Обычно при радиолокационном зондировании ледяного покрова основным информативным параметром становится сечение обратного рассеяния, что не всегда позволяет однозначно определить тип рассеивающей поверхности (лёд/вода).В настоящей работе обсуждается возможность использования доплеровского спектра отражённого радиолокационного сигнала для оценки площади ледяного покрова.Впервые построена полуэмпирическая модель доплеровского спектра радиолокационного СВЧ-сигнала, отражённого ледяным покровом, для радиолокатора с широкой диаграммой направленности, установленного на движущемся носителе, при малых углах падения зондирующего излучения (0-19°).Было рассмотрено несколько конфигураций антенной системы и показано, что для измерений необходимо применять широкую или ножевую (по углу падения) антенну.Вычисления подтвердили предположение, что при измерении с движущегося носителя доплеровский спектр выступает надёжным индикатором перехода от одного типа рассеивающей поверхности к другой для случая сплошного ледяного покрова и в качестве критериев

Сложность построения теоретической модели рассеяния электромагнитных волн СВЧ-диапазона обусловлена тем, что морской лёд -многокомпонентная среда с особыми электрофизическими и физико-химическими свойствами.В области малых углов падения задача осложняется ещё тем, что экспериментальных данных крайне мало, что затрудняет верификацию моделей.Вывод на орбиту спутника GPM (Global Precipitation Measurement) с двухчастотным дождевым радиолокатором (Ku-и Ka-диапазоны), выполняющим измерения при малых углах падения (0-18°), открывает возможности для верификации существующих моделей и построения новых.В качестве тестового объекта было выбрано Охотское море, которое попадает в полосу обзора двухчастотного дождевого радиолокатора (Dual-frequency Precipitation Radar).Исследовалась зависимость сечения обратного рассеяния от угла падения для сформировавшегося однолетнего ледяного покрова при отрицательной температуре воздуха (сухой лёд

The research is aimed at the development of original methods of obtaining new information for the study of surface waves in marine conditions. The paper discusses a method for measuring the short-wave part of the wave spectrum (wavelengths in the range from about 50 cm to 2 cm in 6 intervals) in marine conditions. The information obtained will make it possible to study the interaction of wind with the short-wave component of the wave spectrum and will be in demand by scientists engaged in numerical modeling of the global wave climate and interested in refining the model of the interaction of the surface wind with waves. In addition, this will improve the accuracy of retrieving the surface wind speed from remote sensing data. In this paper, a theoretical study of the algorithm for retrieving the slope variance from the shape of the reflected pulse is carried out. The multifrequency underwater acoustic wave gauge will measure the slope variance at the selected surface wave frequency intervals, which will allow us to consider their correlation with the wind speed. A new approach to retrieving the height spectrum of the short-wave part has been developed, using measurements in several frequency intervals.

In October 2018, the Chinese-French satellite CFOSAT was launched into low-earth orbit, which carries the French SWIM spectrometer and the Chinese scatterometer RFSCAT. The SWIM (Ku-band) spectrometer measures the normalized radar cross section (NRCS) at small incidence angles (0 degrees-11 degrees) and for the first time measurements are performed at different azimuthal angles in a size-limited area where sea waves can be considered homogeneous. At small incidence angles, the backscattering mechanism is quasi-specular and the reflected field is calculated in the Kirchhoff approximation. Due to this, it becomes possible to retrieve the parameters of large-scale, in comparison with the length of the electromagnetic wave, sea waves (hereinafter referred to as large-scale waves). It has been shown that the common approach to measuring the mean square slopes (mss) of large-scale waves leads to large errors. For the measurement scheme at three azimuthal angles, an algorithm was developed for determining the direction of propagation of the dominant wave system, the mss of large-scale waves along the sounding direction and the unnormalized correlation coefficient between the slopes of large-scale waves along the.. and.. axes (hereinafter referred to as the correlation coefficient), which requires solving a system of transcendental equations. If we neglect the correlation coefficient in comparison with the mss of large-scale waves, then the solution is in an analytical form. The results of the simplified algorithm are compared with the exact solution. The values of the mss of large-scale waves obtained with the simplified approach can be used both to estimate the intensity of sea waves and as the initial conditions for solving the transcendental equation. The developed algorithms will be used to process the data of the SWIM spectrometer, which will allow for the first time to retrieval a two-dimensional field of large-scale wave slopes.

