ИСТИНА

Radio occultation technique is a very effective tool of remote sensing of planetary atmospheres. Practice of development of new advanced methods of experimental data analysis, suitable for lower ionosphere investigations, has shown the need for high ratio between refractive effects and instrumental errors. To investigate fine structure of the lower ionosphere utilization of decameter wave band is preferable. In this case, an informative variation of the signal parameters in the planetary plasma environment is significantly larger than instrumental phase fluctuations due to limited accuracy onboard local oscillator. However, strong refraction of the lower frequency signal can lead to violation of the geometrical optics applicability conditions, on which radio occultation interpretation techniques are based. There are presented the results of numerical simulations of radio occultation experiment with ground based transmitter and on-board receiver of coherent signals. By direct numerical solution of the parabolic wave equation variations of the wave field parameters along the spacecraft trajectory are investigated. The modeling results are validated with the experimental data from the Venera 15 and 16 spacecrafts. Basic attention is paid to the analysis of radio physical effects caused by the wave diffraction in thin ionized layers near the lower boundary of the Venusian ionosphere. Experimental data, acquired with Venera 15 and 16 spacecrafts, suggest the possibility to study multi ray wave propagation and diffraction effects, appearing at 32 cm wavelength in the spherically symmetric ionosphere. Simulation results revealed the criteria, allowing checking validity of geometrical optics approximation from the experimental data of thin ionospheric layer sounding. It has been shown that the linear relation between periodical frequency and energy deviations observed in the experiment of the lower ionosphere sounding, proves the absence of multi-ray wave propagation or diffraction effects, so it is caused by the wave refraction during the wave propagation through the stratified multi-layered periodical plasma structure.