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.

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.

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.

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.

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.

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.

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.

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.