西南日本における水蒸気ラジオメータ観測に基づいたWet Mapping Functionの包括的な解析

Abstract

The major error source that limits the accuracy in the Global Positioning System (GPS) as well as other modern space techniques, such as Very Long Baseline Interferometry (VLBI), is the mis-modeling of the tropospheric delay. This delay is experienced by radio waves in propagating through the electrically neutral atmosphere. The tropospheric delay is usually divided into two components, called hydrostatic (or dry) and wet delays. Both hydrostatic and wet delays are usually described as a product of the delay at the zenith and a mapping function. The mapping function models the elevation dependence of the propagation delay. In the couple of decades, a number of mapping functions have been developed for use in the analysis of space geodetic data. Using the water vapor radiometer observation at the south-west Japan, an evaluation of the accuracy of most of the wet delay mapping functions has been performed. This analysis includes those developed by Moffet, Black, Davis (CfA-2.2), Ifadis, Herring (MTT), Niell (NMF) as well as a locally optimized one (Mousa-1). The water vapor radiometer measurements cover selected elevation angles down to 5°.These measurements are mainly taken at Uji (as an inland site) and Shionomisaki (as a seashore site). Small campaigns at both Shigaraki and Izu-Hanto are also included. Except Mausa-1, all the wet mapping functions studied here are globally designed. Since the Japanese climate condition (especially the vapor content of the troposphere) is not taken into account properly in the design of the global wet delay mapping functions, their performance is very poor especially at elevation angles lower than 15°.It is to be noted here that Mousa-1 was optimized based on data taken at Shionomisaki only. But its performance is superior for other Japanese sites to the global mapping functions.