The altimeter is an effective instrument for remote sensing of the sea surface. Existing theoretical models of the waveform of the reflected impulse are used in the retracking algorithms for determination the distance from radar to a scattering surface and measuring the significant wave height (SWH). However, these models cannot be used for an altimeter with a wide antenna beam. In the latter case the Karaev model may be used and in this case the waveform of the reflected impulse depends on two parameters: the SWH and the mean square slopes (mss) of the large-scale waves. To verify the model of the waveform of the reflected impulse, two approaches are used: a numerical experiment and an experiment on a cable car. For the measurement, a microwave radar capable of operating in altimeter mode was used. The numerical experiment simulated the measurement scheme and used a model sea wave spectrum to generate the reflecting sea surface. Comparison of the waveforms obtained during the numerical experiment and calculated using the Karaev model showed good agreement. To form the reflected impulse in the experiment, a special procedure was suggested. The retracking algorithm based on the Karaev model was developed. Processing showed that all three parameters can be retrieved from such a waveform: the distance from the radar to the reflecting surface, the SWH, and themssof large-scale waves.

Altimeter can measure the height of sea waves. Such measurements are carried out at zero incidence angle. The Brown model was developed to describe the waveform of the reflected impulse. There is no theoretical model for the case of oblique probing. In the Kirchhoff approximation the theoretical task about waveform of the reflected impulse at oblique probing was considered. In the result of the investigation the analytical formula for the waveform of the reflected impulse for oblique probing at the small incidence angles (< 12°) for a microwave radar with a narrow antenna beam was obtained. The dependence of the waveform of the reflected impulse on the width of the antenna beam, the significant wave height (SWH), the incidence angle and the distance from the radar to the sea surface was investigated. It is shown that the waveform of the reflected impulse depends on the SWH.

Currently, there is a lack of theoretical models that describe the characteristics of the signal reflected by the ice cover, which covers a significant part of the inland waters in the North Region. This paper represents a new method for obtaining the dependence of the backscattering radar cross section for ice cover on the incidence angle based on measurements of the Doppler spectrum of the reflected microwave signal. To analyze the performance of the new method, we use data from an experiment conducted with an X-band radar on a cable car across the Volga River in Nizhny Novgorod on 17/02/22 with freshwater ice cover and compare the obtained dependences with the results of GPM measurements over Lake Baikal.

The composite method for sea ice detection using the data at low incidence angles is suggested. At incidence angles above three degrees simple clustering based on the value of reflected signal is applied. At lower incidence angles performance of this method is low. To overcome this issue, alternative approach based on surface slopes statistics is applied. Probability density function of surface slopes is obtained and kurtosis coefficient is calculated. For incidence angles below three degrees, classification of surface type is performed based on the value of kurtosis coefficient is applied. The composite method for sea ice detection based on clustering and slope statistics was validated using AMSR-2 radiometer data and performed good accuracy.

This work is aimed at constructing a semi-empirical model of the Doppler spectrum of a microwave signal bistatically reflected by an ice cover. The model operates in the quasi-specular region of reflection. The model allows changing the grazing angles of the axes of the transmitting and receiving antenna patterns, the velocity of movement and the height of the location above the reflective surface of the receiver and transmitter. You can also set the transmitting and receiving antenna patterns, and these patterns can be asymmetrical. The paper compares the Doppler spectra reflected by ice cover and sea waves. The study uses experimental dependencies of the radiation reflectance of ice cover and open water on the angle of incidence. They were obtained using a dual-frequency precipitation radar installed on the GPM mission satellite over the Okhotsk Sea. As a result of the work, it was proposed to determine the type of underlying surface by the kurtosis coefficient of the Doppler spectrum of reflected radiation.

At small incidence angles the dominant microwave backscattering mechanism is quasi-specular. In this case, reflection occurs from facets of the wave profile oriented perpendicular to the incident electromagnetic waves. A new theoretical algorithm was developed for determining the mean square slopes (mss) of large-scale, in comparison with radar wavelength, sea waves from the azimuthal dependence of the backscattering radar cross section (RCS). In this paper the results of an experiment on measuring the azimuthal dependence of the backscattering RCS during the experiment on the Oka River within the city of Nizhny Novgorod is discussed. For measurements, two Ka-band radars installed on the metro bridge were used. To determine the mss of large-scale waves, the azimuthal dependence of the RCS, measured at one incidence angle was used. The comparison of the mss retrieved by the new algorithm, with the results of processing by the well-known algorithm using the dependence of the RCS on the incidence angle, confirmed the effectiveness of the new approach.

Microwave radars are used to classify the type of reflective surface, and usually the power or spectral characteristics of the reflected signal are analyzed as an information parameter. In this study, to classify the type of scattering surface (ice/water), it is proposed to use the waveform of the reflected pulse. It is expected that a significant difference in the statistical characteristics of the ice cover and the rough water surface will manifest itself in the waveform of a reflected pulse. To check this assumption, an experiment was performed on the Nizhny Novgorod cable car, which crosses the Volga river. An altimeter with a knife-like antenna beam (3.6° x 30°) was mounted on a technological trolley and performed sensing vertically downwards while moving over ice and open water. A comparison of the reflected pulses showed that for the ice cover the trailing edge of the reflected pulse is shorter than for the open water with waves. This is due to the difference in the mean square slopes (mss) of the ice cover and the water surface and, thus, the waveform of the reflected pulse can be used to determine the type of the scattering surface.

