In radar, the main source of scattering surface data is the backscattered radar cross section. The dependence of the backscattering normalized radar cross section (NRCS) for ice cover can be ambiguous, which does not always make it possible to determine the kind of scattering surface (ice/water). This paper describes an experiment on measuring the microwave signal backscattered from the ice cover on a river. During the experiment not only the RCS is measured, but air temperature as well. The photos of the ice cover are taken so that we understand what kind of surface and in what conditions correspond to the curtain radar response. The experiment started in January 2022 and will continue thought the winter and spring. The end result of the experiment is the dependence of the RCS of the ice cover on the air temperature. The goal of this paper is to provide experimental data to remote sensing scientists, to discuss the results obtained and future experiments.

Active and passive remote sensing techniques can be used simultaneously and their results complete each other. This paper introduces the results of the experiment that took place in October 2019 on the offshore platform in the Black Sea. Two unique pieces of equipment developed in the Institute of Applied Physics were used in the experiment: set of original optical devices for recording of range - time - intensity (RTI) images of sea surface and an underwater acoustic wave gauge. The devices were installed close to each other, optical devices were measuring slope spectrum, acoustic wave gauge can measure significant wave height and variance of slopes of the large-scale (in comparison with the wavelength of the incidence acoustic wave) waves, also the Doppler spectrum of the reflected signal can be measured. The string wave gauge is working on the platform constantly and provides measurements of the omnidirectional spectrum.

A simple method to calculate the wave spectrum in the presence of the constant river current is presented. The formation of the Doppler spectrum of the microwave radar signal backscattered from the water surface at low incidence angles is modeled. Measurements of the water surface backscatter at low incidence angles were performed using a Ka-band Doppler radar from the bridge over the Oka river in Nizhny Novgorod in June-October of 2019. Incidence angles varied from nadir to 25° during experiments. In this paper the results of comparison of experimental data processing and the model Doppler spectrum are presented. It is shown that the “simple” model that describes the wave spectrum and Doppler spectrum in the presence of the constant current allows to get results that are similar to experimental measurement yet differ especially in the case of sounding along the current. New measurements for different conditions will help to modify the model.

The sea surface wave spectrum and the sea surface wave parameters are retrieved from acoustic wave gauge measurements and CCD receivers. The measurements took place on the offshore platform in the Black Sea. The acoustic wave gauge was installed near the platform in August-October 2019. In September-October 2019 the optical measurements were taken. The string wave gauge is working on the platform constantly. The first results of simultaneous measurements of the sea surface waves using active and passive remote sensing are presented. An algorithm for variance of slopes of the large-scale (in comparison with the wavelength of the incidence radiation) waves retrieval is introduced.

2 Научно-исследовательский центр космической гидрометеорологии «Планета» Росгидромета

The dual-frequency precipitation radar (DPR) was launched in 2014 to make precipitation measurements. Simultaneous measurements at two frequencies open up new vistas in monitoring sea waves and near-surface wind speed. At small incidence angles, the DPR can measure the variance of large-scale slopes of sea waves (or large-scale mean square slopes - mss). A slope retrieval algorithm was developed for Ka-band. Comparison of the large-scale mss in Ka-band and Ku-band has shown that the large-scale mss in Ka-band is larger than the large-scale mss in Ku-band. This result will permit to improve the two-scale model applied for calculation of the signal reflected from sea surface.

This study continues our previous explorations of the new space-borne scatterometer concept. Using a fan-beam antenna and simultaneous RCS measurements at the horizontal and vertical polarizations, one can improve the accuracy of wind speed and wind direction retrieval.

Backscattering of a microwave signal is considered in the Kirchoff approach and in two-scale model of the scattering sea surface. The effective reflection coefficients in Ku (0.021 m), C (0.055 m), and S (0.09 m) bands are calculated. Dependences of the radar cross section on wind speed, wind fetch, and swell height are studies and closeness of numerical estimations and experimental data is shown. The dependence of difference radar cross section on wind speed or one of the backscattering radar cross sections in two-frequency measurements is analyzed. It is shown that the developed model allows describing the nonmonotonic behaviour of the difference radar cross section observed in the experiments. The proposed model enabled us to predict the behaviour of the difference radar cross section for the new pair of frequencies (C - S bands).