Mikhail E. Gorbunov

Microlab‐1 experiment: Multipath effects in the lower troposphere

Interpretation of the Doppler-shifted radio frequencies from the proof-of-concept GPS/MET radio occultation experiment is impeded by the effects of multipath propagation caused by strong variations of atmospheric refractivity. In this paper, we describe and validate a method of deciphering the ray structure of the measured electromagnetic field in regions of multipath propagation. The method is based on diffraction theory and the principles of the synthesized aperture, which allows for the refinement of the vertical resolution beyond the Fresnel scale as well as for the improvement of the signal/noise ratio. The validation of the method is based on artificial occultation data simulated using global fields from a NCEP analysis. An example of the application of the diffraction correction algorithm for the processing of real GPS/MET data is given. The comparison of the inversion results with and without diffraction correction indicates a very significant role of multipath propagation in the lower troposphere, which suggests that the diffraction correction can significantly improve the quality of the data processing.

Comparative analysis of radioholographic methods of processing radio occultation data

Two methods for the determination of refraction angle profiles from radio occultation measurements in multipath areas are analyzed and compared: (1) the radio-optic method based on the analysis of the local spatial spectra of the measured wave field and (2) the back propagation of the received wave field to a single-ray region. The basic limitations of the radio-optic method are (1) the restriction of the resolution of refraction angle profiles due to the uncertainty relation of refraction angle and impact parameter and (2) diffractive effects in subcaustic areas, where the spatial spectra cannot be interpreted in terms of geometric optical rays. The basic limitation of the back propagation method is related to ray and caustic structures, which may not contain single-ray areas. It is shown that strong refraction reduces the uncertainties of refraction angle and impact parameter. On the other hand, strong refraction or superrefraction is responsible for complicated caustic structures that cannot be resolved by the back propagation technique. The two methods are thus complementary to each other and can be combined for processing lower tropospheric occultation data. This analysis is corroborated by numerical simulations based on global fields of atmospheric variables from analyses of the European Centre for Medium Range Weather Forecast.

Three‐dimensional satellite refractive tomography of the atmosphere: Numerical simulation

Refractometric sounding of the Earths atmosphere using a multiorbit system of Global Positioning System (GPS) and low-Earth orbit (LEO) satellites is modeled on the basis of data from the model of global atmospheric circulation (ECHAM3). The errors of the tomographic reconstruction of meteorological fields are investigated because of insufficient measured refractometric data. For a system of 100 LEO and 18 GPS satellites the estimates of errors of geopotential and temperature is obtained, which are 3—5 m and 0.3 K, respectively, in the pressure range 10—200 mbar.

Comparison between refraction angles measured in the Microlab-1 experiment and calculated on the basis of an atmospheric general circulation model

The differences between the refraction angles measured and calculated for the reanalyses of the European Centre for Medium-Range Weather Forecasts were statistically analyzed on the basis of 64 radio occultation events recorded by the Microlab-1 satellite. It is shown that, for minimum ray heights below 20 km, the main contribution to the differences is made by spatial refractive-index fluctuations neglected by the model. The power spectral density of these fluctuations is mainly concentrated within the vertical wave-number range 0.5–10 rad/km. For heights above 30 km, the deviations are mainly determined by ionospheric disturbances and may vary several times during changes of the site and time of observations. This suggests that the results of satellite radio-occultation sounding of the neutral atmosphere can be used as an indirect quantitative estimate of local discrepancies between the actual field of the refractive index and its values calculated on the basis of a hydrodynamic atmospheric general circulation model.

Cosmos-243 as the Starting Point for the Development of Microwave Radiometry Methods of the Earth’s Atmosphere and Surface

On September 23, 1968, the Cosmos-243 satellite was launched into orbit with four radio telescopes directed to the nadir on board. They were designed to measure the microwave radiation of the Earth’s surface and its atmosphere at wavelengths of 0.8, 1.35, 3.4, and 8.5 cm. The onboard infrared radiometer measured radiation in the band of 10–11 µm in the same solid angle as the radio telescopes. This experiment, which was initiated by scientists from the Institute of Radioengineering and Electronics (IRE) and Institute of Atmospheric Physics (IAP) and, in particular, academicians V.A. Kotel’nikov and A.M. Obukhov, broke new ground in the remote sensing of the Earth from space, which is being actively developed.

Statistically average atmospheric bending angle model based on COSMIC experimental data

The retrieval of profiles of meteorological variables from radio occultation observations requires knowledge of bending angle profiles up to heights of no less than 60–70 km. Because of the residual error of the ionospheric correction, retrieved profiles become too noisy by a height of about 40 km. In order to invert the bending angle profiles, the statistical optimization is used. This makes it possible to construct an optimal linear combination of the a priori estimate of the average bending angle profile and a posteriori noisy estimate based on observations. The estimate of the average bending angle profile for the given coordinates and the time of year is usually based on the climatological atmospheric model. MSIS and CIRA models that have been used for this purpose are now obsolete and do not describe the global changes in the atmospheric state. The model of average bending angles BA–IAP (Bending Angle–Institute of Atmospheric Physics) is built based on the processing of the array of COSMIC radio occultation observations during 2006–2013. The proposed model is statistically validated based on the COSMIC database. It is shown that our model describes the average bending angle profiles more accurately than the MSIS model.

Asymptotic methods of calculating the propagation of centimeter radio waves in the atmosphere in space-space paths

Asymptotic methods of calculating the propagation of centimeter radio waves in a neutral atmosphere in space-space paths are considered. The methods are based on the technique of Fourier integral operators. The approximations that allow the representation of the corresponding operators as compositions of nonlinear coordinate changes, multiplications by reference signals, and Fourier transformations are constructed. The approximations are based on a technical procedure using a linearized canonical transform. This approach makes it possible to devise fast numerical algorithms. Numerical simulations are conducted with the use of realistic global gridded fields of meteorological parameters including the turbulence. The numerical simulations show high accuracy and efficiency of the proposed methods.