The paper presents preliminary results of sea ice detection method using the data of Ku-band radar at low incidence angles in the range 2◦— 10◦. The data on sea ice concentration from AMSR-2 measurements are used for validation. The simple classification approach to detect sea ice, based on Otsu method was applied. This unsupervised thresholding method demonstrates good accuracy for all incidence angles and can be used as an alternative method for ice/water discrimination.

In the radar remote sensing of sea ice, the main informative parameter is the backscattering radar cross section (RCS), which does not always make it possible to unambiguously determine the kind of scattering surface (ice/sea waves) and therefore leads to errors in estimating the area of the ice cover. This paper continues a line of articles that explore the possibility of using the Doppler spectrum (DS) of the reflected signal at near-nadir probing for ocean remote sensing. Experimental DS of the microwave signal reflected from water surface and ice cover are compared. The experiment took place on the Nizhny Novgorod cable car. Pulsed X-band radar was installed on a technological trolley and performed measurements in two modes that worked sequentially while moving. In the Doppler mode the DS of the reflected signal was measured. It contains information about the statistical parameters of the surface. Data processing was carried out and the first results confirmed the assumption that the DS can be an effective tool for classifying the type of the underlying surface according to the «ice/water» criterion. Comparison with model of DS for sea ice has shown that the general behavior of the DS for ice and waves in experiment is in agreement with theory. A development of the theoretical description of the DS for freshwater ice and comparison with experimental data is planned for the future research.

To solve the problem of measuring the significant wave height in the bistatic formulation of the problem, it is proposed to use an approach based on measuring the cross-correlation function of two reflected signals close in frequency. This approach is well known and has been successfully tested in experiments on aircraft carriers. The advantage of this approach over the traditional one based on the analysis of the shape of the reflected pulse is a simpler theoretical description, within which, in the bistatic formulation of the problem, an explicit relationship is obtained between the significant wave height and other wave parameters from the measured parameter, the modulus of the cross-correlation coefficient. The formula for the shape of the pulse reflected in the bistatic formulation of the problem has not been obtained to date. The proposed approach has difficulties when implemented on a satellite carrier in a monostatic formulation of the problem, since it requires an ultra-narrow antenna pattern. In this paper, we consider a variant of the implementation of a satellite bistatic formulation of the problem for measuring the significant wave height based on the approach of measuring the cross-correlation function of two reflected signals spaced in frequency.

To solve the problems of measuring wave parameters on the water surface, it is proposed to use an underwater acoustic wave gauge. Underwater observation of surface waves has several advantages over sea buoys and string wave gauge. Underwater instruments can operate anywhere in the World Ocean and do not require mounting on the rigid structure of offshore platforms. Underwater methods are not sensitive to any atmospheric conditions, for example, they can work in hurricane winds and even in freezing waters. There are various devices for underwater measurement of wave parameters. However, they are united by the desire to determine the wave spectrum, which requires making the antenna beam as narrow as possible, and the measurement results will depend on the installation depth. Because of this, it is necessary to limit the region of the measured spectrum only to low-frequency (long) waves, usually the upper limit is about 0.35 Hz. In this work, experimental testing of a new approach for determining the integral parameters of waves by the shape of the reflected acoustic pulse averaged over space and time, is carried out. A feature of this approach is the insensitivity to the instrument installation depth (deeper installation will even provide shorter time averaging). Another important advantage of this approach is the ability to measure the same surface parameters that affect the characteristics of microwave backscattering with the same wavelength.

Orbital radars are widely used to investigate ice cover, but measurements are usually made at the middle incidence angles (20°-60°). In this study the problem of ice cover monitoring at the small incidence angles (0°- 18°) was considered. Two operating radars (DPR - Dual-frequency precipitation radar and SWIM - Surface Waves Investigation and Monitoring) and one promising radar (SKIM - Sea surface KInematics Multiscale monitoring) was analyzed. Numerical simulations of all radars were carried out and the assumption was confirmed that all radars can determine the kind of scattering surface (ice/open water) at the incidence angles less than 12°. The disadvantage of the DPR measurement scheme is that at incidence angles of 1 °_3° the problem may not have a solution. It deals with fact that ice and open water backscatter normalized radar cross sections (NRCS) will be the same. Due to the rotation of the antenna system around the vertical axis, this problem is solved in the SWIM and SKIM. However, the spatial resolution of SWIM is several times worse than that of DPR. Additional information is required to assess sea ice concentration (SIC).

At the radar probing of the sea surface, the main informative parameter is the backscattering normalized radar cross section (NRCS), which does not always make it possible to unambiguously determine the kind of scattering surface (ice/water). In this paper, we discuss the possibility of using the Doppler spectrum (DS) of the reflected radar signal to solve this problem. For the first time, a semi-empirical model of the DS of a microwave radar signal reflected by an ice cover was developed for a radar with a wide antenna beam mounted on a moving carrier. Radar is measuring at the small incidence angles (0° – 19°). Several configurations of the antenna system were considered and it was shown that it is necessary to use a wide or knife-like (in incidence angle) antenna beam for measurements. Calculations confirmed the assumption that, when measured from a moving carrier, the DS is a reliable indicator of the transition from one kind of scattering surface to another. The shift and width of the DS, as well as the skewness and kurtosis coefficients for the shape of the DS can be used as criteria in a selection algorithm.

