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Do Annual Geopotential Variations Affect IGS Products ?

Test effect of GRACE RL05 annual model fits from CSR consider terms (2,0), (2,1), (2,2), & (3,1) Compare GPS results for two extreme weeks 1668 = 25 - 31 Dec 2011 1694 = 24 -30 Jun 2012 Impacts at levels up to several mm Other ACs should test & consider using in Repro2.

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Do Annual Geopotential Variations Affect IGS Products ?

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  1. Test effect of GRACE RL05 annual model fits from CSR • consider terms (2,0), (2,1), (2,2), & (3,1) • Compare GPS results for two extreme weeks • 1668 = 25 - 31 Dec 2011 • 1694 = 24 -30 Jun 2012 • Impacts atlevels up to several mm • OtherACsshould test & considerusing in Repro2 Do Annual Geopotential Variations Affect IGS Products ? J. Ray NOAA/NGS with major help from S. Bettadpur, J. Ries U. Texas/CSR T.-S. Bae Sejong U. X. Collilieux IGN/LAREG T. van Dam U. Luxembourg K. Choi, J. Griffiths NOAA/NGS IGS Workshop 2012, AC Splinter Meeting, Olsztyn, Poland, 26 July 2012

  2. Annual Geopotential Terms Considered wk 1668 wk 1694 • Pick two extreme weeks 6 months apart for testing: 1668 & 1694 • Difference NGS solutions WITH & WITHOUT adding annual terms

  3. Compare Test Orbits

  4. Compare Test Terrestrial Frames • Orbit & TRF frames both shift by about -1 mm in Z component • probably due to N/S network asymmetry • recall that current IGS Z bias wrt SLR origin is ~10 larger • global WRMS impact on stations positions at level of ~0.5 mm

  5. Distribution of dU Shifts Week 1668 (25-31 Dec 2011) TASH - (IGS-load)

  6. IGS Repro1 Residuals (TASH – Loads) • TASH heights are too low each December • annual geopotential effect might partially compensate ?

  7. Distribution of dU Shifts Week 1694 (24-30 Jun 2012) Sometimes regions of good correlation - (IGS-load)

  8. Distribution of dN Shifts Week 1668 (25-31 Dec 2011) - (IGS-load)

  9. Distribution of dN Shifts Week 1694 (24-30 Jun 2012) - (IGS-load)

  10. Distribution of dE Shifts Week 1668 (25-31 Dec 2011) - (IGS-load)

  11. Distribution of dE Shifts Week 1694 (24-30 Jun 2012) But also sometimes areas of poor correlation - (IGS-load)

  12. Compare Test ERPs

  13. Conclusions & Recommendations • Annual geopotential variations have small but non-negligible impacts for IGS products • DZ component of orbit & terrestrial frames shifted by ~1 mm • LOD is biased by few µs • subdaily orbit residuals differ up to ~4 mm WRMS • station positions shift by up to ~0.7 mm horizontal, ~3 mm vertical, probably seasonally • systematic geographic shifts may significantly alias inferred GPS load signatures • however, annual geopotential effect generally appears to be smaller than annual (GPS – load) residuals, esp for dN & dE • Recommend further testing by other ACs • need longer spans of results & further comparisons • Recommend possible adoption for Repro2 • if preliminary NGS results confirmed, IGS should consider adopting a conventional model for annual geopotential variations for Repro2 • must coordinate with GRACE, SLR, & IERS groups • Srinivas Bettadpur working on GRACE fit to degree 15

  14. Models Used from S. Bettadpur & J. Ries (1/2) Subject: Estimates of non-tidal degree-2 annual geopotential variability Author: Srinivas Bettadpur Date: June 27, 2012 Version: v 0.0 The total variability at the annual frequency is a sum of many processes. Not all of these are included in the estimates here. Total_Annual = 3rd Body Pert (relevant only for orbits) <<-- This is NOT included below + All tides (solid, ocean, solid+ocean pole tide) <<-- This is NOT included below + Atmosphere + non-tidal oceans (AOD1B contents) <<-- This is included below + Everything else left over (GSM contents) <<-- This is included below The estimates for "Everything else left over" depends on what was modeled for the parts labeled "NOT included below". This list is included below: 3rd Body Pert: DE405 for luni-solar positions Solid Tide: Eq. 6.xx from IERS2010, with anelastic earth klm Ocean Tide: Self-consistent equilibrium Solid Earth pole tide: IERS C04 pole series with an-elastic earth klm Ocean pole tide: IERS C04 pole series with self-consistent equilibrium model of Desai To calculate the contributions to the Clm/Slm, in the same normalization as in the Conventions: omega = 2*pi/365.2426 theta = omega*( t_mjd - 54101.0 ) dClm( t_mjd ) = CBAR_cos * cos(theta) + CBAR_sin * sin(theta) dSlm( t_mjd ) = SBAR_cos * cos(theta) + SBAR_sin * sin(theta)

  15. Models Used from S. Bettadpur & J. Ries (2/2) Table below gives the values of the annual amplitudes for all the degree-2 harmonics. The GRACE+GAC values are labeled as "ANNUAL". For the (2,0) harmonic, the SLR+GAC based estimates are also provided. name N M CBAR_cos CBAR_sin SBAR_cos SBAR_sin ========== ========== ========== ========== ========== ANNUAL 2 0 0.1103E-09 0.8033E-10 0.0000E+00 0.0000E+00 SLRGAC 2 0 9.9868E-10 1.1105E-10 0.0000E+00 0.0000E+00 ANNUAL 2 1 0.7377E-11 -.2024E-10 0.7651E-10 -.2273E-10 ANNUAL 2 2 -.1394E-10 -.7749E-11 0.5471E-10 -.4229E-10 ------------------------------------------------------------------------------------------------- Subject: Re: degree-2 annual coefficients Date: Wed, 27 Jun 2012 15:40:36 -0500 From: John C. Ries <ries@csr.utexas.edu> Hi Jim, I imagine that degree 2 is the 'tall pole' for GPS, but I'm curious about the effect of an odd-degree order 1 term. I think it will be too small for GPS, but it has shown to be important for lower satellites. A quick fit to RL05 gets, in the same convention as Srinivas: name N M CBAR_cos CBAR_sin SBAR_cos SBAR_sin ========== ========== ========== ========== ========== ANNUAL 3 1 0.22E-10 -0.08E-10 0.31E-10 0.39E-10 I have to suspect that the higher degrees are not very important. JR

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