New Estimates of Tide Gauge Vertical Velocities Based on Nearby GPS Sites

Land motion rates in the vicinity of tide gauges play a significant role in determining global sea level. A global set of GPS sites with three to nine years of data, low repeatabilities, and position within 10 km of a tide gauge were chosen to study the vertical motion at the tide gauges. Using the JPL GIPSY-OASIS software, precise point positioning was performed to determine the position of each site for the duration of each time series. The non-fiducial point positioning method allowed for a consistent analysis of a large number of stations while linking all of the solutions to the ITRF2000 reference frame. In addition to geodetic height, a number of other metrics were examined including formal error, number of pseudorange observables, number of good phase residuals, percentage of observations at low elevation, and mean total zenith tropospheric delay. The majority of the data that was used had formal errors less than 2 cm and at least 1000 pseudorange observables for each day analyzed. Breaks were inserted at the points of large offsets in the metrics and the site rate was estimated again. The rate of each site was computed by de-trending the position time series, computing the dominant periodicities, and then computing the rate again while taking into account the computed periods. Using this technique, the formal error as well as the repeatability of each new rate estimate was capable of being reduced as compared to the estimates of rate in which biases such as these were not estimated. A list of the new estimated trends at 26 sites as well as a thorough analysis of the associated errors will be shown in greater detail.