Evgeniy Meshkov

Remote sensing methods are widely used to monitor ongoing climate change, and the area of sea ice in the Arctic and Antarctic is used as one of the criteria. The duration of ice cover on inland waters can be used to assess processes within continents. The small size of internal waters does not allow the use of traditional methods that have proven themselves well in marine conditions. This study considers the possibility of using dual-frequency precipitation radar data to detect ice formation and destruction on small inland waters. Due to the features of backscatter at small incidence angles (< 18∘), inland waters with sizes smaller than the radar resolution (5 km) are “visible” in radar images. Using the Volga River as an example, it is shown that water-ice and ice-water transitions can be detected when analyzing radar images, and thus, possible to estimate the duration of ice cover on inland waters.

Currently, methods of radar remote sensing at small incidence angles (from the vertical to 15∘) are actively developed. An important application of these methods is the determination of the presence and sea ice concentration. This paper presents an approach to numerical simulation of an experiment in which a reflecting surface with different sea ice concentrations is modeled and then the characteristics of the radar signal reflected by this surface for a given measurement geometry are modeled. Without loss of generality, in this paper we will consider a specific geometry of the DPR (Dual-frequency Precipitation Radar) radar on the GPM (Global Precipitation Measurement) mission satellite and only the Ku-band of this radar. The signal reflected by sea waves will be calculated within the Kirchhoff approximation. Since there is no generally accepted model for the signal scattered by the sea ice at small incidence angles, an empirical formula obtained from DPR data will be used as a model. The paper discusses a method for determining sea ice concentration using radar sensing data at low incidence angles.

The height of sea waves is one of the most important characteristics describing the wave climate of the ocean. At the present, the main radar for remote measurement of wave heights is an altimeter. Measurements are performed at the vertical sounding (incidence angle equal to zero). The Brown model was developed to describe the waveform of the reflected impulse at the vertical sounding. There is no theoretical model for the case of oblique sounding. In the Kirchhoff approximation, the theoretical task about waveform of the reflected impulse at oblique sounding was considered. In the result of the investigation, the analytical formula for the waveform of the reflected impulse for oblique sounding at the small incidence angles (< 12◦) for a microwave radar with a narrow antenna beam was obtained. The waveform of the reflected impulse depends on the width of antenna beam, incidence angle, impulse duration, significant wave height (SWH), altitude of the radar, mean square slopes of large-scale, in comparison with radar wavelength, sea waves. It is shown that possibility exist to retrieve SWH using waveform the reflected impulse at the oblique sounding.

The height of sea waves is one of the most important characteristics describing the wave climate of the ocean. At the present, the main radar for remote measurement of wave heights is an altimeter. Measurements are performed at the vertical sounding (incidence angle equal to zero). The Brown model was developed to describe the waveform of the reflected impulse at the vertical sounding. There is no theoretical model for the case of oblique sounding. In the Kirchhoff approximation, the theoretical task about waveform of the reflected impulse at oblique sounding was considered. In the result of the investigation, the analytical formula for the waveform of the reflected impulse for oblique sounding at the small incidence angles (< 12◦) for a microwave radar with a narrow antenna beam was obtained. The waveform of the reflected impulse depends on the width of antenna beam, incidence angle, impulse duration, significant wave height (SWH), altitude of the radar, mean square slopes of large-scale, in comparison with radar wavelength, sea waves. It is shown that possibility exist to retrieve SWH using waveform the reflected impulse at the oblique sounding.

At the small incidence angles, the dominant backscattering mechanism for sea waves is the quasi-specular backscattering mechanism. The power of the reflected signal depends on the distribution function of the slopes of large-scale waves (in comparison with radar wavelength) and on the effective reflection coefficient, which is introduced instead of the Fresnel coefficient. In this paper, we discussed a new method for calculating the effective reflection coefficient from the wave scatterometer SWIM data. For the first time, measurements are performed by a radar at different azimuth angles at small incidence angles. This makes it possible to measure the effective reflection coefficient. An original algorithm was developed for data processing and determination of the total mean square slopes of large-scale sea waves and the azimuth dependence of the backscattering radar cross section at zero incidence angle. In the result of subsequent processing, the azimuth dependence of the effective reflection coefficient is retrieved. SWIM data were used to evaluate the developed algorithm. Processing of the test data set confirmed the efficiency of the algorithm. The azimuth anisotropy coefficients for the mean square slopes of large-scale waves and the effective reflection coefficient are calculated

The paper presents the results of a series of experiments performed in February-March 2023. An X-band radar was installed on the metro bridge across the Oka River in Nizhny Novgorod. The radar worked in the radio altimeter mode and the waveform of the reflected pulse was recorded (the dependence of the power of the reflected signal on time). As a result of processing, the distance to the reflection area on the surface was determined. Dependences of the power of the radar signal reflected by the ice cover on the incidence angle were obtained. The air temperature was recorded synchronously with the measurements. The change in the distance to the area of reflection on the surface was associated with a change in the water level in the Oka River. Dependences of the power of the reflected signal on the angle of incidence were plotted for a radar with a knife-like antenna beam for different days of measurements. Based on empirical data, a formula is obtained for the angular dependence of the power of reflected microwave signal for fresh ice cover.

The first results of data processing of the experiment on the Nizhny Novgorod cable car are presented. A pulsed X-band radar was installed on a technological trolley and performed measurements while moving in two modes that worked sequentially. In the radio altimeter mode, the reflected waveform was measured and the distance to the scattering surface was determined. In the Doppler mode, the Doppler spectrum of the reflected signal was measured, which contains information about the statistical parameters of the surface. Data processing was carried out and the first results confirmed the assumption that the Doppler spectrum can be an effective tool for classifying the type of the underlying surface according to the "ice/water" criterion. Subsequent data processing will allow us to evaluate the accuracy of the developed algorithms.