GPS tropospheric delays for meteorological applications

Although developed for positioning, Global Navigation Satellites Systems (GNSSs) are increasingly used for atmospheric sounding. With the aim of improving the positioning accuracy, the delays affecting the GNSS signals in the tropospheric layer are partly modelled and partly estimated and removed from the observations. These estimates related to the water vapour content of the troposphere can be exploited for meteorological applications and possibly for precipitation nowcasting forecasting. This thesis represents a follow on of an experimental activity, called MisT (Misure Troposferiche), carried out within the European Space Agency funded project METAWAVE (Mitigation of Electromagnetic Transmission errors induced by Atmospheric Water Vapour Effects). This experiment, designed to study the stochastic behaviour of the GNSS tropospheric Zenith Wet Delays (ZWD) estimated from the adjustment of a local multi-scale network of permanent stations, was used as a test set for the implementation of a data analysis strategy then refined and applied to a Japanese network explicitly designed for meteorological applications. Thanks to a cooperation with the Research Institute of Sustainable Humanosphere (RISH) of Kyoto University, the data of a dense, regular local GNSS network, whose stations are coupled with meteorological devices, as well as the data of ground based radar sensors were made available to us. High resolution time series of ZWDs derived by a Precise Point Positioning (PPP) adjustment of this network were used to compute anisotropic variograms, describing the spatial correlation of the ZWDs themselves. The relationship between these second order statistics, weather station observations and radar derived precipitation models was investigated by means of an ad-hoc built software able to integrate and compare all these data types.

Global Navigation Satellite Systems, nati per il posizionamento, sono sempre più utilizzati per l'osservazione dell'atmosfera. I ritardi prodotti sui segnali GNSS dalla presenza di vapor d'acqua nella troposfera, trovano applicazione in ambito meteorologico per la previsione di precipitazioni. La tesi sviluppa una metodologia di analisi statistica delle stime di ritardo troposferico ottenute dalla compensazione di reti locali, e implementa un software per lo studio della correlazione tra eventi di pioggia e statistiche del secondo ordine derivate dalle predette stime. I risultati relativi a una campagna sperimentale condotta su una rete di stazioni permanenti, gestita dall'istituto giapponese di ricerca RISH, sono promettenti e costituiscono un primo passo nella definizione di una relazione quantitativa tra i prodotti GNSS e le precipitazioni.