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SNARF Ver 2.0 Construction: Current Status

SNARF Ver 2.0 Construction: Current Status. Thomas Herring, Department of Earth Atmospheric and Planetary Sciences, MIT Cambridge, MA. Overview. Analysis of GPS solutions submitted for SNARF Version 2.0 Initial Vers 2.0 generated using stations which appear in at least two analyses.

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SNARF Ver 2.0 Construction: Current Status

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  1. SNARF Ver 2.0 Construction:Current Status Thomas Herring, Department of Earth Atmospheric and Planetary Sciences, MIT Cambridge, MA

  2. Overview • Analysis of GPS solutions submitted for SNARF Version 2.0 • Initial Vers 2.0 generated using stations which appear in at least two analyses. • There are 465 total sites, 283 unique locations. • Height rate sigmas: 222 sites < 5 mm/yr; 213 < 4 ; 200 < 3 ; 167 < 2 and 59 < 1 mm/yr • Motions in the SNARF realization. Iteration now needed to generate GIA model for use in final alignment. SNARF TAH

  3. Submissions to SNARF Ver 2.0 Code AC File Stat UStat Rew CBN CBN/NRCAN CBN06P30v4uc.snx 200 190 8.8 NAR NAREF/NRCAN NAR06P44v6uc.snx 724 578 621 PUR EC/Purdue calais_nov06.snx 677 675 3.6 UAF JF/UAF UAF_SNARF.stacov 132 132 7236 UNR GB/UNR UNR_SNARF_0208.stacv 45 45 2653 Stat -- number of stations in solution. Multiple entries for stations with breaks. Ustat -- number of unique stations Rew -- Multiplier of covariance matrix to make velocity sigmas realistic. • Reweighting factors were determined from the square root of chi^2-per-degree of freedom (normalized root mean square scatter, NRMS) of the differences between the velocity estimates and IGS05 North America reference stations SNARF TAH

  4. Analysis Method • Each center is converted from SINEX or STACOV to Globk binary format. Centers which have frame constrained have ±10 mas and ±1 m rotation and translation covariance added. • Variance scaling factors are determined by velocity RMS fit to IGS05. • During combination of results, rotations, translation and scale values and rates are estimated. • At end of combination, reference frame is realized by rotation/translation and scale onto reference system (could be SNARF V 1.0 GIA, IGS05 or SNARF V 2.0 (for comparing analysis center results). • In current combination, UAF analysis is not used due to numerical stability problems. • Final version of SNARF will use a GIA model derived from preliminary version. • Velocities at collated sites are equated. SNARF TAH

  5. RMS Velocity difference between contributions and combined Ver. 2.0 Code # WRMS (mm/yr) V02 NRMS V02 N E U N E U CBN 73 0.23 0.55 0.61 0.29 0.51 0.13 NAR 311 0.55 0.58 1.51 0.34 0.37 0.25 PUR 204 0.17 0.15 0.44 0.33 0.34 0.41 UAF 96 0.78 0.75 2.59 0.18 0.24 0.29 UNR 36 0.29 0.33 1.96 0.56 0.66 1.09 The reweighting coefficients were set to make the average NRMS from the IGS05 comparison be approximately unity. All sites here should appear in at least 2 analyses but not always with equal sigmas. SNARF TAH

  6. RMS Position differences between analyses and SNARF Ver 2.0 Code # WRMS (mm) NRMS N E U N E U CBN 77 0.48 0.95 2.12 0.23 0.34 0.17 NAR 228 0.88 1.24 4.00 0.44 0.61 0.51 PUR 199 1.12 1.36 2.62 0.43 0.73 0.77 UAF 83 1.28 1.66 3.89 0.07 0.12 0.10 UNR 36 1.11 1.84 5.69 0.94 1.60 1.38 There is variability here possibly due to antenna height problems. 76 site renames were needed. Positions do not agree as well as velocities in general. SNARF TAH

  7. SNARF Ver 1.0 GIA Model SNARF TAH

  8. SNARF Version 2.0 Preliminary • Figures that follow show vertical and horizontal motions for SNARF Ver. 2.0 and the contributing solutions. • In these figures, all sites with vertical rate sigmas < 6.5 mm/yr are shown and sigmas are used to weight the vertical interpolations • Sites with horizontal velocity sigmas less than 1.0 mm/yr are shown with 95% confidence ellipses. SNARF TAH

  9. Preliminary Version 2.0 aligned with SNARF Ver 1.0 GIA Model SNARF TAH

  10. Velocities aligned with IGS05 SNARF TAH

  11. CNB contribution SNARF TAH

  12. NAR contribution SNARF TAH

  13. PUR contribution SNARF TAH

  14. UAF contribution SNARF TAH

  15. UNR Contribution SNARF TAH

  16. Summary • With height errors accounted for, rigorous combination of analyses is possible (except UAF). 47 sites have significant height errors and there could be problems at other sites. • Noise models could be improved. There are large variance re-scaling factors which compensate from correlated noise at stations. Development of site specific noise models would improve the overall reliability of the final solution • New model needs to be incorporated into the GIA ensemble model for the final frame alignment. Consistency of horizontal and vertical motions could be a problem. • Numerical stability problems when UAF analysis is included which needs to be resolved. • Some small remaining editing of sites needed. • Results so far are all pre-absolute phase center models and so frame will need to re-done when IGS re-processing is complete. SNARF TAH

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