Modeling the ship white water wake in the midwave infrared

Laser forward and backward scattering characteristics and experimental study of bubbles in ship wake

The detection and tracking of ships can be realized by using the laser forward and backward scattering characteristics of ship wake bubbles. In this paper, the detection ability of two kinds of scattering to wake bubbles is studied. Based on the distribution characteristics of ship wake and bubble targets, typical bubble targets are selected to study from both micro and macro aspects. The light scattering model of water is established from the microscopic aspect, and the forward and backward scattering light intensity equations of water are derived. The circumferential scattering characteristics of a single bubble are analyzed based on the Mie scattering theory. According to the transmission characteristics of light in wake bubbles, the secondary scattering model of wake bubbles is established, and the forward and backward scattering light intensity equations are derived. In the macroscopic aspect, the laser scattering simulation model of wake bubbles is established by Monte Carlo, and the forward and backward scattering characteristics of wake bubble clusters with different radii, densities, and thicknesses are analyzed emphatically. A laser forward scattering and backscattering detection system under typical bubble characteristics was built, and the composite scattering characteristics of wake bubbles with different parameter characteristics were experimentally analyzed. The theoretical and experimental results show that with the increase of bubble radius, density, and thickness, the amplitude of laser forward scattering signal of bubble groups decreases gradually, the amplitude of backward scattering signal increases gradually, the change rate of forward and backward scattering amplitude increases, and the change rate of backscattering is obviously larger than that of forward scattering. The detection of wake bubbles by backscattering has more characteristic changes than that by forward scattering, and the detection success rate is higher. The research results can provide theoretical and experimental support for the design of a ship wake laser detection system.

Modeling the short wavelength infrared laser radiance reflection on the sea surface

The sea surface is a complex dynamic structure dependent on atmospheric conditions, and for which physical and chemical properties change from water to foam. Its roughness determines how the surface reflects, absorbs, and emits radiance, and depends on multiple parameters such as wind speed and direction, and foam and turbulence induced from natural waves or from object displacement. In this paper, a model description is given for laser reflection on the sea surface in open water driven by the wind. The model allows calculation of the reflected laser radiance from the sea surface toward a receiver as a function of the incoming laser radiance with a known beam intensity profile. Each subarea of the sea surface seen by one pixel of the receiver is considered as an ensemble of facets, where each facet is defined by its x and y directional slopes. The wind speed and orientation determine the probability density function of the sea surface facet slope occurrence. In this paper, we have analytically expressed the reflected radiance on the sea surface as a function of the wind speed, receiver range, receiver heading, laser position, laser output aperture, and laser incoming radiance. Using the tolerance ellipse, the reflected radiance expression was approximated, and both direct and approximated results were compared. The richness in behavior of the reflected radiance and its dependence on the geometry of the problem were studied showing the impact of the receiver position, the laser position, heading, and beam divergence.

Polarization reflection distribution characteristics of wakes on the sea surface

Research on the polarization reflection distribution characteristics of wakes on the sea surface can provide a theoretical basis for ocean wake target detection and has important research value in the field of ship and underwater moving target monitoring. The Kelvin wake model and the Cox-Munk model are used to describe a wake on a rough sea surface. Considering the atmospheric Rayleigh scattering and the reflection characteristics of a rough sea surface, a visible spectrum band wake polarization characteristic model based on the Stokes vector and Mueller matrix is established to explore the polarization reflection distribution characteristics of wakes on the sea surface under skylight background at different wind speeds, wind directions, and sun angles. A simulation is done of the airborne polarization reflection imaging of wakes on a rough sea surface. The results show that under the determined observation angle, the polarization distribution characteristics of wakes on a rough sea surface are mainly related to the angle of the sun. The polarization contrast of simulated wakes in typical scenes is acceptable, and it is feasible to detect sea wake targets by the polarization method. The analysis and simulation of the wake polarization characteristics model can provide a theoretical basis for ocean wake target detection.

Optical Design of a Common-Aperture Camera for Infrared Guided Polarization Imaging

Polarization and infrared imaging technology have unique advantages for various applications ranging from biology to ocean remote sensing. However, conventional combined polarization camera and infrared camera have limitations because they are constrained to single-band imaging systems with rotating polarizers and cascaded optics. Therefore, we propose a common-aperture mode based on multi-band infrared guided polarization imaging system (IGPIS) in this paper, which consists of infrared wide-area sensing and polarization features acquisition for accurate detection of ship targets. The IGPIS can provide images in visible polarization (0.45–0.76 μm), near-infrared polarization (0.76–0.9 μm), and long-wave infrared (8–12 μm) bands. Satellite attitude parameters and camera optical parameters are accurately calculated by establishing a dynamic imaging model for guidance imaging. We illustrate the imaging principle, sensors specifications and imaging performance analysis and the experimental results show that the MTF is 0.24 for visible and near-infrared, and 0.13 for long-wave infrared. The obtained multi-band images have an average gradient of 12.77 after accurate fusion. These results provide theoretical guidance for the design of common-aperture cameras in remote sensing imaging field.

Simulation of the interaction of a high energy laser beam with the sea surface in the short wavelength infrared

The knowledge of the interaction of a high energy laser beam with a dynamic sea surface is of great practical interest in maritime environments. The components transmitted into the sea and reflected at the sea surface have to be considered. The calculation of energy transfer into the sea is fundamental to the prediction of upper-ocean heating and temperature-dependent optical properties of the sea, which in turn influence its reflectance characteristics. In addition, the spatial energy (or power) distribution of the laser beam reflected at the dynamic sea surface is also of high significance. For the estimation of the laser light energy reflected into a specific spatial direction, several parameters need to be considered, e.g., wind speed, wind direction, and fetch. The calculated amount of light energy reflected into a specific direction varies statistically and depends largely on the dynamics of the wavy sea surface. A 3D simulation of a dynamic sea surface is presented interacting with a high energy laser beam in the short wavelength infrared spectral band. The simulation computes the upper-ocean heating, the temperature-dependent Fresnel reflectances, and the absorption in seawater considering the laser geometric configuration. For the reflectance calculations, a bistatic configuration of the laser source and receiver is regarded, where the receiver positions are on a virtual hemisphere having the laser spot center as the center point. The specular reflection of the laser beam at the sea surface is modeled by an analytical statistical bidirectional reflectance distribution function of the sea surface. The simulation is restricted to sea surfaces heated to the boiling point to avoid complex phase transition effects between water and gas. For a high energy laser beam focused on a small laser spot on the evolving wavy sea surface, the maximum expected reflected laser power is calculated for the specular forward- and back-reflection direction for glints. The probability of occurrence and temporal occurrence of those glint events is estimated for both directions.