There are a number of methods of currents measurements, including both in situ and remote sensing methods. HF Doppler radars are widely used to monitor currents from the shore. They are operating in the middle incidence angles, when the scattering is assumed to be resonant. The influence of currents on the Doppler spectrum in case of sounding at small incidence angles is not so well studied. In this paper the result of data processing of the experiments with Doppler radars installed at the opposite sides of the bridge over river are presented. The measurements were carried out at small incidence angles, when the backscattering mechanism is quasi-specular. The dependences of the main parameters of the Doppler spectrum on the azimuth angle were constructed for three schemes of measurements. It is shown that the use of the kurtosis coefficient and the coefficients of skewness to describe the Doppler spectrum makes it possible to determine the direction of wave propagation relative to the direction of the current. The goal of this paper is to provide experimental data to remote sensing scientists, to discuss the results obtained and future experiments.

Currently, the task of measuring and monitoring the global field of sea currents is becoming relevant and attracts the attention of scientists around the world. However, in order to correctly account for the influence of the current on the signal reflected from the water surface, it is necessary to better understand what the wave looks like on the current. In this paper the results of experimental measurement of the wave spectrum for different wind speeds are presented. A string wave gauge was installed on the Oka River in Nizhny Novgorod. The device was installed at a distance of 70 meters from the shore, measurements were carried out synchronously with measurements of the wind profile using anemometers, as well as with measurements of the Doppler spectrum of the reflected signal from the metro bridge located downstream. The goal of this paper is to provide experimental data to remote sensing scientists, to discuss the results obtained and future experiments.

Presented study describes the method for numerical estimation of upwelling process caused by passing of tropical cyclones and typhoons in North-Western Pacific. Authors use the synchronous dataset of the active (satellite altimetry) and passive (biooptical parameters of water column) remote sensing data in order to explore variability of seawater parameters and get certain statistical estimates from global-scale data analysis.

Reliable and up-to-date information on the parameters of sea waves is in demand in solving a wide range of fundamental and applied problems, in particular, those related to meteorology and oceanology. Expanding the possibilities of remote sensing can be achieved using dual-frequency radars. During the study, an analytical formula was obtained for the cross-correlation function of two radar signals reflected in the quasi-specular region, which are close in frequency. The problem is considered in a general form for the bistatic sensing using the Kirchhoff approximation. The solution considers the radiation patterns of the emitting and receiving antennas, which can be set as asymmetric. The reflecting water surface is described by 4 statistical parameters: height variance, slope variance in two perpendicular planes, and correlation coefficient of slopes in two perpendicular planes. Also, an expression is given for the case of emission and reception of signals strictly along the normal to the undisturbed water surface, which is like the measurement scheme of a satellite radar altimeter. In the experiment, it is proposed to measure the modulus of the correlation coefficient, the expression for which, in a particular case, was obtained in the work. The resulting expressions can be used to create new remote sensing schemes for retrieving the variances of wave heights and slopes, as well as the correlation coefficient of slopes in two perpendicular planes.

The total mean square slope (mss) of large-scale (in comparison with the scale of radar wavelengths) ocean waves can describe the roughness of the ocean surface. This important parameter has found many applications and can be determined by the dependence of large-scale mss on azimuthal angles. The Surface Waves Investigation and Monitoring instrument onboard the China France Oceanography Satellite (CFOSAT) can provide a two-dimensional (2D) normalized backscattering radar cross section (NRCS) for 2D global ocean-surface wave detection. In this paper, the 2D NRCS is used to calculate the total large-scale mss via two methods. The first method is constructed from regression, while the second scheme is derived from a three-solution method. Finally, the total large-scale mss are calculated using these methods, and the map of the distribution of the global total mss is presented for the first time. Our results show that the two sources of total large-scale mss obtained are consistent with each other. CFOSAT can provide a 2D total large-scale mss map and be used as a new data resource for global applications.

Sea ice is a complex structure and theoretical models have been developed to describe the scattering of electromagnetic waves. The area of small incidence angles remains the least studied both theoretically and experimentally. With the launch of a Dual-frequency Precipitation Radar (DPR) into orbit as part of the GPM mission, it became possible for the first time to analyze the dependence of the backscattered radar cross section (RCS) on the incidence angle for small angles (<20 degrees). The Okhotsk Sea was chosen as a test site. For three well-known scattering models, the performed comparison showed that, at the small incidence angles, the model constructed within the framework of the perturbation method best describes the dependence of the RCS on the incidence angle. However, synchronous measurements in the Ku- and Ka-bands showed that there is an unexplained change in the RCS in the framework of this theoretical model when passing from one frequency range to another. Research will continue to explain this effect and to improve the scattering model.

