The possibilities of using ground-based microwave radiometry data to estimate the tropospheric phase delay of signals under cloudy atmosphere conditions are discussed. A comparative analysis of some approaches for calculating the phase delay in a colloidally stable cloud, including for signals with a frequency from 10 to more than 300 GHz, is carried out. The features of utilizing a single and double Debye models for complex dielectric permittivity while cloud induced phase delay estimation are considered. The retrievals of the phase delay in water vapor and cloud layer from measurements of atmospheric downwelling self-radiation performed by microwave radiometer-spectrometer P22M are presented.

The article explores systematic errors in atmospheric moisture content parameters microwave radiometric retrieval. The errors under consideration arise due to the nonlinearity of brightness temperature (BT) level on liquid water and effective cloud temperature dependencies, and due to neglecting the spatial distribution of cumulus clouds in the satellite microwave radiometer antenna field-of-view (FOV). It is shown that when liquid-drop moisture is assumed to be uniformly distributed over the FOV area, a systematic underestimation of FOV-averaged liquid water content (LWC) and an overestimation of total water vapor (TWV) are observed as the result of retrieval. An attempt is made to relate specific statistical parameters, which describe spatial distributions of inhomogeneous cloudiness observed in an atmospheric cell, with TWV and LWC errors arising from this assumption.

The paper deals with studying the atmospheric characteristics based on spectral microwave radiometric measurements in the 18.0-27.2 GHz band close to the water vapor absorption line. The results of analyzing the ground-based measurements of clear-sky and cloudy atmosphere radiation with the P22M microwave radiometer-spectrometer performed at Fryazino Branch of Kotelnikov Institute of Radioengineering and Electronics of Russian Academy of Sciences are presented. The measurements have been carried out almost continuously since July 2017. The seasonal variability of clear-sky self-radiation spectra in the 18.0-27.2 GHz frequency band is analyzed. Estimates are made for the error in the linear approximation of the dependence between clear-sky brightness temperature and atmospheric integrated water vapor retrieved from radiosonde data. A possibility of studying the variability of liquid water content in clouds of various types with a spatial resolution about 200 m and a temporal resolution of 10 s is demonstrated. The time series of the brightness temperature spectra may be a basis for an operational analysis of the temporal variability of atmospheric integrated water vapor and cloud liquid water content.

The paper presents methods for describing distortions of signals during their propagation in transionospheric transmission lines, which cause inter-symbol and inter-channel interferences. A technique for estimating the statistical characteristics of the interference noise has been developed and their estimation for digital phase-shift keyed signals is carried out by simulating transionospheric lines. The probabilistic characteristics and asymptotic probabilities of erroneous reception of the considered signals with an increase in their frequency band are estimated using the calculated statistical characteristics for the lines of the P range.

A model of a radio line with fog, which is used to represent distortions of complex envelopes of digital signals because of the absorbing and dispersive properties of the propagation medium, has been considered. It is shown that with an increase in the frequency band of digital signals and with an increase in their order of manipulation (when using frequency-efficient signals), the influence of these distortions leads to energy losses in relation to propagation in free space. The quantitative estimation of energy losses for the digital signals used in applications with multi-position phase, quadrature-amplitude, and amplitude-phase manipulations and for a radio line with fog with a variation of its parameters has been carried out. It has been shown that for signals with multiposition phase and amplitude-phase manipulations (the frequency efficiency coefficient of the signals is 4 bits/s/Hz), the energy losses reach 3.5 and 1.0 dB, respectively.

Monitoring the atmosphere characteristics is an important task in remote sensing of the environment. In this paper, the question is raised about taking into account the influence of clouds on the characteristics of thermal radiation. The aim is to analyze the applicability of models for cloud correction during microwave radio sounding of thermal radiation. The result of the work is to determine the parameters that best correspond to the cloud fields of the Earth's atmosphere. For the applicability of the calculated values to account for the structure of clouds in the atmosphere, it is necessary to generate model cloud fields comparable in size to real cloud fields in the Earth's troposphere. This results in significant computational costs required for their modeling and analysis. Numerical modeling of the statistical properties of thermal radio emission of separated cloudiness field models implies the need to perform calculations with an ensemble of simulated random fields of a sufficiently large size, and the use of projections with good resolution, therefore, the optimal use for computing high-performance computers. As a result, the developed algorithms and computer models of cloud fields significantly form a practical basis for further studies of radiation processes in the cloud atmosphere. Two-dimensional (2D) distributions of the brightness temperature of the outgoing thermal radiation of the separated cloudiness fields were calculated. For simplified statistical models of separated cloudiness fields, numerical estimates of the radio brightness temperatures of microwave radiation of the cloud layer are obtained depending on the model statistical parameters.

