Toward an ITRF2000 Combined Solution for 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 GIPSY-OASIS II processing software and the Scripps Institution of Oceanography (SIO) uses the GAMIT/GLOBK/GLORG software. The first comparison of JPL and SIO results revealed discrepancies of several mm in station position and several mm/yr in station velocity; these differences were the result of inconsistent antenna heights, different methods of estimating velocity, and, especially, different reference frames. To combine JPL and SIO solutions in a consistent reference frame, we chose 75 stations with at least 500 days of data and horizontal RMS of 1 mm or less. Rotating and translating the frames to align these 75 stations generates 0.5 mm/yr horizontal RMS velocity difference. Adding the vertical component, the 3-dimensional RMS velocity difference is 1.2 mm/yr. In a consistent reference frame the SIO and JPL horizontal RMS position difference is less than 1 mm. Vertical differences, due to antenna height discrepancies, still have to be resolved. Current combinations are in a North American reference frame, but using stations from the global network will allow us to align the SCIGN stations in the ITRF2000 reference frame.