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Sub-Commission 1.3 - Regional Reference Frames
Regional Sub-Commission 1.3c - North America

North American Reference Frame Densification (NAREF)
Working Group

The purpose of this working group is to densify the ITRF and IGS global reference frame in the North American region by organizing the computation of weekly coordinate solutions and associated accuracy information for continuously operating GPS stations that are not part of the current IGS global network. A "cumulative" solution of coordinates and velocities will also be determined on an annual basis. The Subcommission will organize, collect, analyse and combine solutions from individual agencies, and archive and disseminate the weekly and cumulative solutions.

Map of NAREF Contributions as of November 2006

NAREF Data Archive

All of the NAREF WG coordinate solutions, together with the regional solutions used to produce it, as well as data for GPS stations operated by NRCan/GSD are available from the NAREF data archive at the following FTP server. Access to the server requires a password. Please contact Mike Craymer for access.



NAREF Combinations

Regional Solutions

Regional and local network solutions are obtained from various agencies. Presently, only solutions from the northern half of North America are being contributed. We are always seeking new solutions, especially from Mexico and Central America.

Current Contributions

We would most welcome your contribution if you have any regional solution in North American that meet the criteria described in the above publications. For further information, please contact Mike Craymer.

NAREF Combinations

The NAREF weekly combination of regional solutions is obtained using SINEX Software version 1.0 by Remi Ferland. It is the same software used to produce the official IGS coordinate solutions. The combination procedure is based on that used for the IGS solutions and is performed in two steps: alignment of each regional solution to the IGS weekly solution and combination of the (aligned) regional solutions. The procedure is summarized as follows:

    Alignment of Each Regional Solution
  1. A priori datum constraints are removed.
  2. Solution is aligned to the IGS weekly solution of the same week using 3 translations, 3 rotations and a scale change.
  3. Covariance matrix is scaled by the weighted root mean square (WRMS) of the residuals from the above transformation.
  4. Residuals are tested for outliers. If any outliers are found, they are removed and steps 2 to 4 are repeated.
  5. Combination of Regional Solutions
  6. Regional solutions are combined together by summation of the normal equations for the (scaled) regional solutions.
  7. Combined solution is aligned to IGS weekly solution of same week using 3 translations., 3 rotations and a scale change.
  8. Covariance matrix is scaled by the WRMS of residuals from the above transformation.
  9. Residuals are tested for outliers. If any outliers are found, they are removed from their solution and steps 2 to 8 are repeated (only steps 2 to 4 for the regional solution with the outlier needs to be repeated).
  10. A minimum constraint is applied to the combined solution for further testing and comparison with the IGS weekly solution. Presently, IGS station DRAO is constrained to itŐs IGS97 coordinates and weights at the epoch of date to facilitate comparisons among all the solutions (itŐs the only station common to all solutions).

The combination results are analysed using the combination residuals for each regional solution and comparing the NAREF combations to the IGS weekly solutions. It is also planned to use other combination software for further quality control.

Recent NAREF Results

Results of the most recent cumulative solution in the ITRF2005/IGS05 reference frame using data up to GPS week 1399 (when the IGS changed it's processing strategy to use absolute antenna phase centers) can be found in the following paper. Maps of the contributed networks and resulting velocity fields from this solution are provided. The horizontal velocity field is dominated by the tectonic motion of the North American plate while the vertical velocity field in Canada is dominated by post-glacial rebound.

Maps of the velocities fields for the combined NAREF and Canadian Base Network solutions. The CBN solution is based on repeated occupations of highly stable, forced centering pillars over the years 1994 to 2006. It provides greater spatial sampling of the velocity field in Canada where there are fewer continuous GPS sites then in the US.

Last revised: 2013-11-17
Web Site Contact: Michael Craymer