Проведены измерения доплеровского спектра отражённого радиолокационного сигнала при углах падения меньше 30° в случае стационарного речного течения.Для описания доплеровского спектра помимо традиционно используемых ширины и смещения применялись коэффициенты асимметрии и эксцесса, а также ширина доплеровского спектра, вычисляемая через 4-й и 2-й центральные моменты.Обработка данных показала, что в отличие от морского волнения коэффициент эксцесса максимален при малых углах падения и уменьшается при увеличении угла падения.Были построены зависимости ширины и смещения доплеровского спектра, сечения обратного рассеяния, коэффициентов асимметрии и эксцесса от угла падения и направления зондирования (азимутального угла).Для вычисления доплеровского спектра отражённого сигнала по теоретической модели необходима информация о статистических характеристиках волнения, которые находились по спектру волнения

В условиях изменяющегося климата актуальной задачей становится мониторинг ледяного покрова.Для мониторинга используются сенсоры, работающие в оптическом, инфракрасном и микроволновом (МВ) диапазонах.Современные орбитальные радиометры обеспечивают глобальное покрытие и оперативное получение информации.Из-за влияния атмосферы восстановление параметров поверхности выполняется с погрешностью, поэтому применение радиолокационных систем способно обеспечить повышение точности.В работе рассмотрены результаты зондирования ледяного покрова двухчастотным дождевым радиолокатором DPR (англ.Dual-frequency Precipitation Radar) и многоканальным сканирующим МВ-радиометром GMI (англ.GPM Microwave Imager) со спутника GPM (англ.Global Precipitation Measurement).Показано, что по радиолокационному контрасту «лёд -вода» при малых углах падения надёжно определяется кромка льда.Впервые предложен алгоритм определения сплочённости ледяного покрова по радиолокационным измерениям в Ku-диапазоне (частота 13,6 ГГц) при малых углах падения, который протестирован по измерениям в Охотском море

Проведено исследование проявления в радиолокационном изображении двухчастотного дождевого радиолокатора, установленного на спутнике GPM (Global Precipitation Measurement), процесса формирования и разрушения ледяного покрова на озере Байкал в осенне-зимний период 2015/2016 г.Для обработки были выбраны три временных интервала, связанные с тремя типичными состояниями поверхности озера: 1) открытая вода в ноябре -декабре 2015 г.; 2) сформировавшийся ледяной покров при отрицательной температуре воздуха («сухой» лёд) в феврале -марте 2016 г.; 3) ледяной покров при положительной температуре воздуха («влажный» лёд) в апреле -мае 2016 г.Впервые была детально исследована зависимость сечения обратного рассеяния ледяного покрова от угла падения для малых (0-18°) углов падения в Kuи Ka-диапазонах.Было показано, что при переходе от взволнованной водной поверхности к ледяному покрову при малых углах падения происходит изменение вида функции, описывающей зависимость сечения обратного рассеяния от угла падения

The launch of the Dual-frequency Precipitation Radar (DPR) opens up new opportunities for studying and monitoring the land and inland waters. It is the first time radar with a swath (±65°) covering regions with cold climate where waters are covered with ice and land with snow for prolonged periods of time has been used. It is also the first time that the remote sensing is carried out at small incidence angles (less than 19°) at two frequencies (13.6 and 35.5 GHz). The high spatial resolution (4–5 km) significantly increases the number of objects that can be studied using the new radar. Ilmen Lake is chosen as the first test object for the development of complex programs for processing and analyzing data obtained by the DPR. The problem of diagnostics of ice-cover formation and destruction according to DPR data has been considered. It is shown that the dependence of the radar backscatter cross section on the incidence angle for autumn ice is different from that of spring ice, and can be used for classification. A comparison with scattering on the water surface has shown that, at incidence angles exceeding 10°, it is possible to discern all three types of reflecting surfaces: open water, autumn ice, and spring ice, under the condition of making repeated measurements to avoid possible ambiguity caused by wind.

