卷 61, 编号 2 (2018)

We present the results of radar investigation of scattering of electromagnetic radiation in the 3-cm wavelength range by the sea surface in the case of horizontal polarization of the transmitted and received signals and wide incidence angles. Full-scale measurements were performed on the Oceanographic Stationary Platform in the Black Sea in the range of incidence angles from 84.0° to 87.5° at wind velocities from 4 to 19 m/s. Only the conditions of wind waves with no swelling were considered. It is shown that the specific scattering cross section is almost invariant, when the incidence angle varies, but has a strong wind dependence. It can be described by the power function with the exponents 2.8 and 3.8 at the upwind and downwind probing directions, respectively. At weak and moderate winds, the azimuthal dependence of the specific scattering cross section is characterized by the unimodal function with one maximum in the upwind direction and a minimum in the downwind direction. At wind velocities exceeding 10 m/s, the azimuthal dependence of the scattering cross section becomes bimodal and has a minimum at the azimuth being close to the direction perpendicular to the wind (crosswind). The measured values of the specific scattering cross section are approximated by a finite Fourier series with respect to the azimuth, in which the expansion coefficients depend on the wind velocity and inclination angle.

We develop the theory of a three-mirror echelette traveling-wave resonator with frequency tuning, in which one of the mirrors is an echelette reflective diffraction grating. The theory makes it possible to allow for the diffraction loss related to finite sizes of the mirrors, the loss due to the existence of a mirror lobe at the grating (coupling loss), and the ohmic losses. The possibility of significant rarefaction in the spectrum of the resonator eigenfrequencies is demonstrated. The structure of mode fields allows one to ensure efficient input of the radiation to the resonator. An experiment performed in a frequency range near 140 GHz confirmed the theoretical calculations.

We present a detailed study of the characteristics of the exact decision statistic for the optimal incoherent signal detection in a multielement antenna array. The exact decision statistic is derived from a rigorous expression for the likelihood ratio in the case where the useful-signal amplitudes at the antenna-array elements are identical, whereas the phases are random. The analysis is carried out in the space of the variables which are observed at the outputs of the incoherent matched filters performing the initial signal processing at each antenna-array element. Variation in the signal-to-noise ratio is shown to deform the boundary of partitioning of the multidimensional observation space, which results in a variation in the threshold values of the exact decision statistic when the Neyman–Pearson criterion is used. The characteristics of the exact decision statistic and various approximate decision statistics are compared in detail. For the antenna arrays with a large number of elements, the influence of the used decision statistic on the target-missing probability is shown to become rather significant. A new combined decision statistic with the characteristics that are close to those of the exact decision statistic is introduced for a wide signal-to-noise ratio range.