Annotation of the results obtained in 2021
A theoretical analysis and modeling of sea wave parameters based on space-time diagrams using a two-dimensional Fourier transform were carried out, and radar portraits of the sea surface at different polarizations have been studied. The applicability of the maximum covariance analysis (MCA) for the analysis of matrices characterizing the spatiotemporal models of sea surface backscatter signals for different types of sea waves was investigated.The application of MCA to the comparison of the characteristics of different models of the sea surface backscatter signals at different types of sea waves results in the dimensionality reduction eventually leading to their replacement by a single correlation coefficient. Statistical analysis of the correlation coefficients determined by the MCA method, carried out with respect to the backscatter signals matrices at very deep water, deep water and finite depth, indicated that the final correlation coefficients are relatively small, and this tendency takes place at different degrees of sea roughness. With an essential increase in the degree of the roughness, the median of the correlation coefficients and their interquartile range increase insignificantly. We assume that weak correlations between these scenarios make it possible to reliably distinguish between backscatter signals in deep water, the appearance of shoals, and changes in the seabed topography, which can be useful for bathymetric measurements based on the radar backscatter signal analysis. Two different approaches to the analysis of correlated random fields are considered: using correlation and phase metrics. Analytical relationships are established between the distributions of phase shifts of individual harmonics used for synthesis of random fields and the coefficients of mutual correlations and RMS of the difference in Hilbert phase of the cross sections of the random field formed by them. The dependence of the amplitudes of the local maxima of the random field on the indicated characteristics is studied. The results obtained confirm the dependence of the considered metrics, and also indicate the possibility of estimating the local maxima of sea wave amplitudes under different conditions, which in the future will allow using the obtained dependencies in modeling the dynamics of both normal and anomalous sea waves. Theoretical basis for creating a methodology for measuring the height of sea waves by means of a linear regression analysis of the spectrum of sea waves based on Doppler information obtained using a coherent radar were investigated. The analysis and generalization of analytical models of the dispersion characteristics of the sea surface as applied to the question of the implementation of algorithms for radar bathymetry in the centimeter wave range were carried out. An analysis of the technical characteristics of airborne surveillance radars was carried out in order to expand their functionality when they operate in the scatterometer mode, and corresponding concepts were developed to expand the functionality of such airborne radars to measure water/sea ice backscatter signatures that can be used to recover the wind vector over the sea surface, determine the sea ice parameters, and provide sea ice/water discrimination from aircraft. In order to reveal the potential and possible optimization of the geometry for measuring the wind speed and direction under various conditions, the cases of antenna location under and above the aircraft were considered, and the corresponding geometries and algorithms for measuring the wind vector over the water surface using airborne surveillance radars were developed. Under the condition that the antenna is located over the aircraft, the cases of narrow, medium and wide shading elements of the aircraft (nose, wings, tail) were also considered. The results of the simulation of the near-water wind speed and direction measurements have shown that scatterometers installed over the aircraft, despite the presence of shading from the aircraft elements (nose, wings, tail), make it possible to measure the wind speed and direction over the water surface with almost the same measurement errors, as in the absence of shading (the antenna is installed under the aircraft). Under the condition that the antenna is located under the aircraft, the influence of the simultaneous use of several incidence angles on the accuracy of the wind speed and direction measurement was considered. The simulation results have shown that the simultaneous use of several incidence angles up to 3–4 makes it possible to increase the accuracy of measuring the wind speed and direction in comparison with the case of using only one incidence angle, while the highest accuracy is achieved in the region of incidence angles closer to 60°. At the same time, an increase in the number of incidence angles used leads to a decrease in the maximum height of applicability of the method compared to the case of using only one incidence angle. Simultaneous use of 2 incidence angles is a simpler procedure compared to using 3–4 incidence angles, combining both improving the accuracy of measuring wind speed and direction, and ensuring the maximum altitude of the method applicability approaching the maximum possible one when using only one incidence angle. The results obtained can be used to optimize the geometry and accuracy of measurements in the creation of new airborne radars used for oceanographic, meteorological and navigation purposes at their joint and standalone use. A conceptual structure of the hardware-software prototype of the 2-station coherent X-band radar system radar has been developed. As a single radar at each position, it is proposed to use navigation radars with adapted hardware for tasks of the sea surface monitoring. The coherentization of the Furuno radar was carried out. The features and operation principles of the Garmin radar were studied. Technical proposals have been developed for the modernization of the Garmin radar for the tasks of the sea surface monitoring. The software for controlling, recording receiver signals, visualization and processing of the data recorded by coherent Furuno radar was developed. New hardware and software solutions have been developed for the implementation of a purely coherent radar for the sea surface monitoring. A technique has been developed for remote sensing of the sea surface with a 2-station coherent X-band radar system. An algorithm for remote measurement and construction of vector fields of surface currents by a 2-station coherent X-band radar system has been developed. The algorithm has been tested using experimental data in application to a single radar. Experimental data of signals from the sea surface were obtained for different radar locations in the Black Sea and the Gulf of Finland, for different probing azimuths under different meteorological and sea surface conditions. The advantages of the Doppler radar in increasing the surveillance range due to coherent integration are confirmed.The possibility of revealing the structure of the spectrum and the possibility of using the radar for various purposes (operational measurement of currents and wind speed) of the sea surface monitoring are demonstrated. The limitations of the Doppler radar in range due to the insufficient potential of the Furuno 1623 radar have been determined. The operability of the proposed method of remote sensing of the sea surface has been confirmed. An algorithm for measuring currents using a single radar was tested, and qualitative estimations of the fields of surface currents were obtained. Further steps have been determined for carrying out experimental work in the waters of the Gulf of Finland and the Black Sea.

