The GPS signal passing through the troposphere is subjected to bending of the ray path and slowing of the propagation speed - which in result lead to delays in signal arrival. The delay is proportional to the refractivity along the ray path which in turn depends upon meteorological parameters - pressure, temperature and humidity. The refractivity can be separated into two components (Hopfield, 1969); wet part Nw – caused by the varying amount of water vapour and temperature gradient in the troposphere, and the hydrostatic (dry) part Nd - result of the pressure gradient. Thus from system architecture it is possible to calculate inversion of delays - meteorological parameters - which is the tomography issue. The principle of tomography follows that the results of the measurements of the parameters, the a priori information on model parameters, and the information on the physical correlations between observable parameters and model parameters can all be described using probability densities (Tarantola, 2005). The measurements of parameters are GPS delays, the a priori information on model parameters is temperature, pressure and water vapour, and the physical correlations are dependences between amount of water vapour, laps rate, pressure gradient and refractivity. From 24th of August till 27th of August (48 hours of continuous observations) a GPS tomography experiment was performed on the Karkonosze network. The sites of geodynamic network "Karkonosze" are located on the main faults of the Karkonosze Mountains (in southwest of Poland) - mountains that are well know for fast changes of meteorological parameters and relatively high elevation (approximately 1300 m above MSL). The meteorological data originates from ground measurements - with approximate planar resolution of 5 km. Moreover there are soundings on adjacent area available 4 times a day. The GPS measurements were performed with the use of equipment and strategy developed during 10 years of study (Bosy, 2005). In the paper there are presented: the methods of calculation the delays from GPS measurements, the meteorological data network calibration, the transformation from point meteorological data into data referring to area, gradient calculation based on adjacent soundings and early results of tomography derived water vapour.
keywords: tomography, water vapour, GPS