The use of GPS for the estimation of precipitable water vapour for weather forecasting and monitoring in South Africa

The propagation of the Global Positioning System (GPS) signal from the satellite to the receiver is affected by, among other factors, the atmosphere through which it passes and, whereas the effects of the ionosphere can be eliminated by the differencing of the two transmitted frequencies, the effects of the troposphere remain one of the major sources of noise in traditional geodetic and positioning applications of GPS. This noise can, however, be turned into a signal for the meteorologist and, by applying suitable constraints and processing strategies, it is possible to estimate the amount of precipitable water vapour (PWV) in the atmosphere. The application of the GPS data for the estimation of PWV in the atmosphere is not a new concept and has been described in many publications and reports since the early 1990s. This project is, however, an attempt to test the technique using the South African network of permanent GPS base stations. This paper describes the fundamental principles and the testing of the estimation of PWV using GPS and meteorological data from a network of nine permanent GPS base stations in South Africa. The results of the GPS derived estimates have been verified against upper air measurements from radiosonde ascents from a sub-net of five of the nine stations as well as with PWV estimated from a numerical weather model (NWM).