This work is devoted to the study of the influence of the Earth’s ionosphere on the polarization characteristics of radio waves in the megahertz range. The relevance of the work is determined by the need to improve methods for calculating radiation passing through ionospheric plasma layers and reflected from them, in order to solve modern problems of radio communication, radar and radio navigation, as well as problems of remote sensing of the Earth from space. The megahertz range is interesting in that it allows subsurface sounding of the earth’s covers. In this paper, we consider the frequency range that is borderline for the applicability of the concept of “Faraday rotation”, that is, we consider the frequencies for which the idea of the propagation of radio waves with ordinary and extraordinary polarization along the same trajectory is very conditional. The analysis of the influence of the Earth’s ionosphere on the parameters of high-frequency radio signals is carried out depending on the spatial model of the ionospheric plasma, geographical coordinates, orientation of the magnetic field, and ionospheric irregularities. These characteristics include rotation of the polarization vector (angle of Faraday rotation), phase shift (phase deviation), deviation of the aiming angle, deviation of the radio signal trajectory from a straight line, and others. To calculate the above characteristics, a bicharacteristic system of equations was used in the work. In the process of numerical simulation, it was assumed that the radiation source is located on a moving spacecraft at a distance of several hundred kilometers from the Earth’s surface, the angle of inclination of the rays varies relative to the positive direction of the horizontal axis, and the receiver is located on the Earth’s surface. Other measurement schemes are considered, in which the ionosphere is located on the signal path between the transmitter and the receiver. Previously, the authors performed a numerical simulation of the influence of the rotation of the polarization vector in the ionospheric plasma on radio waves of a higher, decimeter range. The relevance of these studies was associated with the creation of a space P-band synthetic aperture radar (SAR) for ground and subsurface remote sensing of the Earth, as well as with the problems of reconstructing the electron density profile of the ionospheric plasma by radio tomography methods. A comparison is made of the results of modeling the polarization characteristics of the decameter and decimeter ranges.

One can count on significant increase in efficiency of algorithms used for solving the inverse problem of atmospheric total water vapor (TWV) and liquid water content (LWC) microwave radiometric retrieval through introducing into the related computational models such information as cloud top height, ground-level and cloud top temperatures, binary cloud mask or also cloud phase distributions obtained in the radiometer antenna’s field-of-view. This supplementary information, in turn, may be separately pre-reconstructed by means of data on infrared (IR) observations carried out simultaneously and in accordance with microwave ones. But it also makes sense to consider the possibilities and advantages of developing unified (single) algorithms, which involve synchronous IR and microwave data processing (using all available spectrum at once) for solving various inverse problems. We suppose such algorithms could be implemented on the basis of deep convolutional neural networks (CNNs) using data assimilation approach. Herewith, to demonstrate capacities of CNNs in combining estimates made by analyzing data of various wavelength ranges, we consider the problem of cloud phase spatial distribution IR-reconstruction in more detail, because of both near and far IR subranges are sensitive to cloud phase, but two distinct and independent reconstruction algorithms are usually utilized for them.

We consider the results of an experiment on measuring the brightness temperature of the atmospheric downwelling radiation near the resonant line of water-vapor absorption at the frequency 22.235 GHz. The brightness-temperature spectra are obtained using a special multichannel microwave radiometer spectrometer with an improved fluctuation sensitivity and high temperature stability of the antenna gain, which allowed one to perform long-term sessions of continuous measurements. The frequency spectra of intensity of the temporal fluctuations of the brightness temperature of the downwelling radiation are computed for a wide range of temporal intervals under the conditions of clear sky and cumulus clouds of various vertical development in summer. In addition to radiometry, video recording of the current weather conditions is carried out as well as recording of the near-surface meteorological parameters. Taking these parameters into account, the values of total water vapor mass and integral liquid water content in clouds are retrieved from the brightness-temperature spectra. The temporal fluctuations of the retrieved liquid water content are considered for the summer-time cumulus clouds.