Methods for measuring of sea currents are currently being actively developed. Significant success has been achieved at the middle incidence angles, when the scattering mechanism is presumed to be resonant. This paper presents results from experiments with Doppler radar installed on the bridge over a river. The measurements were carried out at small incidence angles, when the backscattering mechanism is quasi-specular. The dependences of the main parameters of the Doppler spectrum on the azimuth angle were constructed for three scheme of measurements. It is shown that the use of the kurtosis coefficient and the Doppler spectrum width ΔF42to describe the Doppler spectrum make it possible to determine the direction of wave propagation relative to the direction of the current. The paper tabulates detailed experimental data for interested scientists, discusses the results, and outlines future plans for experiments.

Synoptic scale and mesoscale cyclones and atmospheric rivers (AR) are key elements of atmospheric circulation over the polar oceans. They determine the weather conditions in the Southern Ocean, impact strongly on evolution of the sea ice surrounding Antarctica and on the variations of Antarctica ice balance. The increased values of total water vapor content$V$, total cloud liquid water content$Q$, near surface wind speed$W$and precipitation are observed in cyclones and AR areas. In our research, these phenomena were considered using the brightness temperatures$T_{\mathrm{B}}(v)$at frequencies$v$in the range of 6–190 GHz acquired by AMSR2, GMI and MTVZA-GY microwave radiometers in 2019–2020. The fields of$V,\ Q,\ W$, and the mean sea surface slopes were retrieved from the measured$T_{\mathrm{B}}(v)$and GPM DPR by application of the known and original algorithms. The ancillary data consisted of scatterometer-derived wind fields, visible and infrared satellite images, radiosonde data and surface analysis charts.

The algorithm to retrieve wind speed in a wide swath from normalized radar cross section (NRCS) is developed for the data of Ku-band radar operating in scanning mode installed onboard the Global Precipitation Measurement (GPM) satellite. Wind speed is retrieved within a wide swath and with high resolution. The algorithm is based on wind speed retrieval for satellite altimetry. The sea buoys data are used to obtain the dependence between NRCS and the wind speed.

Numerical simulation of sea waves in Persian Gulf is performed using WAVEWATCH III model for the two different source terms. Comparison of mean square slope obtained from the Ka-band radar data and mean square slope from the model spectrum is performed. The comparison is complicated by the following details. The part of the sea wave spectrum with wavelength of decimeter band and shorter significantly contributes to the mean square slope from Ka-band radar data, while the shortest wavelength in the spectrum from WAVEWATCH III model is of the order of meter. The method to calculate mean square slope from model data is presented. It implies integrating of the composite spectrum, which long-wave part is obtained by WAVEWATCH III model, while the descending part is from the sea wave spectrum model.

The acoustic wave gauge was developed by the Institute of Applied Physics of the Russian Academy of Sciences (IAP RAS) for long-term all-weather monitoring of wind waves. The instrument can be used to validate remote sensing data. Currently the instrument is being tested in various conditions and compared with different buoys. The acoustic wave gauge “Kalmar” was installed at the Gelendzhik test site of the Southern Branch of the Shirshov Institute of Oceanology RAS (IO RAS) in 2019 and has worked successfully till December of 2020. The ADCP RDI WH-600 device was also installed at the landfill. Both instruments are installed underwater and use ultrasound to scan the water surface, they are able to measure omnidirectuinal wave spectrum. The parameters of large waves (up to 12 m) can be calculated from the measured wave spectrum. In the paper both the wave spectra themselves and the significant wave heights are compared for February 1–10, 2020. It is shown that the wave heights measured by the two instruments are close and the difference between the measurements of the acoustic wave gauge and the results of two different ADCP data processing algorithms (Echo and Velocity) do not differ more than the results of the two algorithms differ from each other. The spectra measured by the two instruments differ in the peak region, and fall off in the same way. However, the spectra recovered by different methods from ADCP measurements also differ, so that the shape of the recovered spectrum depends significantly on the processing algorithm. In the future, it is planned to consider this issue in more detail and compare the measurements of the acoustic altimeter with the measurements of the string wave gauge.

This work is devoted to the presentation of an original approach for calculating the characteristics of microwave radiation scattered in forward direction from wind waves formed in the presence of a constant current. Within the framework of this approach, the concepts of effective wind speed and direction, depending on the velocity and direction of the current, are used to define the wavenumber spectrum of surface waves in the presence of a constant current. To set the frequency spectrum of the waves, the harmonic frequencies are additionally recalculated depending on the velocity and direction of the current. The pa per presents the dependence of the wave spectrum on the current velocity and on the angle between the wind and current directions. Next, the wave parameters are calculated, which are necessary to calculate the Doppler spectrum (DS) of the reflected radiation. To calculate the DS, an approach is used that considers the speeds of the receiver and emitter, the antenna patterns of the receiving and emitting antennas, and depends on 6 second-order statistical moments describing the reflecting surface. In this work, the dependences of the bistatic radar scattering cross-section, width, and shift of the DS of reflected radiation on the azimuthal angle of the sensing plane and the wind direction are constructed.

