Requirements for data and positioning accuracy for the study of earthquake and volcanic processes were discussed in two papers, presented by Mike Watkins and Mark Murray respectively. After discussing specific monitoring activities in northern California, Mount St. Helens, Oregon, and the Kilauea volcano in Hawaii, both speakers reported the need for 1 to 3 millimeter horizontal accuracy and 5 to 10 millimeter vertical accuracy. They also noted that more accurate measurements would allow additional geophysical processes to be studied, such as nonsecular motions. This level of accuracy can only be achieved by minimizing the sources of error discussed above.
The monitoring of earthquake and volcanic processes requires a 30-second temporal sampling rate for reliable, autonomous “cleaning” of data (i.e., detection and removal of cycle slips and bad data). A decimated sampling rate can be used for determining position because of the temporal correlations in GPS phase measurements. The analysis centers of most GPS networks decimate data to 2 to 10 minutes. Data latency of 1 to 3 days is adequate, but there are times when near real-time dissemination would be useful (immediately following a large earthquake, for example). For studying postseismic deformations, sampling rates on the order of 1 second would be desirable, even if the resulting positioning accuracy is less precise.
Many of the issues discussed by the working group on networks, data sources, and static positioning applications were described in Chapter 2. However, some of the discussions focused on subjects covered by the presentations described above and were considered particularly important by geophysical researchers.
Although the group agreed that a 30-second receiver sampling rate should be the standard default setting for all GPS networks and reference stations, some applications may require that higher sampling rates be available on request. The group noted that the need for higher sampling rates may grow as ionospheric effects increase with the approach of the solar maximum. The group also agreed that real-time data are not required for most static positioning applications. However, postseismic studies related to earthquakes and volcanic eruptions could benefit from near real-time data. Data derived from networks like the U.S. Coast Guard's are already packaged in hourly files that are available a few minutes past the hour. However, rapid transfers of data with one hour latency may be too expensive for some sites. In addition, the need for data centers that compile older, previously received data sets that can be integrated with current data will continue.
The working group discussed the need to mitigate errors caused by monument instability, keeping in mind that some researchers actually gain useful information about surface conditions by observing antenna displacements. Even in these cases, however, attempts should be made to measure and characterize the stability of monument and antenna placements at as many GPS tracking sites as possible. Information for each site could be provided to users in the comprehensive network catalogue described in Chapter 2. To ensure that this information is accurate and consistent, standard methods of measuring monument stability will have to be developed. Otherwise, researchers will have difficulty separating the effects of monument instability from the effects of other error sources, such as multipath, radio-frequency interference, ionospheric and troposphere delays, and antenna phase center variations.
In order to satisfy the accuracy requirements of many static positioning applications, the group agreed that GPS observations should probably be collected by all sites down to at least 5 degrees elevation angle unless physical limitations, such as natural or man-made obstructions, make this impractical. Low elevation angle data are critical to measuring the signal delay and subsequent positioning errors caused by tropospheric water vapor. However, because errors related to interactions between GPS antennas and the surrounding environment are more prevalent at low elevation angles, characterizing reference sites for both near-field scattering and far-field multipath effects is critical. Information used to evaluate these effects, such as measurements of signal-to-noise ratio, could be made available to users. In addition, geological and geomorphological descriptions of sites and antenna configurations, including photographs and horizon mask information could also be made available. The need for continued testing of antenna phase center variations, especially for networks with mixed antennas was also discussed, as was the need for a better understanding of the effects of radomes on positioning accuracy.