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.

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.

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.

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 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.

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.