THE IMPROVEMENT OF GLOBAL POSITIONING SYSTEM TROPOSPHERIC DELAY BY USING MODIFIED UNBabc MAPPING FUNCTION

Abstract

Global Positioning System (GPS) tropospheric delay refers to the refraction of the GPS signal as it passes through the neutral atmosphere from the satellite to the earth, which causes longer distance traveled by the signal. The issue of atmospheric delay is investigated to minimize the positioning error due to tropospheric and ionospheric delay. The mathematical modeling on the tropospheric model should be revised and also modified to improve the delay. The zenith tropospheric delay can be amplified by a coefficient factor called mapping function to form total tropospheric delay. There are many mapping functions have been established to calculate the scale factor which can affect the total tropospheric delay. However, the mapping functions give large value when the elevation angles less than 5 degrees. The modification of the continued fractions by replacing sine function to cosine function in the popular mapping function called UNBabc has been proposed and tested to ensure the improvement of the result. The result of the modification, show a significant reduction of mapping function scale factor. The modified UNBabc mapping function has improved the tropospheric delay up to 19.1% at 2 degree elevation angle. This modified mapping function could be used as the alternative mapping function.