A STUDY ON THE RELATIONSHIP BETWEEN SEASONAL VARIATION AND POSITIONING ACCURACY BY GPS

摘要

The positioning accuracy of the Global Positioning System (GPS) has been improved considerably during the past two decades. Historically, the main error sources such as ionospheric refraction, orbital accuracy, antenna phase center variation, signal multipath, and tropospheric delay have been reduced substantially or eliminated. With the present state-of-the-art of GPS data analysis in geodesy, positioning accuracy is on the level of 1–2 mm in horizontal coordinates and 5–10 mm in the vertical coordinate [Bock, 1998; Bock, 2000; Johansson, 1998; Schenewerk, 1998]. There are two major reasons for the poor accuracy in the vertical axis. The first one is associated with a theoretical limit due to the satellite geometric distribution in the sky since observations are made within a minimum elevation angle (typically about 15 °) [Santerre, 1991]. The other one is due to tropospheric path delay, especially water vapor (or wet path delay) [Davis, 1985; Dodson, 1996; Emardson, 1999]. In this study, seasonal effect on the GPS positioning accuracy is investigated. Taiwan is chosen for her unique geographic location and complex topography to exhibit abound water vapor in the air but spatially and seasonally dependent. The GPS data were collected from continuously operating reference stations by International GPS Service (IGS), Ministry of the Interior (MOI), Central Weather Bureau (CWB) and Industrial Technology Research Institute (ITRI) of Taiwan. The investigation of the relationship between variance of the vertical coordinate and change of climate is carried out by computing the GPS data collected from July to December, 2003. In addition, the comparison in results from using the Saastamoinen model and Hopfield model for correcting the influence of the atmosphere path delay is analyzed. It is found that the maximum difference in the monthly average ellipsoid height between July and December 2003 is about 20 mm. The corresponding daily maximum difference is 60 mm. Also, the ellipsoid height derived by using the Saastamoinen model is smaller than that by using the Hopfield model.