ИСТИНА

Radio occultation (RO) technique is applied for the atmospheric and ionospheric investigations since interplanetary missions to Mars [1] and Venus [2]. Quite soon the general formulae for the radio occultation data inversion, based on the geometrical optics (GO) approximation and the local spherical symmetry assumption, have been derived [3]. During further progress in this field, new sophisticated approaches for investigations of the fine atmospheric and ionospheric structure have been proposed [4]. Application of new approaches in practice requires significant improvement of the ratio between the effects under investigation and instrumental errors [5]. These errors, which are largely related to the limited stability of the local oscillator onboard the spacecraft, can be reduced, and the informative variations of the signal phase on the ionospheric path can be increased with the decrease of the sounding signal frequency [5]. However, too large distortion of the lower frequency signal in the inhomogeneous plasma can result in the inapplicability of the geometrical optics approximation, on which the data interpretation techniques [3] are based. Motivation of this study is the validation of the geometrical optics approximation for the radio occultation data interpretation on the lower frequencies with the direct simulation of the wave field. During this study, a computer code for direct numerical solution of the parabolic diffraction equation has been developed and tested. Computer simulations of the radio occultation experiments of the Venusian ionosphere, accounting for the diffraction effects, have been carried out. Validation of the experimental data, previously obtained from the interplanetary space missions Venera-15 and Venera-16, is performed. Applicability of the geometrical optics in the focusing regions with the nearly fourfold increase of the signal intensity, which took place in real experiment, has been studied. Deviations from geometrical optics have been detected and specially interpreted. Validity of the conventional RO inversion techniques for Venus RO data analysis has been shown. The research is carried out using the equipment of the shared research facilities of HPC computing resources at Lomonosov Moscow State University [6]. The research has been partially supported by Russian Science Foundation with the grant 17-77-20087. REFERENCES 1. Arvydas Kliore, Dan L. Cain, Gerald S. Levy, Von R. Eshleman, Gunnar Fjeldbo, and Frank D. Drake. Occultation experiment: Results of the first direct measurement of Mars’s atmosphere and ionosphere. Science, 149(3689):1243–1248, 1965. 2. Mariner Stanford Group. Venus: Ionosphere and atmosphere as measured by dual-frequency radio occultation of Mariner V. Science, 158(3809):1678–1683, 1967. 3. R. A. Phinney and D. L. Anderson. On the Radio Occultation Method for Studying Planetary Atmospheres. J. Geophys. Res., 73:1819, March 1968. 4. M.E. Gorbunov and G. Kirchengast. Processing X/K band radio occultation data in the presence of turbulence. Radio Science, 40(6):RS6001, 2005. 5. A.L. Gavrik, Y.A. Gavrik, T.F. Kopnina, and L.N. Samoznaev. Amplitude and frequency variations of coherent radio signals during probing of the daytime venusian ionosphere. Journal of Communications Technology and Electronics, 55(3):256–262, 2010. 6. V. Sadovnichy, A. Tikhonravov, Vl. Voevodin, and V. Opanasenko ”Lomonosov”: Supercomputing at Moscow State University. In Contemporary High Performance Computing: From Petascale toward Exascale (Chapman & Hall/CRC Computational Science), pp.283-307, Boca Raton, USA, CRC Press, 2013. 2 Title: Applicability of the geometrical optics approximation in radio occultation experiments: numerical simulations. Ya. A. Ilyushin12 Moscow State University Leninskie Gory, ulitza LebedevaMoscow 119992 Russian Federation phone +7 495 939 32-52 fax +7 495 932-88-20 email ilyushin@phys.msu.ru Gavrik A.L Kotelnikov Institute of Radio Electronics and Engineering, Fryazino branch Vvedenskii Sq., 141190, RussiaFryazino, Moscow region 141190 Russian Federation phone +7(496)565- 24-00 fax +7(496)565-24-07 email alg248@hotmail.com Corresponding author: Ya. A. Ilyushin Presenting author: Ya. A. Ilyushin Oral presentation is preferred 1Corresponding author 2Presenting author