The influence on GPS estimates of NWP-derived mapping functions

Abstract

When estimating zenith total delays (ZTD) from global positioning system (GPS) signals it is assumed that the atmosphere is symmetric in the horizontal plane and that delays in the signal from one of the GPS satellites to a receiver can be mapped onto the zenith direction through a known mapping function. The generally used mapping function has only a dependence on the day of the year and the latitude of the receiver. The fact that these mapping functions do not depend on the actual weather may introduce errors in estimates of the location parameters and the estimate of the ZTD. In this paper mapping functions from numerical weather prediction (NWP) models are derived and the influence on the accuracy of location parameters and ZTD estimates is investigated during a cold front passage in the Netherlands on the 30th of October 2000. The hydrostatic mapping function computed from NWP was smaller than the Niell mapping function (0.02 at 5° elevation). The differences for the wet mapping function were much larger, and of opposite sign. However, since the hydrostatic delay is larger than the wet delay, we found that the geodetic parameters are mainly influenced by errors in the hydrostatic mapping function. Using an elevation cut-off error of 10°, we observed systematic errors in the estimated ZTD of −2.1 to −3.1 mm, in the clock error of 6.5–9.2 mm and in the height of 4.8–6.5 mm.