Numerical modeling of the effect of the polarization vector rotation in the ionospheric plasma on the radio waves of the high frequency range is performed. The relevance of the work is associated with the creation of space‐based P‐band synthetic aperture radars (SARs) for the Biomass mission and the MKS‐RSA (P) experiment on board ISS station. These instruments are intended for surface and subsurface sounding of the Earth and their readings are strongly influenced by the ionosphere. Another problem of EM propagation is the reconstruction of the profile e‐ionospheric plasma density by radio tomography methods. The source of radiation is located on the moving spacecraft at a distance of 400 km from the Earth's surface. The angle of rays inclination varies from 150 to 30° relative to the horizontal axis. The receiver is located on the surface of the Earth. Three spherical two‐layer models of the ionosphere are compared. To determine the ray paths, a bi‐characteristic system of equations is used. The dependence of the aiming angle deviation on group time is investigated. The phase variation due to the influence of the ionosphere and the angle of the Faraday rotation are studied. It is shown that the changes in the Faraday rotation angle for the selected parameters lie in the range from 6° to 54°, which will have a strong impact on the operation of the P‐band SAR and requires a sufficiently accurate quantitative account of the variability of the angle when probing vegetation and soil covers.

The influence of the equatorial anomaly of the ionosphere on the propagation of radio waves in the P-band (430 MHz), planned for remote sensing of the Earth's surface from space, has been investigated. Calculations at three different points of reception on the Earth's surface are compared. The Faraday rotation of the plane of polarization, phase deviation and polarization coefficients are considered. The values of the angle of Faraday rotation and phase deviation are estimated. It is shown that during propagation, the circular polarization of radio waves at such frequencies practically does not turn into elliptical, except for the insignificant region.

An important problem that arises when planning experiments on remote sensing from space in the P-band is taking into account the influence of the Earth’s ionosphere. We investigated the influence of ionospheric inhomogeneities on the results of remote sensing of the Earth from space, taking into account the curvature of the propagation medium. One- and two-layer models of the ionosphere, both with and without large-scale inhomogeneities of the cold ionospheric plasma, were considered. To obtain numerical results, a bicharacteristic system was used, which makes it possible to adequately take into account the complex structures of ionospheric plasma layers. The dependence of the rate of phase change on the height and the dependence of the total electron concentration on the horizontal distance and group time were investigated. The case was compared when the vector of the strength of the Earth’s magnetic field is perpendicular to the plane of propagation, and the case when this vector lies in the plane of propagation. The dependence of the difference between the refractive indices on the height along the rays was studied. Estimates of the Faraday rotation angle and phase deviation were obtained for various models. The magnitude of the angle of Faraday rotation depends significantly on the orientation of the trajectory relative to the Earth’s magnetic field. Polarization coefficients are investigated. It is shown that the o- and x-waves are separately circularly polarized, and the contribution of the longitudinal component of the electric field in the electromagnetic wave is insignificant.

Results of atmospheric downwelling radiation brightness temperature spectral measurements in the band of water vapor resonant absorption near 22.235 GHz line are considered. The experiment has been going on for over four years. Ground-based observations have been carrying out by means of special multichannel microwave radiometer-spectrometer with improved fluctuation sensitivity and high temperature stability of gain coefficient. Two meteostations located nearby have been registering near-surface meteorological parameters change in addition to radiometric data. All the data obtained have been properly calibrated and divided by seasons. A part of data have been categorized by cloud species observed by video recordings. Atmospheric moisture content parameters (total water vapor mass and cloud liquid water content) have been retrieved using multi-frequency radiometric method. Kolmogorov’s structural functions for brightness temperatures at different frequencies and structural functions of moisture content parameters have been evaluated for a wide range of temporal intervals. The square root of structural function for a parameter is taken as a native measure of the parameter’s fluctuation intensity in this work.

