 2003). • The impact of these periodic gravity variations, which could be applied as a new (hydrological) background model, is analyzed for Jason-1, ENVISAT, CHAMP and GRACE.

  3. EIGEN-GRACE05S modifications • Reprocessed February 2003 to December 2006 with two-step method using GFZ-processed GPS constellations (EIGEN-GRACE05S, RL04). Instrument parameterization, drift rates for C20, C30 and C40 in the background gravity field and the arc length (1 day) were not changed. • Major updates: • Static background gravity model: EIGEN-GL04C (150x150) (Förste et al. 2006). • Inclusion of secular rates for C21, S21 (IERS Conventions 2003). • Reference epoch of models: 01-Jan-2000 (IERS Conventions 2003). • Updated K2 and included M4 tide in FES2004 ocean tides. • Usage of new atmospheric/oceanic de-aliasing product AOD1B-RL04 (mass- conserving baroclinic OMCT ocean model, improved bathymetry, …). • Rectified usage of ocean pole tide model (Desai 2002). • Relativity extended by Lense-Thirring & de Sitter effects (IERS Conventions 2003). • Implementation of IERS 2003 nutation and precession model. • Tidal and nutational corrections to EOP (local spline interpolation). • Integration in CIS (Conventional Inertial System) instead of TDS (True of Date System). • Az/El dependent phase center corrections for GPS-SST of GRACE-A/B (JPL)

  4. GFZ RL04 RMS-Variability for 2005 in the Space Domain RMS surface mass variability (Gaussian averages r = 500 km) GFZ-RL03 GFZ-RL04 Release min. max. wRMS Reduction wRMS [%] RL03 9.5 230.1 41.7 (27.6) RL04 7.4 242.8 39.8 (25.7) 4.5 (6.9) [mm] water column, (only oceans)

  5. GFZ RL04 RMS-Variability in the Space Domain Spurious signal induced by AOD-RL03 … RL03 Localization of continental hydrology … Striping over the oceans …

  6. RMS-Variability in the Space Domain … has gone with AOD-RL04 RL04 … slightly improved ... slightly reduced

  7. GFZ RL04 RMS-Variability for 2005 in the Spectral Domain RMS of deviations of monthly models w.r.t their mean in 2005 Geoid Height [mm] Spherical Harmonic Degree n

  8. RMS-Variability in the Spectral Domain RMS of deviations of monthly models w.r.t their mean in 2005 Geoid Height [mm] Slight reduction of signal power in the medium range confirming further reduction of spurious gravity signals No change in the signal part of the spectrum Spherical Harmonic Degree n

  9. Determination of annual and sem-annual terms (1) • Available GRACE Models: • GFZ RL03 EIGEN-GRACE04S 02/2003-12/2006 (45 monthly fields, n=120) • GFZ RL04 EIGEN-GRACE05S 02/2003-11/2006 (44 monthly fields, n=120) • CSR RL04 01/2003-09/2006 (43 monthly fields, n=60) • GRGS RL04 EIGEN-GL04S 08/2002-05/2006 (128 10d fields, n=50) • Available Hydrological Model (expressed in spher. harm. coeff.): • GFZ RL01 WGHM 01/2003-12/2005 (36 monthly fields, n=100) • Adjustment of 6 parameters for each coefficient up to degree 40: • Bias • Drift • COS(annual) • SIN(annual) • COS(semi-annual) • SIN(semi-annual)

  10. Example of selected GFZ RL04 and WGHM coefficients GFZ RL04 monthly coefficients and fitted periodic signal WGHM monthly coefficients and fitted periodic signal

  11. Correlations Period Terms EIGEN-GRACE05S vs. WGHM Cnm Snm

  12. Correlations Periodic Terms EIGEN-GL04S vs. WGHM Cnm Snm

  13. Correlation per Degree of WGHM vs. different GRACE solutions • All GRACE models are • highly correlated (>0.6) with • WGHM for n<20 • GRGS shows smaller • correlation for small degrees • but highest correlation for • higher degrees

  14. Influence of Gravity Field on Precise Orbit Determination (POD) Results • Four satellites tested: • ENVISAT • JASON • GRACE • CHAMP • Test period: 1 year (July 2005 – June 2006) • Four models tested: • Static model: mean EIGEN-GL04S (from GRGS) • EIGEN-GL04S time series: GRGS 10d solution over 4 years • Mean monthly models: Mean monthly coefficients (GRGS) • from 4 years • Periodic Model: GFZ RL04 + GRGS combined periodic terms • (i.e. adjustment of the periodic terms commonly from the • GFZ and GRGS derived spheric. harm. coefficients)

  15. POD Results for ENVISAT 30.5 mm 30.6 mm 30.6 mm 30.6 mm .3301 mm/s .3303 mm/s .3299 mm/s .3303 mm/s 70.0 mm 70.0 mm 69.8 mm 70.0 mm RMS values of one arc per month of the obervations residuals after orbit adjustment

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

  17. POD Results for GRACE 31 mm 30 mm 29 mm 30 mm 7.4 mm 7.2 mm 7.1 mm 6.4 mm 9 μm 12 μm 11 μm 12 μm .18 μm/s .14 μm/s .14 μm/s .14 μm/s RMS values of one arc per month of the obervations residuals after orbit adjustment

  18. POD Results for CHAMP metre 21.7 mm 21.1 mm 21.4 mm 21.2 mm metre 10.7 mm 10.6 mm 10.6 mm 9.0 mm RMS values of one arc per month of the obervations residuals after orbit adjustment

  19. Conclusions (1) • GFZ has reprocessed a homogeneous EIGEN-GRACE05S (RL04) time series for almost the complete GRACE mission period based on improved standards and background models showing reduced striping and no longer spurious signals. RL04 is available from the GRACE-ISDC since February 2007 (http://isdc.gfz-potsdam.de) ! • GRGS has prolonged it 10-daily time series EIGEN-GL04S to about 4 years. • The coefficients of all GRACE gravity time series (GFZ RL03, GFZ RL04, CSR RL04, GRGS RL04) show a clear annual and semi-annual signal. • These periodic signal is highly correlated (>0.6) with the hydrological WGHM model up to degree and order 20 or so.

  20. Conclusions (2) • The GFZ and GRGS combined periodic terms as well as three other gravity field models (static, 10d GRGS time series, mean monthly coefficients) have been tested for 1 year of ENVISAT, JASON, GRACE and CHAMP POD. • Especially for Jason and GRACE the (usually applied) static field gives larger residuals than any of the 3 other time-variable solutions. Therefore the analysis of these satellites could benefit from a time-variable gravity background information, such as annual and semi-annual period terms. • Further tests will be made analyzing the significance of these terms and the corresponding maximum degree of the model. For GRACE, also gravity field solutions should be derived using this new background information.

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