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

2. Overview Analysis of GPS solutions submitted for SNARF Version 2.0
Vertical motions in the SNARF realization
Application to Plate Boundary Observatory Reference Frame
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3. Submissions to SNARF Ver 2.0
Code AC File Stat UStat Rew
CBN CBN/NRCAN CBN06P30v4oc.snx
NAR NAREF/NRCAN NAR06P44v6uc.snx
PUR EC/Purdue calais_nov06.snx
UAF JF/UAF UAF_SNARF.stacov
UNR GB/UNR frame_ITRF0--.stacov
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 SNARF version 1.0
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4. RMS difference between new solutions and SNARF Ver 1.0
Code # WRMS (mm/yr) V NRMS V01
N E U N E U
CBN
NAR
PUR
UAF
UNR
The reweighting coefficients were set to make the average NRMS from the V01 comparison be approximatey unity.
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5. RMS difference between new solutions and SNARF GIA model
Code # WRMS (mm/yr) GIA NRMS GIA
N E U N E U
CBN
NAR
PUR
UAF
UNR
Notice new solutions match the GIA height rates better than SNARF Ver 1.0
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6. Center to Center comparison
Comparison between centers for velocity estimates
NAR PUR UAF UNR
# WRMS NRMS # WRMS NRMS # WRMS NRMS # WRMS NRMS
(mm/yr) (mm/yr) (mm/yr) (mm/yr)
CBN
NAR
PUR
UAF
# -- Number of common stations.
The NRMS < 1 suggests re-scaling may be too large, although similar values for all comparisons suggests one over all scale factor.
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7. Center to Center Position comparison
Center # NE RMS # NEH RMS
CBN.NAR mm mm
CBN.PUR mm mm
CBN.UAF mm mm
NAR.PUR mm mm
NAR.UAF mm mm
NAR.UNR mm mm
PUR.UAF mm mm
PUR.UNR mm mm
Differences show height errors up to 6 meters in one case. 47 sites have height differences greater than 10 cm.
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8. SNARF Ver 1.0 GIA Model
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9. SNARF Ver 2.0 Preliminary
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10. PUR contribution
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11. CBN+NAR Contribution
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12. Summary
Height errors in current files make rigorous combination impossible at the the moment. 47 sites have significant height errors and there could be problems at other sires.
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 seems to be a problem.
Need for additional non-North America sites (Pacific plate) to control the rotation of the frame should be considered.
RMS differences between solutions are smaller than RMS to SNARF Ver. 1.0 and its GIA model indicating that SNARF Ver. 2.0 will be more accurate and contain many more stations than SNARF Ver. 1.0
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