To study and use the short-wave part of the wave spectrum in model constructions, it is necessary to learn how to measure them. However, in marine environments, the measurement of the short-wave part of the wave spectrum is difficult. The development of new instruments for measuring the “roughness” of the short-wave part of the wave spectrum (wavelength up to 2 cm) is extremely urgent and important. This information is in demand by specialists studying the interaction of the atmosphere and the ocean, developing numerical models of wind waves. This paper presents a method for calculating the dependence of the reflected radiation power on the angle of incidence without changing the orientation of the sonar antenna. The new method is based on the geometry of the interaction of the emitted acoustic pulse with the sea surface. For verification, the developed analytical model of the shape of the reflected acoustic pulse and the conducted numerical study of the shape of the pulse from the parameters of surface waves are used. Further, the dependence of the reflected radiation power on the angle of incidence can be used to determine the slope variance. The slope variance retrieval method is like that used for processing data from the GPM and CFOSat mission satellites. The paper investigates the efficiency of the approach for determining the dependence of power on the incidence angle at different depths, pulse duration, wind speeds and wind fetch. Even though the problem is considered for sonar, the proposed approach will be valid for radar as well.

Measuring the global field of sea currents is an important task, therefore, several projects of orbital radars designed to solve the problem are developing in parallel. To measure sea currents, it is proposed to use Doppler radars that will operate at small or middle incidence angles. However, there are virtually no measurements of the Doppler spectrum (DS) at small incidence angles, thus the series of DS measurements carried out in the fall of 2019 in the range of incidence angles of 0°-25° is unique. The paper presents the results of measuring the DS of the backscattered microwave signal for a stationary river current. Doppler radar was mounted on a bridge over the Oka River (Nizhny Novgorod) at an altitude of about 30 m. The first results of the radar data processing show that the river current strongly affects the DS shape and leads to an additional shift of the DS compared to that in the absence of current. Comparison of the theoretical model of the DS with the measured DS has shown good results.

Probing of the typhoon Hagibis in stage of maximum intensification with satellite active and passive microwave sensors onboard GPM (Global Precipitation Measurements), GCOM-W1, Meteor-M No. 2-2 and Sentinel-1 satellites permitted retrieving the ocean surface and atmospheric parameters and investigate evolution of their spatial structure. Wind speed W, cloud liquid water content Q and total atmospheric water vapor content V were estimated from GMI and AMSR2 data with original algorithm based on total atmospheric absorption at frequency of 10.65 GHz τ(10). Mean square slope (mss) was retrieved from Ku-band channel of DPR radar. The location of typhoon center was traced due to the difference of retrieved parameters between the eye and surrounding area. Hagibis warm core was found by MTVZA-GY radiometer at several frequencies at oxygen absorption band. The position of the sharp gradient zones in V, Q and τ(10) fields served as an indicator of the change of typhoon area and structure. The calm cloudless region in the western Japan Sea was used as the reference area for matching to zero level of the retrieved wind speed and mss.

This paper presents the results of experimental measurements of the Doppler spectra in bistatic scheme of observation. The measurements were carried out on one of the bridges over the Oka River in Nizhny Novgorod in the spring and autumn of 2019. The experiments were organized to verify the original approach proposed in previous works for calculating the Doppler spectrum of radiation reflected by the water surface in the quasi-specular directions. In the framework of this approach, the sea surface is described by 6 second-order statistical moments and the received radiation can be found in the Kirchhoff approximation taking into account the radiation patterns of the emitting and receiving antennas. An important feature of the approach is the explicit dependence of the characteristics of the Doppler spectrum on the parameters of the sea surface. The paper presents bistatic measurements of the Doppler spectrum of a signal reflected by a water surface performed under various conditions of wave formation, as well as at different angles of incident and received radiation. A comparison of field measurements with theoretical estimates is carried out, which confirms the operability of the proposed approach for describing the Doppler spectrum of reflected radiation in a bistatic formulation of the problem.

Significant progress has been made in the study of electromagnetic wave scattering from the sea surface. Essential results were obtained in solving applied problems, but there are still unsolved problems in the scattering theory. The radar backscatter cross section is usually used to solve remote sensing tasks, although the Doppler spectrum (DS) of the scattered microwave signal contains more information about sea waves. The concept of a two-scale model of the scattering surface is used in this article. Correct allowance for large-scale waves, in comparison with radar wavelength, in deriving theoretical formulas enabled us to develop a modified Bragg scattering model. It is based on the previously developed scattering model that is refined here to take into account the contribution of the horizontal component of the orbital velocity to the DS. It is shown that allowance for large-scale waves limits the growth of the radar cross section of the Bragg component with a decrease in the incidence angle, which allows using the resonant scattering model at small incidence angles. An additional result is that the modified model of the DS can also be used in the range of middle incidence angles because it takes into account the contribution of the horizontal component of the orbital velocity. It is shown that if the correlation between large-scale and small-scale components of sea waves is neglected, the new formulas pass into the known formulas for the DS within the framework of the Bragg scattering model.

