ИСТИНА

Fluctuations of the GPS signals on the tangential paths in the low terrestrial atmosphere: in°uence of the small-scale structure. Both low stratosphere and troposphere of the Earth are characterized by complicated inhomogeneous structure of the refractive index ¯eld. This irregu- lar structure results from turbulent °ows, non-uniform distribution of the water vapor in the air, orographic perturbations of the wind °ows and so on. Char- acteristic scales of these structure is on the order of hundreds of meters - units of kilometers, which is comparable to Fresnel zone size for GPS signals, while amplitude of the °uctuations of the refractivity may reach several N-units, i.e. units per cent of regular refractive index of the terrestrial atmosphere. First 3D reconstructions of small-scale water vapor °uctuations were recently published. In this research, a model of irregular structure of the refractive index ¯eld has been formulated on the basis of the previously published observational results. The parabolic di®raction equation has been solved for the global cross-section through the inhomogeneous atmosphere (2D approximation). It has been shown that the di®ractional e®ects of the small-scale structure play signi¯cant role in the formation of the wave ¯eld in the atmosphere. Amplitude °uctuations of the signal, caused by this small-scale structure of the low atmosphere, may be quite signi¯cant and comparable with the amplitude °uctuations due to ionospheric irregularities. On the other hand, these °uctuations do not result in complete degradation of the coherent structure of the signal ¯eld, observed at the low orbits of typical satellites, used for radio occultation observations. The in°uence of scattering of the signals by the terrestrial terrain has not been considered in the present work and is still to be studied. This research has been in part supported by RFFI grants 05-05-65145 and 06-05-64988. [1] BRAUN, J.J. and C. ROCKEN. WATER VAPOR TOMOGRAPHY WITHIN THE PLANETARY BOUNDARY LAYER USING GPS. International Workshop on GPS Meteorology - GPS Meteorology: Ground-Based and Space- Borne Applications - Tsukuba, Japan, 2003. XY0845; EGU2007-A-01853; ST14-1TH3P-0845;