This paper focuses on the comparison of the measured characteristics of radar backscattering by the water surface with the theoretical model which includes the scattering model and the surface model. In the experiment, the backscattered signal at small incidence angles was studied at the different azimuth angle of antenna rotation. Comparison of the experimental data with the model confirmed the accuracy of the sea wave spectrum model and the electromagnetic scattering model. It is shown that the analysis of the reflected signal is necessary to take into account the parameters of the surface rather than wind speed as is usually done.

This paper presents the results of field measurements of surface waves parameters obtained in the expedition to a fixed offshore platform in the Black Sea near the Katsiveli settlement in 2015. For the first time experimentally confirmed the efficiency of the new method of directly measuring the slope variances and other surface waves parameters affect the scattering of ultrasound and microwaves. The method is based on the analysis of spectral and energy characteristics of the reflected acoustic waves at underwater surface waves observation. A feature of this method is the simultaneous use of multiple antennas with different radiation patterns. The scheme of measurements with a single radiating and three receiving antennas with different radiation patterns implemented in the acoustical wave gauge manufactured in the IAP RAS. Sensing of the water surface occurs vertically, so that the scattering occurs in quasispecular area. This measurement scheme allows retrieving all the parameters of surface waves that effect on scattering cross-section, the width and the shift of Doppler spectrum of the reflected radiation. String wave gauge used in the experiments to control the effectiveness of a new method. Using of string wave gauge is a standard for precision measurements of surface waves.

The new concept proposed during the development of the first Russian orbit-borne scatterometer SCAT-3 requires an additional study for estimating its efficiency and comparison with the current scatterometer concepts. Using the fan antenna pattern (with angular dimensions 1◦ × 6◦), we have reduced the antenna rotation speed by about a factor of three compared with the prototype (the “SeaWinds” scatterometer) and measured the backscattering cross section for each wind cell at the horizontal and vertical polarizations. The numerical model of the scatterometer was developed with allowance for the technical characteristics of the radar, orbital parameters, and observation scheme. The scatterometer operation is simulated with the subsequent swath formation and partitioning into the wind cells. It is shown that using the fan pattern in the scatterometer, one can improve the accuracy of the wind-direction r5etrieval in a wind cell due to employing the radiometric resolution in the processing algorithm. The main error in determining the wind direction is related to the ambiguity ±180◦, which is caused by the type of the azimuthal dependence of the backscattering cross section. With the help of the two-dimensional median filtering, we can significantly reduce the wind-direction retrieval error. This error can probably be smaller than that for the current scatterometers.

We have chosen a “SeaWinds” scatterometer with an orbital altitude of about 800 km as a prototype of the first Russian orbital scatterometer. An involuntary decrease in the orbit altitude to 650 km made us choose between conservation of the initial swath width 1800 km or the incidence angles with the swath-width decrease to 1500 km. A wider swath width has the advantage of a better coverage of the world-ocean surface. However, it leads to an increase in the local incidence angles and, hence, a decrease in the reflected-signal power. As a result, the signalto-noise ratio decreases and an error in the wind velocity and direction reconstruction because of the equipment noise increases. The error of the wind-velocity vector reconstruction for the same drive and antenna is the choice criterion. During the study, the mathematical model of the scatterometer is developed, the numerical simulation for both swath widths is performed, the data are processed, and the reconstruction accuracies of the wind velocity and direction are compared. It is shown that the reconstruction accuracy can significantly be improved if the measurement for two polarizations is used. The results obtained also show that the wind velocity is sufficiently well reconstructed for both swaths, while the wind-direction reconstruction accuracy in the case of a wider swath is worse than that required by the technical specifications for the scatterometer. Therefore, the swath width of the new scatterometer should be 1500 km.