The Doppler spectrum (DS) of the signal backscattered at the low incidence angles from the sea surface was measured by the Ka-band radar in an experiment on a marine oceanographic platform. The directional elevation spectrum of sea waves was measured simultaneously by a six-string wave gauge mounted on a platform. At low incidence angles the backscattering mechanism is quasi-specular, and DS of the backscattered signal is calculated in the Kirchhoff approximation with the antenna pattern being taken into account. In the case of pure wind sea, the parameters of the sea waves spectrum model (wind speed and fetch) were retrieved by 2-D optimization method from measured DS shift and width. Sea wave propagation direction was obtained from the azimuthal dependence of Doppler shift. Analytical dependence of DS shift and width on incidence angle calculated from the spectrum model is in a good agreement with the experiment for incidence angles from 0° to 12°. Close values of significant wave height, the average wave period, and the variance of orbital velocities were obtained from the retrieved model spectrum and from the spectrum directly measured by the string wave gauge, which also proves the correctness of DS model.

Dual-frequency precipitation radar (DPR) is a powerful instrument for remote sensing of sea waves but its application is yet limited. Measurements of a radar backscattered signal are made at small incidence angles and retrieval algorithms permit to retrieve mean square slopes (mss) of the sea surface. The combination of the DPR product and the sea surface wind speed (SSW) field retrieved from microwave radiometer and scatterometer data with a resolution of 5-10 km can be used to estimate the relationships between wind and waves, reveal the spatial features of the air-sea interaction in cyclones, atmospheric fronts, cold air outbreaks, and other meso- and synoptic-scale weather systems. For the first time, the DPR and microwave radiometer data were processed for extratropical cyclones. Changes of mss and wind in the areas of cyclonic activity were analyzed. A strong correlation of mss and near-surface wind characteristic was shown. Short wind-generated waves strongly affect mss and they rapidly respond to changes in the wind speed.

The underwater acoustic wave gauge "Kalmar" (Ka-band) for the sea wave parameters measurements was developed in the IAP RAS. Since December of 2017 a number of test experiments were conducted. Since spring of 2019 the wave gauge was installed on the "Gelendzhik" site of the Southern Branch of the P.P. Shirshov Institute of Oceanology in the Black sea. The wave gauge measures Doppler spectra and significant wave heights, the data will be available through web-interface. This paper presents the results of the test measurements that were conducted on the "Gelendzhik" site. The modeled Doppler spectrum and a measured one are in agreement with each other. The variance of the vertical component of the orbital velocity was retrieved from the Doppler wave gauge measurements and compared with the string wave gauge measurements. The results are in good agreement.

This work is devoted to the development of the original approach of multi-static remote sensing laid down in previous works. The basis of the new approach is the analysis of the shape of the Doppler spectrum of the signal reflected in the forward direction from the sea surface. The Doppler spectrum contains more information about the reflecting surface than the radar cross section, since the radar cross section is an integral characteristic of the Doppler spectrum. The advantage of the Doppler spectrum is also its greater versatility when comparing the measurements of various instruments comparing to the radar cross section. An important feature of the proposed approach is the possibility of obtaining explicit dependencies for the surface waves parameters on the characteristics of the Doppler spectrum of reflected radiation obtained using receivers with different antenna patterns. In this paper, special cases of the potential implementation of the developing approach are considered.

This paper presents the results of experimental measurements of the sea surface parameters during the ship expedition to the Gorky water reservoir in the Nizhny Novgorod region on September 22, 2018. The measurements were carried out using underwater sonar at small angles of incidence so the scattering mechanism remains mostly quasispecular and is well described by tangent plane approximation (Kirchhoff approximation). The method is based on the analysis of spectral and energy characteristics of the reflected acoustic waves at underwater surface waves observation. An important feature of this method is the simultaneous use of multiple antennas with different radiation patterns. The underwater sonar antenna system consists of one radiating antenna with a symmetrical radiation pattern and three receiving antennas of various shapes: 1 - symmetric, 2 and 3 - fan beam antennas oriented perpendicular to each other. Within the framework of this expedition, the original configuration of the antenna system was first used: the width of the symmetric receiving antenna, equal to the widest axis of the fan beam antennas.

In this paper, we present an algorithm for retrieving all the second statistical moments of the sea surface that affect the scattering of waves at small angles of incidence. A feature of this algorithm is the analysis of the spectral characteristics of the reflected waves only, without taking into account the backscattering cross section. The analysis of the width and the shift of the Doppler spectrum (DS) allows us to use the obtained algorithm without the preliminary instrumental calibration, which is necessary for the backscattering cross section. We give final expressions for calculating the width and the shift of the DS in the monostatic problem statement for small angles of incidence. We obtain the expressions within the framework of the Kirchhoff approximation, which makes them reliable only in the quasi-specular reflection region. Features of these formulas are the description of the surface by five statistical moments of the second order and taking into account the anisotropic antenna pattern (AP). To solve the inverse problem of retrieving sea surface parameters, we propose a measurement scheme including three motionless transceiver antennas with different APs. For this measurement scheme, we give analytical expressions for all five unknown parameters of surface waves. Also, we give a theoretical analysis of the accuracy of the algorithm obtained for different wind speeds and wind directions. All the results are valid for the scattering of both acoustic and electromagnetic waves.

