Testing monument stability with short-baseline GPS measurements

The two permanent GPS sites at Pinon Flat Observatory (PFO)- called, naturally, PIN1 and PIN2 -are among the oldest permanent GPS sites in southern California. These sites were built in 1989 as prototypes for a continuous GPS network, an idea only just being implemented in a few places. The monumentation (how the GPS antenna was attached to the Earth) used at these sites was based on long experience operating several laser strainmeters at PFO, and is the basis for the drilled-braced monument design currently used by networks such as SCIGN and PBO. The record of GPS measurements between these sites provides long-term data between two closely spaced and well-anchored monuments, and so gives us a unique perspective on potential sources of noise such as monument stability, near field environmental effects and equipment effects. Because the two sites are only 50 meters apart, many sources of noise (and signal) are common to both, and cancels when we estimate the baseline vector between them. The day-to-day scatter in the baseline series (1-sigma 0.07 mm in the horizontal) is almost a factor of 10 smaller than is typical for a globally-referenced position series, and significantly less than the scatter from differencing the global series between these sites (0.35 mm). We present the results of a Maximum Likelihood Estimation (MLE) of the noise content which we assess in the context of monument stability. We also discuss an unexpected annual signal in the north-south component, a change in the white noise content attributable to an equipment change and outliers and offsets due to known local environment changes.