GNGTS 2013 - Atti del 32° Convegno Nazionale

probe (Champollion et al. , 2004). The basic idea behind this topic is that when travelling through the atmosphere to Earth-based antennas or other satellite, microwave radio signals transmitted by GPS satellites experience changes in velocity and deviation (bending) from the Fermat’s straight lines depending on the refractive index gradients and related physical conditions of the atmosphere. All these effects increase the travel time of the GPS signals. GPS meteorology is then concerned with inverting this delay time to shed light on the atmospheric conditions. Two main branches of GPS meteorology deal with “radio occultation”, which is a satellite-to-satellite technique for limb sounding, and “ground-based GPS” measurements. The propagation problem of the GPS radio waves develops in a different way in the troposphere and ionosphere. Fortunately ionosphere is dispersive (i.e. frequency dependent) for microwave radiation and propagation effects can be reduced by just collecting a linear combination of both L1, L2 frequencies transmitted by the GPS satellites, the so called “LC-ionofree” observable (Hoffman-Wellenhof et al. , 2001). On the contrary, the troposphere is non-dispersive, so the consequent delay cannot be directly eliminated from GPS observations. The tropospheric delay in GPS signals is due to the permanent electric dipole moment of the molecule of water so that it is nearly proportional to the quantity of water vapor (WV) integrated along the signal path. Being directly responsible for the unusually large latent energy associated with water’s change of phase, WV plays a fundamental role in the transfer of energy through the atmosphere and in the formation and propagation of weather systems and tropospheric fronts (Stephens et al. , 1991; IPCC, 1992). Because of its extremely variable distribution both in time and space as well as a poor correlation with surface humidity, the WV is one of the most difficult meteorological parameter to quantify. Routinely WV content in the atmosphere is measured by means of standard synoptic radio soundings (water vapor radiometers –WVR-balloons), but they are too sparsely distributed in space and time to support reliable forecasting. The societal impact of extreme weather events such as floodings and associated effects (rock fall and mud flow) set a claim for an improvement of our skill to monitor moisture-fluxes able to trigger such severe weather. The nowadays extensive use of geodetic permanent GPS networks offers a powerful tool for a high resolution remote sensing of atmospheric WV and “precipitable water” (PW), which is the total quantity of WV overlying a point on the Earth’s surface expressed as the height of an equivalent column of liquid water. Duan et al. (1996) demonstrated that a pure GPS solution was possible for PW retrieval on a regional network by incorporating few remote (distance > 200 km) stations into the geodetic analysis. Modelling of GPS atmospheric delay. The observed tropospheric delay (TD) of a generic GPS signal is customarily described as resulting from three contributing components according to the equation: TD( , ) =STD sym ( )+STD az ( , )+S (1) A spherically symmetric component STD sym ( ) , only depending on the elevation angle ( ) of the satellite to the station; a second component allowing to account for the atmosphere anisotropy and dependent on both elevation and azimuth ( ) angles and last a generic “residual” term ( S ), which could be defined as the difference between a model daily solution and the observed one. Both of the first two terms in the Eq. (1) are customarily implemented in software packages devoted to GPS analysis in the following form: (2) (3) 201 GNGTS 2013 S essione 3.3