IGS Analysis Center Workshop - IGS - NASA

impossible task for 1000 days of data). Furthermore, this approach allows for thecombination of data from many regional clusters, in combination with the global trackingstations (i.e., distributed processing).The system developed at S10 for GAMIT/GLOBK users can be viewed as a prototype forone possible way in which IGS can support campaign-type surveys and regional clusters.S10 archives each day the following information:(i) “clean” RINEX files for the PGGA and global tracking stations. Although GPSsoftware packages provide “automatic” cycle-slip fixing algorithms, there are frequentanomalous conditions when human interaction is required. Providing access to cleanRINEX data for a robust global (and California regional) network provides a considerabletime savings for principal investigators, regardless of the software package, but particularlyfor packages that examine doubly -differenced phase measurements (e.g., Bernese,GAMIT).(ii) Coordinates and velocities for the IGS and PGGA stations in the current standardglobal reference frame.(iii) The GAMIT/GLOBK solution files. These files are an invaluable resource for theanalysis of regional networks since they contain the complete geodetic content of the globaltracking network, as described in this proposal. Once a standard software-independentexchange format is adopted for solution files this information will be readily available to allusers and GPS data processing packages.(iv) precise ephemerides in a short ASCII file that includes a set of 9 initial conditions (sixstate vector elements of position and velocity, and three radiation pressure parameters) foreach satellite at the midpoint of each day (12:00 UTC), Since all GAMIT users use thesame orbital integration program, it is a simple manner to generate a tabular ephemeris for aparticular survey. This of course reduces the amount of orbital data that needs to betransmitted via electronic mail, compared to the transfer of many times larger NGS SP3formatted files which are made available to the general user. An Earth orientation file basedin the S10 analysis is also circulated,4.4 Distributed Analysis — Four ScenariosScenario 1. This is a zeroth order example of distributed processing and the IGS concept,A surveyor goes to the field in southern California with one receiver knowing that thePGGA/IGS network is operating continuously, The current distribution of stations insuresthat any user in southern California will be within 100 km of a PGGA station. Thesurveyor occupies a set of stations in some GPS surveying mode (e.g., static, rapid-static)for a specified time depending on the application and accuracy requirements. At the end ofthe survey, the following information is copied electronically from the PGGA: RINEX datafor the one or two closest PGGA sites, satellite ephemerides in NGS SP1 or SP3 format,time-tagged station coordinates (ITRF) for the reference PGGA site. Using thisinformation and any of the scientific or commercial GPS software packages, the usercomputes station positions using the satellite orbits and PGGA station coordinates as fixed.This is a si~ple straightforward procedure in which PGGA/IGS plays strictly a servicerole. Depending on the length of time of a particular survey and assuming that standardfield procedures have been followed, the station coordinate information can be used bygeophysicists to study crustal deformation. This information could be retrieved in the formof solution files, in this case, strictly the solutions and variance-covariance matrices forstation coordinates which could easily be integrated with the PGGA/IGS solution files.82