The raw and RINEX data files are imported in a timely fashion into the UCB/USGS Northern California Earthquake Data Center (NCEDC) data archive maintained at UCB (Romanowicz et al., 1994), where they are immediately available to all BARD participants and other members of the GPS community through Internet, both by anonymous ftp and by the World Wide Web (http://quake.geo.berkeley.edu/bard). Contact Mark Murray (phone: 510-642-5333; email: firstname.lastname@example.org) for more information.
Data from the 15 BSL sites, and the 20 other sites run by BARD collaborators are archived at the NCEDC. Before being submitted to the archive each daily data set is checked to verify data integrity, quality, completeness, and conformance to the RINEX standard. Any data that fail to meet these criteria are held back from the public until the problem can be corrected or it can be established that no more data is forthcoming.
As part of the attempt to verify data quality and integrity, we have begun periodic checks of the signal-to-noise ratio associated with the incoming GPS signals. Currently we create skyplots that show the signal-to-noise associated with each region of the sky for each of our stations (Figure 2). We will use these plots to give us early warning of antenna or receiver failure, or other problems such as changes in the multipath environment at the sites due to alterations in buildings, vehicles and other local structures.
Data and ancillary information about BARD sites are also made compatible with standards set by the International GPS Service (IGS), which administers the global tracking network used to estimate precise orbits and has been instrumental in coordinating the efforts of other regional tracking networks. The USGS imports data daily from their stations into the NCEDC. In addition, the BSL automatically retrieves data from other continuously operating stations in northern California from other GPS archives, such as at SIO, JPL, and NGS, using modified UNIX scripts provided by SIO. The NCEDC currently archives nearly all high-precision continuous GPS measurements collected in northern California.
Many of the BARD sites have been added by the NGS to their database of CORS sites, which are used as references stations by the surveying community, after having established their locations in the WGS84 coordinate system. All permanent sites operational in July 1998 will be included in a new statewide adjustment of WGS84 coordinates, which makes the data more useful to the surveying community. Members of the BARD project regularly discuss these and other common issues with the surveying community at meetings of the Northern California GPS Users Group.
The BARD Project and the NCEDC are cooperating with UNAVCO and others in the GPS community to establish a seamless data archive. This project will enable a user to find and access GPS data no matter where they are archived and to be assured that this data is correct and up to date. In 1997-98 members of the BARD project participated in a working group organized by UNAVCO to define the GPS Seamless Archive Centers (GSAC) proposal and necessary meta-data file structures, and work is underway to create the databases that will describe the holdings of the NCEDC to the GSAC community.
Figure 2: Skyplots and L2 signal-to-noise ratios for four BARD stations. Skyplots show satellite azimuth and elevation at each 30-sec epoch for 24 hours on 23 May 1998. The zenith direction is at the center of the circle, and circle grids are elevation angles down to the horizon. The absence of low elevation data in the north direction is due to the geometry of the satellite orbits. All receivers can track down to the horizon, as is mostly possible at SUTB. Satellite tracks that cut off above the horizon indicate local obstructions. The cutoff in the NE and NW directions at DIAB are due to the summit of Mount Diablo itself, which is to the north of the receiver. The greyscale of each symbol shows the signal-to-noise ratio of the L2 phase observable (SN2). High SN2 (darkest greys) are normally near the zenith and the low SN2 (lightest greys) are normally near the horizon. Unusual deviations from this pattern might indicate signal multipathing due to reflectors in the local site environment.