Results of the first experiment on measuring the Doppler spectrum of the microwave signal backscattered by the sea surface at low incidence angles (<; 20°) made from a sea platform are presented. At small incidence angles (<;10°), the reflection is quasi-specular and the Kirchhoff approximation is used to calculate the Doppler spectrum. Comparison showed a good agreement between the experiment and numerical estimates of the Doppler spectrum model. The measurements have shown that in the transition range of incidence angles (10°-20°), one should take into account the resonant scattering contribution to explain the features of the measured Doppler spectrum. To calculate the Doppler spectrum in the transition range, an advanced Bragg scattering model was used. It is shown that the employed quasi-specular and Bragg scattering mechanisms qualitatively explain the dependences of the width and shift of the Doppler spectrum on the incidence angle. In passing to quantitative estimates, tilt modulation and hydrodynamic modulation of the Bragg scattering mechanism should be taken into account, which will be done in a future study. The first measurements of the dependences of the width and shift of the Doppler spectrum on the incidence angle allow comparing well-known scattering models.

In this paper we present a new directional spectrum based on both oceanological and remote sensing data. The model is compared with popular ocean wave models used for remote sensing problems of T. Elfouhaily, P.A. Hwang and V. Kudryavtsev. It is shown that the modified wave spectrum is a good choice for L band RCS modeling for moderate incidence angles. It is also shown that the modeling results with the modified wave spectrum are in agreement with experimental data. In a region after spectral peak the model is presented in a form of power-law dependency of spectral density on frequency, where power is different for different wave ranges. It allows carrying out analytical transformations easily. An asymmetrical angular spreading function is used in a spectral model.

The work is aimed at developing a new approach to bistatic remote sensing of the sea surface. The basis of the new approach is the analysis of the shape of the Doppler spectrum of the signal reflected in the forward direction from the sea surface. The Doppler spectrum contains more information about the reflecting surface than the scattering cross section, since the cross section is an integral characteristic of the Doppler spectrum. The advantage of the Doppler spectrum is also its greater versatility when comparing the measurements of various instruments comparing to the scattering cross section. The paper presents the final expressions for the width and shift of the Doppler spectrum of microwaves reflected by the sea surface during bistatic sensing in the case of a moving receiver and a transmitter. An important feature of the proposed approach is the consideration of the antennas patterns of the receiving and transmitting antennas. And it is taken into account that the antennas patterns of the receiver and transmitter can be different and asymmetrical. The correct choice and use of antennas with different patterns allows directly expressing the sea surface parameters of interest from the measured characteristics of the Doppler spectrum.

Flood monitoring and forecasting are the key challenges to ensure the population safety. Modern spaceborne instruments significantly extend the capabilities of the terrestrial hydrological network, since they enable one to perform measurements over large areas. In 2014 the Japan Aerospace Exploration Agency (JAXA) launched spaceborne dual-frequency precipitation radar (DPR) and increased the possibilities of remote sensing. The main objective of mission is the measurements of the precipitation. However, the radar cross section (RCS) measured at the maximal range contains the information about the scattering surface. The advantage of radar systems is their capability of all-weather/day-night measurements. DPR measurements made over the ground of the territory of Russia have been processed for the first time. The Moksha River basin were chosen as test area. The spring flood of 2015 was explored in the radar image in Ku- and Ka-bands. Data processing shown that the spring flood can be observed in DPR image.

The Precipitation radars onboard the Tropical Rainfall Measurement Mission (TRMM) satellite (1997-2014) and onboard the Glopal Precipitation Measurement (GPM) satellite which was loanched in 2014 operate in scanning mode at low incidence angles. It allows to obtain in the frameworks of Kirchhoff approximation the normalized radar cross section (NRCS) at nadir and the slope variance of sea waves along the scanning direction. Here we mean the slope variance of waves being large-scale compared to the probing wave length in the frameworks of the two-scale model of the sea surface. Processing of real precipitation radar data revealed possibilities of the radar to detect large-scale oceanic and atmospheric processes.

The paper deals with the solution of the inverse problem of remote sensing of the sea surface. The solution is obtained for the problem statement with the monostatic quasispecular acoustic sensing from under water. Sensing is supposed to carry out with the help of a fixed Doppler sonar with the antenna unit consists of two transceiver antennas with asymmetrical beam patterns oriented perpendicular to each other. The method of the inverse problem solution is based on the analysis of spectral and energy characteristics of the waves reflected from the sea surface. The paper gives the accuracy estimation of the proposed method of sea surface parameters retrieval for different wind speeds and angles of incidence. Also, the paper gives the expressions for the scattering cross-section, the width and offset of Doppler spectrum of the waves reflected from the sea surface in monostatic quasispecular problem statement taking into account the asymmetrical beam patterns of transceiver antennas.

The Global Precipitation Measuring (GPM) was proposed jointly by US and Japan and satellite equipped with a dual-frequency precipitation radar (DPR) and a microwave radiometer (GMI). GPM is partly a follow-on mission of the Tropical Precipitation Mission (TRMM), but GPM will extend the observation to cold regions. Our team has experience in developing of retrieval algorithms for precipitation radar from TRMM therefore we intend to use DPR data for analysis and retrieval of sea wave parameters and wind speed. In this paper the features of the DPR data is discussed and the first examples of data processing are shown.

