Comparison and Combination of Solutions From the Southern California Integrated GPS Network

The Southern California Integrated GPS Network (SCIGN) has two independent centers for precise processing of data from 250 stations. The Jet Propulsion Laboratory (JPL) uses the processing software GIPSY-OASIS-II, and the Scripps Institution of Oceanography (SIO) uses the GAMIT/GLOBK/GLORG software. In earlier comparisons of JPL and SIO results, we found discrepancies of several mm in station position and several mm/yr in station velocity and traced these differences to inconsistent antenna heights, different methods of time series analysis, and different reference frames. Current work focussed on correcting antenna heights, comparing baselines, investigating noise models and offset detection, and defining a common reference frame in which to combine JPL and SIO solutions. For baselines, which are relative positions between two stations and hence do not depend on reference frame, the mean difference in JPL and SIO relative position is about 0.1 mm in the horizontal and 0.5 mm in the vertical. Proportional error in baselines is 0.9, 2.5, and 1.0 parts per billion (ppb) for the north, east, and vertical components, respectively, and 1.2 ppb for baseline length. Offset detection and estimation depends on the error model; the best current noise model is a combination of white and flicker noise. Estimated offset uncertainties are then 0.5 mm in the north, 1 mm in the east, and 2 mm in the vertical. To combine JPL and SIO solutions, we defined a preliminary SCIGN reference frame by choosing 16 stations with good geographic distribution and a fairly complete time history back to 1996. The JPL and SIO groups reprocessed data from these stations with loose constraints on station coordinates. We used GLOBK/GLORG to combine the JPL and SIO daily solution and covariance files, aligning to the IGS97 positions of the global stations. These results provide a reference frame (SCIGN97 v.1.) for the entire SCIGN array. For data back to 1996, the mean difference between JPL and SIO solutions in the same reference frame is typically less than 1 mm in the north and east coordinates and 1 to 3 mm in the vertical coordinate. We are combining JPL and SIO solutions from all SCIGN tations in this reference frame.