The capabilities of a new instrument for measuring the surface wave parameters are studied in detail. An acoustic wave gauge with a single antenna is designed to measure the backscatter intensity and the Doppler spectrum of the reflected acoustic signal and to retrieve the variance of the vertical orbital velocity component. The new instrument is numerically simulated, and field experiments using a prototype acoustic wave gauge are carried out. The comparison of the variances of the vertical orbital velocity component measured by acoustic and string wave gauges under field conditions, as well as the comparison of measurements and numerical simulation results, has confirmed the high accuracy of the retrieval algorithms. The advantage of the instrument is its capability of making measurements in any water basin without the use of a fixed platform.

A model for determining the reflected-pulse shape during vertical sounding of the sea surface by an underwater ultrasonic sonar with a wide antenna pattern is developed. The reflected-pulse shape dependence on the parameters of an underwater acoustic altimeter and the measurement schemes is studied. The wave-height retrieval algorithm whose input parameter is the reflectedpulse leading-edge duration is developed. The algorithm efficiency is confirmed by numerical simulation. The obtained formula for the wave height involves the coefficients which depend on many characteristics among which the submersion depth, the beamwidth, and the soundingpulse duration are the main parameters. Therefore, the formula should be specified according to variations in the measurement scheme and the acoustic-system parameters.

Orbital scatterometry is briefly overviewed and its trends are indicated. Two scatterometer concepts are currently considered for trade-offs: with fixed and rotating antenna systems. The concept with a rotating antenna system was selected and SeaWinds was chosen as the prototype for the first Russian scatterometer. The scatterometer concept was then further developed and instead of two pencil beams, a fan-beam antenna was proposed about 1° × 6°. The fan-beam antenna allows successive measurements for horizontal and vertical polarization in each wind vector cell WVC. This increases the number of observations of the WVC at different incidence and azimuth angles during flight. The scatterometer parameters required to implement the proposed measurement geometry for an orbit altitude of 650 km and a swath width of 1525 km are discussed. A numerical scatterometer model that accounts for both the specifications and the observation geometry is developed. The scatterometer performance, with subsequent formation of a swath and splitting into WVCs, is simulated. The procedure of wind vector retrieval includes two stages: 1 determining wind speed and wind direction in a single WVC; and 2 using the information from adjacent WVCs to correct wind direction. It is shown that the accuracy of wind direction retrieval by a WVC can be increased by simultaneous radar cross-section RCS measurements at vertical and horizontal polarization. The basic error in determining wind direction is due to a 180° wind direction ambiguity caused by the form of RCS azimuth dependence. Two-dimensional median filtering is commonly employed in scatterometry to increase the accuracy of wind direction retrieval. In this study, two-dimensional angular median filtering was employed and it is shown that the error in wind direction retrieval significantly decreased. The results of the research indicate that wind field can be retrieved by the new scatterometer with the level of precision required.

Classification of the types of surface waves with respect to the problem of radar sensing of the sea surface is discussed. A combined array of radar and buoy data was used to determine the type of sea waves using the oceanographic age of ocean waves. Four types of sea states are defined, and it is shown that the strongest correlation between the backscattered radar cross section (RCS) and sea waves is observed in the case of dominant wind waves. In the development of the radar description of sea waves, the dependence of the backscattered RCS on the variance of large-scale wave slopes (retrieved from the precipitation radar data) has been studied. It is shown that the sea state classification with allowance for the wave parameters that affect the process of radar-signal scattering allows one to more precisely calculate the backscattered RCS. For a complete radar description of sea waves, it is also necessary to measure the variance of the vertical component of the orbital velocity and determine the coefficient of correlation of the surface wave slopes and the vertical component of the orbital velocity.