Fluctuations of radio occultation signals in sounding the Earth's atmosphere
Abstract. We discuss the relationships that link the observed fluctuation
spectra of the amplitude and phase of signals used for the radio occultation
sounding of the Earth's atmosphere, with the spectra of atmospheric
inhomogeneities. Our analysis employs the approximation of the phase screen
and of weak fluctuations. We make our estimates for the following
characteristic inhomogeneity types: (1) the isotropic Kolmogorov turbulence
and (2) the anisotropic saturated internal gravity waves. We obtain the
expressions for the variances of the amplitude and phase fluctuations of
radio occultation signals as well as their estimates for the typical
parameters of inhomogeneity models. From the GPS/MET observations, we
evaluate the spectra of the amplitude and phase fluctuations in the altitude
interval from 4 to 25 km in the middle and polar latitudes. As
indicated by theoretical and experimental estimates, the main contribution
into the radio signal fluctuations comes from the internal gravity waves. The
influence of the Kolmogorov turbulence is negligible. We derive simple
relationships that link the parameters of internal gravity waves and the
statistical characteristics of the radio signal fluctuations. These results
may serve as the basis for the global monitoring of the wave activity in the
stratosphere and upper troposphere.