 

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Annotation of the results obtained in 2022
An algorithm has been developed for determining significant sea wave height from the radial Doppler wave velocity measured by a coherent centimeter range radar. The performance of variants of the algorithm for determining significant sea wave height was tested using records of real signals reflected from the sea surface obtained as a result of experiments. An optimal type of algorithm for the solution of the above problem was determined, assuming the calculation of significant wave heights by the RMS of the measured Doppler wave velocity. The results of wave height distribution in the investigated water area during the experimental works have been obtained. Limitations of the method of determining the significant wave height are revealed. The operability and applicability of the developed algorithm, taking into account the identified limitations, is confirmed. The distribution of local maxima of sea surface models at different depths, wind strength, and the presence of ripple waves was investigated. Models of sea waves were obtained by harmonic synthesis based on the Pearson-Moskowitz energy spectrum. It is established that the distribution of local maxima in the absence of swell waves is a type III GEV distribution (Weibull distribution). The scale and location parameters of the GEV distribution, as well as its median, increase with increasing wind speed, while there is practically no dependence on depth. In the presence of swell, the distribution of local maxima refers to type II GEV distribution, i.e. is a Fréchet distribution. The distribution median, as well as the scale and location parameters, depend on the angle between wind waves and swell waves, taking the largest values at an angle close to 90 degrees. The obtained results allow us to conclude that it is expedient to use the distribution parameters of local extrema for the characteristics of sea waves, and, first of all, for predicting anomalous situations at sea associated with the influence of swell waves. In this case, the most undesirable situation for safe navigation is the approach of the angle between wind waves and swell waves to 90 degrees, which leads to a significant increase in the amplitudes of the total waves. An analysis of the possibilities of exploitation of third-party transmitters of illumination for the tasks of radar hydrography was carried out. The analysis results showed that digital satellite TV transmitters have the highest effective radiated power in the centimetre range of electromagnetic waves. The results of analysis of the influence of bistatic geometry formed by receiver-transmitter pair on the parameters of the reflected signal spectrum at small sliding angles showed a strict dependence of the Doppler spectrum parameters on the observation angle, i.e. the relationship between the wave direction and the bistatic geometry. An algorithm for measuring bathymetric parameters in coastal waters based on the processing of sequences of radar images of the water surface has been developed. The dependence of sea surface condition on depth is expressed by a dispersion relation, which describes the dynamic relationship between wave number and angular frequency and determines the phase velocity of wave propagation. Parameters of the dispersion relation vary in the presence of a current and/or depth change. The proposed processing algorithm allows obtaining complex spatial single-component images showing the intensity and phase of individual parts of the wave field and estimating parameters of the sea conditions, namely the depth and current vector. An analysis of the technical characteristics of the high-altitude conical scanning radar was carried out in order to expand their functionality when it operates in the scatterometer mode, and corresponding concepts were developed to expand the functionality of such airborne radars to measure water/sea ice backscatter signatures that can be used to recover the wind vector over the sea surface, and provide sea ice/water discrimination from aircraft. In order to reveal the potential for increasing the maximum altitude of the method applicability and optimizing the geometry for measuring the wind speed and direction under various conditions, the cases and geometries for measuring the water surface backscattering signatures and the water-surface wind vector retrieval were considered according to the semicircle scheme at a single incidence angle, and simultaneously at several angles sightings. Also, appropriate algorithms for retrieval of the wind vector over the water surface using the high-altitude conical scanning radar were developed. The applicability of the proposed approach is supported by the results of mathematical modeling. The simulation results for semicircular geometries of measuring the water surface backscattering signatures at a single incidence angle and simultaneously at several neighboring incidence angle showed that the maximum errors in measuring wind speed and direction have a general tendency to decrease with increasing the incidence angle. The tendency in the case of semicircular geometries is less pronounced than in the case of circular geometries, with which the comparison was made. In these cases, all the errors are within the range of typical wind measurement errors with a scatterometer. Also, the simulation results have shown that the simultaneous use of several neighboring incidence angles up to 3–4 at the semicircular geometries, as well as for the circular geometries with which the comparison was made, makes it possible to increase the accuracy of measuring the wind speed and direction in comparison with the case of using single incidence angle, while the highest accuracy is achieved in the region of incidence angles closer to 60°. At the same time, an increase in the number of incidence angles used leads to a decrease in the maximum altitude of the method’s applicability compared to the case of using only one incidence angle. Simultaneous use of 2 incidence angles is a simpler procedure compared to using 3–4 incidence angles, combining both improving the accuracy of measuring wind speed and direction, and ensuring the maximum altitude of the method applicability approaching the maximum possible one when using only one incidence angle. Measurement geometries with the semicircular scheme allow not only to measure the wind speed and direction with an accuracy similar to measurements with the circular geometries, but also doubling the maximum altitude of the method applicability, but requires the tripled number of integrated normalized radar cross sections compared to the circular scheme. The results obtained confirm the validity of the proposed concepts, the efficiency of the algorithms, the applicability of the high-altitude conical scanning radar with the considered configurations and extended functionality for measuring the underlying surface signatures and the wind vector in the scatterometer mode, and can be used to optimize the geometry and accuracy of measurements in the creation of new airborne radars used for oceanographic, meteorological and navigation purposes at their joint and standalone use. On the basis of two coherent navigation radars, a prototype of a 2-position coherent radar complex of the X-band for measurements of hydrographic parameters of marine areas has been created. The concept of a true-coherent radar for sea surface monitoring has been developed that enables flexible selection of a probing signal by applying the technology of direct digital synthesis of quadrature signals. To provide a high range resolution, a wide second IF bandwidth of 35 MHz was selected, which allows obtaining a spatial resolution of 4.5 m providing detailed mapping of the sea surface. A methodology for the design of control and data acquisition software in a sea surface radar monitoring system has been developed.

 

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