Yuriy Titchenko

This paper presents the results of a laboratory study of ultrasonic pulse reflection from freshwater ice during a complete freeze–thaw cycle of the ice cover. Measurements were conducted in a small ice tank at the Arctic and Antarctic Research Institute. The maximum ice thickness was 57 cm, and the freeze–thaw cycle lasted 90 days. A sonar with a carrier frequency of 200 kHz operated continuously at the bottom. The transceiver antenna was oriented vertically upward toward the water surface. The underwater sonar measured and stored the reflected pulses throughout the experiment. Analysis of the reflected pulse data made it possible to study ice thickness dynamics and to obtain estimates of the average speed of sound in ice and the sound attenuation coefficient in ice over the entire measurement period. A simple empirical model for the shape of an acoustic pulse reflected from a freshwater ice cover is proposed. In this model, the reflected signal is represented as the sum of reflections from the undisturbed water surface, shifted by the travel time of the acoustic wave in the ice, with the reflection from the upper boundary of the ice cover additionally multiplied by the sound attenuation coefficient in ice.

Freshwater ice cover on inland water bodies is a sensitive indicator of climate change, and its monitoring is hampered by the decline of in situ observation networks since the 1980s. Previous applications of GNSS-Reflectometry (GNSS-R) have focused mainly on sea ice detection, while its capability for seasonal freshwater ice monitoring has remained insufficiently explored. The aim of this study is to assess the sensitivity of spaceborne L-band bistatic measurements to the processes of freshwater ice cover formation, growth, and break-up on large inland lakes, and to compare the scattering behavior with that of sea ice. A full annual cycle (September 2023 – August 2024) was analyzed using data from the GNOS-II instrument onboard the Feng-Yun 3E (FY-3E) satellite at two test sites: Great Slave Lake (Canada) for freshwater ice and the Sea of Okhotsk for sea ice. Statistical moments of the Doppler spectrum – width and kurtosis coefficient – were computed from delay-Doppler maps. The results were validated against meteorological records from the Yellowknife station, MODIS imagery, and ERA5 ice thickness reanalysis. A monotonic decrease in Doppler spectrum width with decreasing air temperature was established for freshwater ice, consistent with a reduction of effective surface roughness. For freshwater ice, peak power and kurtosis coefficient show a distinct dependence on ice thickness, reflecting signal attenuation within the ice volume and dominant reflection at the ice-water interface; for sea ice these parameters remain stable, since reflection occurs at the air-ice boundary owing to high brine salinity. The findings demonstrate that L-band bistatic reflectometry provides an effective all-weather tool for operational monitoring of freshwater and sea ice, including detection of freeze-up and break-up phases, ice thickness estimation, and support for Arctic navigation.

The spectral and energy characteristics of the backscattered radar signal contain information about the scattering surface parameters. The backscattered normalized radar cross section (NRCS) is determined by the geometry of the underlying surface, while information on the sea surface movement is contained in the Doppler spectrum (DS) of the backscattered microwave signal. The paper deals with the analysis of the properties of the DS model at small incidence angles, when the quasi-specular backscatter mechanism is dominant. The dependences of the width and shift of the DS on the speed and direction of wind, nondimensional wind fetch, incidence angle, width of antenna beam and velocity of the radar movement are investigated. It is shown that even for a case of wind waves there is an ambiguous relationship between the wind speed and the DS parameters, which leads to ambiguity in solving the inverse problem. Numerical estimates have shown that the width of antenna beam strongly affects the width and shift of the DS and this can be used to determine the optimal measurement scheme. The case of swell and mixed sea is also considered and it is shown that a large difference in DS is observed even under the condition of equality of the SWH and the dominant wavelength of wind waves and swell. When measuring from a moving platform, the DS parameters remain dependent on the statistical characteristics of the sea waves for a radar with a wide antenna beam and can be retrieved during subsequent processing. When measuring the DS, information about the RCS is saved and can also be used during processing.

This study addresses the challenge of understanding sea ice microwave scattering properties by developing a novel method to retrieve scattering indicatrix and angular dependence of backscattering patterns from Global Navigation Satellite System Reflectometry (GNSS-R) data. Sea ice remote sensing in quasi-specular reflection area requires theoretical models validated using experimental data across different frequency bands. However, only limited observations of quasispecular scattering in the L-band exist. We developed an algorithm to convert Doppler spectra from delay-Doppler maps (DDM) obtained by the TDS-1 satellite into scattering indicatrix and then to angular dependence of the sea ice backscattering patterns, using geometric transformations from bistatic sensing geometry. The method was applied to approximately 100 DDM measurements over the Sea of Okhotsk during February–March 2017, where ice concentration remained at 90–100% with temperatures below 0 °C. Results show that L-band (19 cm wavelength) exhibits broader angular dependence compared to Ka-band (8.45 mm) and Ku-band (2.21 cm) measurements from dual-frequency precipitation radar (DPR), contrary to expectations based solely on surface roughness. This anomalous broadening is attributed to volume scattering effects within sea ice, where L-band signals penetrate up to one meter depth compared to millimeter-scale penetration in Ka- and Kufrequency bands. The findings provide new insights into L-band scattering mechanisms and offer a validated approach for improving theoretical models of sea ice microwave interaction.

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 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 the mss of large-scale waves.

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.

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.