Support for GPS Analysis

Annual Report, 1997

SCEC infrastructure funds for MIT support three principal activities: 1) m~ntenance and distribution of the GAMIT and GLOBK software used for GPS data analysis by MIT, Scripps, UCLA, and the USGS; 2) assistance provided to these groups in their analyses; and 3) processing of GPS phase observations with GAM1T to obtain daily estimates of station position ("h-" or "SINEX" files) to be used by all members of the Geodesy Working Group in their scientific investigations. During the past year we also used infrastructure support to organize, reproduce, and deliver to the SCEC archive data and field logs for the Salton Trough/Riverside County (STRC) surveys of 1991, 93, 95, and 97, and the NGS part of the 1988 survey.

GAMITIGLOBK maintenance and development

GAMIT/GLOBK maintenance and development are tasks which benefit not only SCEC activides but several dozen other geophysical invesdgations by sciendsts throughout the world. Hence support for these activities is now funded primarily by an NSF grant through UNAVCO. Funds from this grant, however, were not available until September of 1997, so we devoted over half (2 mm) of our SCEC infrastructure effort in 1997 to these tasks. The most important software enhancements were more efficient preprocessing, tighter data editing through the use of post-fit residuals, radiation-pressure and transmitting antenna models for Block IIR satellites, new parameters to model atmospheric gradients, improvements in the use of generalized constraints for defining a reference frame, and plotting scripts for GLOBK velocities and time series. Since last January, we have made available to SCEC analysts six new releases of GAM1T and four new releases of GLOBK.

GAMITIGLOBK training and support

With the help of users at Scripps and UCLA we have continued to improve the software documentation, limited so far to the GAMIT and GLOBK manuals and the GLOBK online help, but with plans to extend this to a web page during the coming year. We provided assistance with software issues to Scripps, UCLA, and USGS analysts through e-mail communications. We also hosted Yehuda Bock and Peng Fang from Scripps for two days of discussion of software enhancements to support SCION (and other) processing.

Data Processing

As we noted our report of last December, there were at that time 14 old surveys that required reprocessing to overcome deficiencies in the orbital and/or antenna models, data editing, and/or supporting fiducial data. The most important of these for testing Landersinduced velocity changes are the STRC surveys of 1990, 91, 92, 93, and 95.

completed 1993 and 1995 during the year, along with newly acquired data from a 1997 survey conduced by Rob Reilinger and Javier Gonzales.

We postponed reprocessing of most of the data from 1990, which involved more use of mixed-antennas, in anticipation of improved models for elevation-dependent variations in the antenna phase centers. Although new models are now available for four of the
antennas used commonly in early SCEC surveys (11-4100, FRPA-2, MiniMac, and Ashtech P-12), we are not sufficiently satisfied that will produce the same level of accuracy especially in the vertical but also in the horizontal-as processing the data in subsets of homogeneous antennas. Thus, this is the course we will pursue in 1998 for the 1990 and 1991 data sets (STRC plus TREX 17, 18, and 20).

In our current pre-Landers velocity solutions, STRC91 is the most anomalous of the major surveys in chi-square increment (10.1). There are also larger-than-expected increments the reprocessed TREX10 (6.7) and TREX13 (9.5) data, but these may result from stadon overlaps with STRC91. The small Santa Maria Basin surveys (VF1 and VF2) are outliers (chi-square increments of 13 and 15, respectively) but don't affect the overall velocity field very much. The 1988 LA Basin data (6.6) and the combined Chalome 1989 (8.5) are also problems.

In the post-Landers data set, the 1993 Ventura Basin and 1994 Santa Maria Basin surveys skill need to be reprocessed. Finally, the current in-files for "Constract 92", the 1993 and 1994 HPGN surveys, Inter-county 1993, and the LA Basin 1997 survey have weak external des (SOPAC in-files apparently not included) and should be remade.

A summary of the current status of our reprocessing effort is given in Table 1.