[1] The noisy and impulsive fluctuations in the CHAMP radio occultation (RO) amplitude data are similar to the Ctype and S-type ionospheric amplitude scintillations formerly observed at 1.5 GHz in the mid-latitude region in satellite-to-Earth Inmarsat links. These amplitude scintillations can be associated with different types of ionospheric structures. S-type amplitude variations can be explained by the influence of inclined plasma layers in the ionosphere where the RO signal trajectory is perpendicular to the sharp plasma gradient. Simulation indicates the possibility to reveal the spatial distribution of the electron density in the inclined ionospheric layers from analysis of the S-type RO amplitude variations.

The radio holographic principle is briefly described and tested by using radio occultation data of the GPS/MET and MIR/GEO experiments. Sub-Fresnel spatial resolution ∼12 m/pixel was achieved using focused synthetic aperture radio holographic approach, and direct evidence of multibeam propagation effects in the atmosphere was obtained. The achieved instrumental accuracy in angular distance measurements was near 0.004 milliradian/pixel, and observed angular distance between different rays was equal to 0.3 milliradians. The angular resolution of the radio holographic method depends on the wavelength as λ 1 compared to λ 1/2 in conventional methods. In general case the principal limit of the vertical resolution may be determined using focused synthetic aperture antenna theory and may achieve a value ∼20-40 m under assumptions of spherical symmetry and quiet atmospheric conditions. Wave structures were discovered in the altitude distribution of the gradient electron density at a height interval of 60-95 km with spatial period 1-2 km and vertical resolution 300-500 m. Good correspondence was found between the temperature profiles revealed by radio holographic analysis and those obtained by traditional retrieval using UCAR GPS/MET data.

[1] Wave phenomena in the upper atmosphere can be studied using the high-precision Global Positioning System (GPS) radio navigational field. In this paper, basic principles, accuracy, and vertical resolution of the radioholographic technique for studies of ionospheric wave phenomena are presented for the general case when the orbits of the satellites are arbitrary. Results of testing of the radioholographic method are discussed using orbital station MIR and geostationary satellites (MIR/GEO) and GPS/Meteorology (GPS/MET) radio occultation data. The radioholographic method has high vertical (12-30 m) and angular (4-8 mrad) resolution, which has been validated by directly observing multibeam propagation in the atmosphere and revealing signals, reflected from the sea, in GPS/MET and MIR/GEO radio occultation data. We show that this method allows one to determine the vertical profile of the electron density and monitoring wave structures in the upper atmosphere. As an example of this approach, observations of the summer Antarctic mesosphere on 7 February 1997 are presented. We show, by combining phase and amplitude analysis, that a vertical resolution of 0.3-0.5 km reveals wavelike structures with spatial periods from 1-2 km to 8-10 km in the vertical electron density distribution in the D and E regions. Variations in the gradient of the electron density from ±5 Â 10 3 to ±8 Â 10 3 el/(cm 3 km) at altitudes of 72-95 km were observed. The obtained results demonstrate the high-technology level of the radioholography approach and open new perspectives for radio occultation experiments: measurements of the characteristics of the natural processes in the atmosphere, mesosphere, and ionosphere and observations of the state of the sea surface by measuring parameters of reflected signal simultaneously with radio occultation experiments.

A theoretical analysis of refractive loss of radio waves by the Earth's atmosphere in radio occultation measurements along the satellite‐to‐satellite line for various altitude profiles of the refractive index is given. Experimental results for refractive loss on the orbital spacecraft ‐ geostationary satellite link are presented. Theoretical calculations are compared with experimental data, and a conclusion is drawn that the signal amplitude during radio occultation is strongly dependent on the layered structure of the refractive index profile. Amplitude scintillations of centimeter (λ1 = 2 cm ) and decimeter (λ2 = 32 cm ) radio waves used in radio occultation experiments are described. Dependences of the rms value of the amplitude scintillations versus the minimum altitude of the ray line for the two above wavelength bands are presented. The frequency spectra of the log‐amplitude scintillations are analyzed together with the dependence of the rms amplitude on the wavelength. Experimental data are compared to the theory of scintillations in a turbulent atmosphere, and the altitude model of the structure constant of refractivity fluctuations is determined.

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The relationships between the Doppler frequencies, eikonal acceleration, and refractive attenuations of the direct and reflected signals are established for bistatic and radio occultation experiments. These connections allow recalculating the Doppler shifts and the phase delays to the refractive attenuation (reflectivity cross section) and open a new avenue for potentially measuring the total absorption in the atmosphere at low elevation angles. The fundamental characteristics of bistatic remote sensing of the atmosphere and Earth's surface such as the phase delay, reflection coefficient, reflectivity cross section, and Doppler shift of the reflected signals relative to the direct signals are obtained in analytical forms by taking into account the refraction and absorption effects in the atmosphere. Difference in the Doppler frequencies of the reflected and direct signals is proportional to the difference of the modified refractive index at the radio ray perigee and at the Earth's surface. The obtained analytical results are in good agreement with the measurements data obtained during the MIR/GEO (wavelengths 2 and 32 cm), and CHAMP (wavelengths 19 and 24 cm) radio occultation experiments. Detecting the reflected signals in radio occultation data has opened new perspectives for bistatic monitoring of the atmosphere and Earth's surface at low elevation angles. Experimental results of the propagation effects at low elevation angles are of great importance for fundamental theoretical investigation of radio waves propagation.