Dual-frequency Precipitation Radar (DPR) consists of Ka- and Ku-band radars which operate in scanning mode at low incidence angles. It is installed on the Global Precipitation Measurement (GPM) satellite. Definition of boundary wave number in frameworks of the two-scale model of scattering surface is an important problem for investigation and modelling of radar backscatter from the sea surface. In this research DPR data combined with sea buoys data were used to evaluate slope variance of large-scale (compared to the probing wave length) sea waves along the scanning direction. Then this information was applied to obtain the boundary wave numbers dependences on wind speed for Ka- and Ku-bands. The dependencies were obtained for the case of fully developed sea.

Sea Winds scatterometer with an orbit altitude of 800 km was chosen as a prototype of the first Russian orbital scatterometer. A reduction of the orbit altitude to 650 km has brought about two competing approaches to the choice of swath width: either to remain the original 1800 km swath or the incidence angles by reducing the swath to 1500 km. The advantage of a wider swath is the best covering of the ocean surface. However, this leads to an increase of local incidence angles and thus to a decrease of the backscattered signal power. The selection criterion is the minimal error in wind speed and wind direction retrieval. Numerical simulation was carried out for both swath widths. The data were processed and the accuracy of wind speed and wind direction retrieval was estimated. The results showed that the accuracy of wind speed retrieval is rather high for both swaths, while the accuracy of wind direction retrieval for a wider swath is lower than that required by the scatterometer technical specifications. Thus, the swath 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.

Theoretical model for return waveform was developed for underwater ultrasonic sonar with wide antenna beam at vertical sounding. First time the analysis of return waveform was used in acoustic system for measurement of significant wave height as in radio altimeter. New retrieval algorithm was developed for scheme of measurement and the advanced underwater acoustic “altimeter” can measure the significant wave height of sea waves. The numerical simulation confirmed this conclusion. The first experiment was carried out and data processing confirmed the efficiency of retrieval algorithm. Underwater acoustic system can measure significant wave height at any sea states and wind speeds and therefore exceed the conventional sea buoys.

The new method for wind speed retrieval using the SAR spectrum is proposed. Wind speed and direction are estimated using the SAR spectrum peak corresponding to the dominant wavelength of wind waves. The numerical simulation with the quasilinear SAR imaging model was performed to illustrate the algorithm. Ambiguity +/-180° is avoided by calculating the correlation coefficient between signals at VV and VH polarizations.

By using ten years of collocated Precipitation Radar and buoy data, the relationships between Ku-band normalized radar cross section (σ°) and integrated wind and wave parameters (e.g., significant wave height, wave period, wave steepness, and wave age) at low incidence angles are analyzed for different sea states using correlation and dependence analysis. The results show that the relationships are significantly different for different sea states. Second, the potential for inverting these parameters directly from a single σ° is investigated. The results reveal that the retrieval of wind speed above 10 m (U10) is most suitable for a nonpure-swell sea and that the performance of significant wave height (Hs) retrieval decreases with the ratio of swell to wind waves. For wave period and wavelength, a feasible inversion can only be performed for a wind-wave-dominated sea. Wave steepness (δa) is strongly correlated with σ° in all sea states and can even be higher than that forU10in pure-swell seas. It is suggested from the present data set that real wave age (β) may be retrieved from a swell-dominated sea and, therefore, has a low-value cutoff. Furthermore, the extent to whichU10retrieval depends on sea state is examined by calculating the correlation coefficient between σ° residuals and various standard wave parameters and, then, by determining the partial correlation coefficient between σ° and each wave parameter controlled byU10. Both results indicate that wind direction is better taken into account above 13.5°. Moreover, an auxiliary parameter related to wave slope, such as wave steepness, would help to improve retrieval performance. Significant wave height data are of secondary use, but an implementation with real wave age information might not significantly improve wind speed retrieval. To improve the performance ofHsand wave period retrieval, auxiliary information, such as wind speed, wave steepness, or wave age, is needed. Finally, multivariable empirical models are proposed.

Backscattering of a microwave signal is considered in the Kirchoff approach and in two-scale model of the scattering sea surface. The effective reflection coefficients in Ku (0.021 m), C (0.055 m), and S (0.09 m) bands are calculated. Dependences of the radar cross section on wind speed, wind fetch, and swell height are studies and closeness of numerical estimations and experimental data is shown. The dependence of difference radar cross section on wind speed or one of the backscattering radar cross sections in two-frequency measurements is analyzed. It is shown that the developed model allows describing the nonmonotonic behaviour of the difference radar cross section observed in the experiments. The proposed model enabled us to predict the behaviour of the difference radar cross section for the new pair of frequencies (C - S bands).

This paper extends our theoretical and experimental researches on microwave backscattering at small incidence angles as applied to remote sensing of the ocean. A new concept of a Doppler radar with a knife-beam antenna pattern is proposed. With a nadir-pointing rotating antenna the radar will be able to measure a number of key sea state parameters previously unavailable, such as the long wave slope, the direction of wave propagation. Preliminary theoretical analysis and numerical simulations predict that in a space-borne scenario, signals may be isolated for scattering cells of the order 14/spl times/14 km over a swath approximately 250 km wide.