Development of Tropospheric Correction Techniques to Optimize Vertical GPS Accuracy

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The Global Positioning System (GPS) satellites provide a powerful new tool for accurate geodetic measurements. Vectors between GPS receivers can now be measured with accuracies of one centimeter out of a baseline of 100 kilometers in the horizontal and 3-5 centimeters in the vertical direction. One major contribution to the degradation of vertical positioning is inadequate correction for time delays of GPS signals caused by water vapor in the troposphere. The "wet-path delay" can be measured directly, in principle, using water-vapor radiometers pointed along the line-of-sight to GPS satellites. However, definitive line-of-sight experiments have not yet been performed. This project will perform a definitive radiometer test by operating line-of-sight meters at two permanent GPS sites in California. Tests will be conducted for two separate one-month sessions during different seasons of the year. The object is to develop wet-path delay correction techniques that optimize vertical GPS positioning accuracy.