Large data bank of images has been accumulated during atmospheric cloudinessground-based observations from 2017 to 2020 near Fryazino city, Moscow Region. The problemof obtained images classification into several types is considered. The types are clear sky (noclouds); cumulus cloudiness of various extent (humilis, mediocris, congestus); very powerfulclouds such as cumulonimbus or nimbostratus; stratus cloud cover; stratocumulus clouds; lightand high-positioned clouds (altocumulus, cirrus, cirrostratus and cirrocumulus). A qualitativeanalysis of the key features of gray, blue, green and red level co-occurrence matrices for variouspixel distances and directions (angles) is performed to solve the problem. An image classification algorithm based on these and other key features retrieved during image preprocessingis developed. The quality evaluation is performed. The developed software tool is currentlysuccessfully used for atmospheric radiometry problems support.

We consider the Planck model for generating random discontinuous cloud fields in a three-dimensional computation domain. An algorithm for calculating a two-dimensional pattern of the brightness temperature of the upwelling microwave radiation of the smooth water surface–cloudy atmosphere system taking into account the altitude profiles of the main meteorological parameters and an arbitrary distribution of the water content is developed. The inverse problem of retrieving the distributions of the integral water-content parameters by the two-frequency radiometric method using the obtained brightness temperatures is discussed. Systematic errors in the estimated integral water content of the clouds associated with the use of a homogeneous plane-layered model of the cloud field, which ignores its actual, i.e., discontinuous and heterogeneous structure, are studied.

A study of the change in the polarization characteristics of high-frequency radio waves, caused by the influence of the Earth's ionosphere in the region of the equatorial anomaly, is carried out on the basis of the Hamilton-Lukin bicharacteristic method. The dependence of the group time of signal arrival on the coordinate of the ray exit is considered. The rate of phase change along the rays from height, phase deviation as a function of group time and horizontal coordinates, as well as differences of refraction indicators of ordinary and unusual wave as the function of height along the rays are investigated. The angle of Faraday rotation in the meridian direction at the receiving point is calculated taking into account the change in the magnitude and orientation of the magnetic field and the influence of the equatorial anomaly of the ionosphere and its surroundings. The calculations are made for three frequencies: 336 MHz, 436 MHz and 536 MHz.

Problem formulating. The nature of fluctuations in the microwave radiation of the atmosphere remains poorly understood. A number of questions, related to the nature of radiation fluctuations near the lines of resonant absorption of various atmospheric components, the intensity of fluctuations at large spatial and temporal intervals requires further study. The influence of various meteorological factors on the intensity of radiation fluctuations requires a deeper consideration. Goal. The goal is to reveal the spectral features of spatio-temporal fluctuations in downward radiation of the atmosphere in the region of the water vapor resonance 18−27 GHz and to estimate the influence of cumulus clouds of various vertical extent on the intensity of fluctuations. Result. Based on the data of long-term measurements of down welling radiation brightness temperature 18−27.2 GHz, the analysis of the spectra of structural function for wide range of temporal intervals is carried out. It is shown that the spectrum of radiation fluctuations in a cloudless atmosphere is similar to the absorption spectrum in water vapor. The spectra of fluctuations in atmospheric radiation under conditions of cumulus cloud cover of various vertical extent are presented. Practical meaning. Radiation fluctuations should be taken into account when creating systems for remote sensing of the atmosphere and the Earth’s surface. They affect the quality of radiometric measurements and radio astronomy observations. The data on fluctuations in atmospheric radiation gathered during this research can be useful for assessing the spatial and temporal fluctuations in the phase delay, for taking into account the influence of atmosphere during the radio interferometric measurements and when observing radiation sources in the diagrammatic modulation mode.

The features of the influence of the ionosphere on the propagation of decimeter-wave radio waves are studied, taking into account the curvature of the propagation medium. Single-layer and two-layer models of the ionosphere are considered, both including and not including large-scale inhomogeneities of cold ionospheric plasma. Estimates of the angle of the Faraday rotation and phase incursion for various models are obtained. The effect of ionospheric inhomogeneities on the results of remote sensing of the Earth from space is investigated.