WET TROPOSPHERIC EFFECTS ON PRECISE RELATIVE GPS HEIGHT DETERMINATION

Considerable interest has been generated recently in the use of the Gl obal Positioning System (GPS) for precise height determination. A majo r error source in these measurements is the propagation delay due to a tmospheric water vapour. In order to achieve the high precisions requi red for such applications as absolute sea-level monitoring improvement of wet delay modelling is necessary. Results from a GPS campaign show a significant correlation (0.91) between the variability of the wet d elay measured using a water vapour radiometer (WVR) at the Onsala site and the absolute value of the residual error in the height determinat ion of a 134 km baseline from Onsala to Jonkoping. This correlation in dicates that the atmospheric variability as inferred from the WVR data includes information on the quality of the GPS height estimate. Durin g periods of high atmospheric activity, e.g., during the passage of a weather front, the use of a six-parameter gradient model reduces the s pread for the vertical coordinate from 40 mm to 20 mm (with standard d eviations of 17 mm and 9 mm respectively) over the 134 lan baseline (l ess than 1 x 10(-7)) using 8 hour data spans on 11 different days over a six month period.