# Presentation: 388530

## SNARF 2.0:A Regional Reference Frame forNorth America Based on IGS ProductsCurrent Status

- Thomas Herring
- Department of Earth Atmospheric and Planetary Sciences, MIT
- and the SNARF Working Group
- Presented at the IGS Analysis Center Workshop
- Miami Beach, June 2-6, 2008

## Overview

- Rationale for a “SNARF”
- SNARF GIA model
- Contributing GPS solutions to SNARF 2.0
- Analysis of contributions
- Motions in the SNARF realization
- Iteration needed to generate GIA model for use in final alignment

- SNARF TAH

## Rationale for SNARF

- Define a reference frame fixed to stable interior of North America
- ITRF velocities dominated by tectonic plate motion
- Plate-fixed frame better for studying intra-plate deformations
- Need to account for effects of GIA
- Provide a standard plate-fixed frame to facilitate...
- Geophysical interpretations
- Inter-comparison of solutions
- Primarily for Earthscope/PBO studies

- SNARF TAH

## SNARF GIA Model

- Velocity Model
- VGIA	= GIA velocities (a priori values from stochastic model)
- dΩ	= Rotation rates (plate motion)
- dT	= Translation rates (satellite phase center issues)
- GPS velocities – focus of this talk
- Combination of several solutions (SNARF 1.0 used only 3)
- A priori stochastic GIA model
- Based on ICE-5G loading model (SNARF 1.0 used ICE-1)
- Using ensemble of reasonable Earth/ice models

- SNARF TAH

## SNARF 1.0 GIA Model

- SNARF TAH

**Notes:**
Note: There are some problem spots here.

## Contributions to SNARF 2.0 (1/2)

- Five contributing velocity solutions
- NAREF – combination of 7 regional solutions (4 different s/w)
- Purdue (Calais/DeMets) – combination of GIPSY & GAMIT
- Univ. of Alaska Fairbanks (Freymueller) – GIPSY
- Univ. of Nevada Reno (Blewitt et al) – GIPSY
- Canadian Base Network (NRCan) – 27 campaigns – Bernese
- All solutions based on relative antenna phase centers
- Using data up to GPS week 1399 (Nov/06)
- Need to re-do with absolute phase centers after IGS re-proc.
- Using sites in at least two contributions
- 283 unique sites (many more than SNARF 1.0)
- 465 velocity estimates (many breaks in station time series)

- SNARF TAH

## Contributions to SNARF 2.0 (2/2)

- SNARF TAH

- Contributions to SNARF 2.0 (2/2)

**	****Code AC          File                #Sta  #Vel  Rew**
- 	CBN  CBN/NRCAN   CBN06P30v4uc.snx     190   200  8.8
- 	NAR  NAREF/NRCAN NAR06P44v6uc.snx     578   724  **621**
- 	PUR  EC/Purdue   calais_nov06.snx     675   677  3.6
- 	UAF  JF/UAF      UAF_SNARF.stacov     132   132 **7236**
- 	UNR  GB/UNR      UNR_SNARF_0208.stacv  45    45 **2653**
- 	#Sta - Number of stations in solution. Multiple       entries for stations with breaks
- 	#Vel - Number of unique stations
- 	Rew  - Covariance matrix re-weighting factor to make       velocity sigmas realistic & consistent
- Re-weighting factors determined from the normalized root mean square (NRMS) of differences with IGS05 velocities in North America – may modify procedure

- NAREF Week 1367
- 838 Stations
**Number of Stations in**
- 1 solution	607
- 2 solutions	105
- 3 solutions	101
- 4 solutions	  20
- 5 solutions	    3
- 6 solutions	    2

## Analysis Method

- Loosened constraints in contributions – added ±10 mas and ±1 m rotation & translation uncertainties
- Covariance matrices re-scaled by NRMS fit of velocities to IGS05
- Rotation/translation/scale values & rates estimated for each contribution during combination
- Velocities at collocated sites are equated
- UAF contribution not presently used due to numerical stability problems – being investigated
- Reference frame of combination realized by alignment to desired reference system (eg, SNARF 1.0 GIA, ITRF2005 NA plate)
- Final version of SNARF 2.0 will use GIA model derived from preliminary version – iterative scheme

- SNARF TAH

## RMS of Velocity Differences BetweenContributions and Preliminary SNARF 2.0

- SNARF TAH

- RMS of Velocity Differences BetweenContributions and Preliminary SNARF 2.0

**  ****Code  #      WRMS (mm/yr)          NRMS      **
**              ****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
- Preliminary SNARF 2.0 aligned to SNARF 1.0 GIA
- NRMS < 1 for most contributions, suggesting over-weighting

## RMS of Position Differences BetweenContributions and Preliminary SNARF 2.0

- SNARF TAH

- RMS of Position Differences BetweenContributions and Preliminary SNARF 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
- Horizontal positions agree fairly well
- Vertical variability possibly due to remaining antenna height discrepancies among contributions – needed to rename 76 sites and equate velocities
- NRMS slightly higher except for UAF

## Preliminary SNARF 2.0

- Aligned to SNARF 1.0 GIA
- Figures that follow show vertical and horizontal motions for SNARF 2.0 and the contributing solutions
- Sites with vertical velocity sigmas < 6.5 mm/yr are shown with white triangles
- Vertical velocity contours use sigmas as weights
- Sites with horizontal velocity sigmas < 1.0 mm/yr are shown with 95% confidence ellipses

- SNARF TAH

## Preliminary  SNARF 2.0(aligned with SNARF 1.0)

- SNARF TAH

## SNARF combination alignedwith ITRF2005 N.A. plate

- SNARF TAH

- SNARF combination alignedwith ITRF2005 N.A. plate

**Notes:**
Here the vertical rates increase and Northward motion is cleat

## CBN Contribution

- SNARF TAH

## NAR Contribution

- SNARF TAH

## PUR Contribution

- SNARF TAH

## UAF Contribution

- SNARF TAH

## UNR Contribution

- SNARF TAH

## Summary

- SNARF TAH

- Rigorous combination of analyses is possible with antenna height discrepancies accounted for –  47 sites have significant discrepancies and there could be problems at other sites
- Noise models & re-weighting could be improved.  Large variance re-scaling factors likely due to correlated noise at stations and uneven redundancy in NAREF combination.  Development of better re-weighting & site specific noise models would improve the overall reliability of the final solution.
- Preliminary combination needs to be incorporated into the GIA model for the final frame alignment.  Consistency of horizontal and vertical motions could be a problem.
- Numerical stability problems when UAF combination is included which needs to be resolved.
- Some small remaining editing of sites needed.
- Results are all pre-absolute antenna phase center and so...*Frame will need to re-done when IGS re-processing is complete*