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.

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.

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.

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.