Search

The interaction between a moving submarine and seawater generates characteristic wakes on the sea surface, enabling indirect detection of undersea objects via airborne remote sensing. Here, we demonstrate the feasibility of using visible-light polarization imaging to observe submarine wakes. The key links in the imagine chain are considered separately. These include the polarization patterns of skylight, the elevations and slopes of submarine wakes and sea waves, and the changes in the sea surface polarized bidirectional reflectance characteristics due to modulation of gravity–capillary waves by the wake’s velocity field. A complete model of the airborne optical polarization imaging process is constructed and images are simulated via ray tracing. All theories proposed are verified by a series of terrestrial observation experiments. The results show that both the sea surface roughness modulation by the wake’s velocity field and the sea surface slope formed by wake elevation play significant roles in the imaging process. The wake features in the Stokes vector linear polarization component (Q, U) images are effectively enhanced, and the environmental adverse effect on these images is smaller than that on the intensity images. The degree of linear polarization (DoLP) and angle of polarization (AoP) images exhibit acceptable contrast under certain zenith and azimuth angles. Thus, our analysis confirms that airborne optical polarization imaging has considerable potential for observing wakes and other small- and medium-scale ocean dynamic processes.

Temporal noise and spatial non-uniformity primarily limit the measurement precision of division of focal plane (DoFP) polarimeters, based on which this study proposes an error model for DoFP polarimeters. The closed-form expressions of the estimation error of the main polarization parameters (Stokes vector, degree of linear polarization, and angle of linear polarization) are derived. Compared with the existing error models for DoFP polarimeters in the presence of temporal noise, the proposed model modifies the normalization condition in traditional calibration methods of DoFP polarimeters and clarifies the selection rule of the coefficient matrix leading to more accurate precision estimation; and experiments using linearly polarized light on a real-world DoFP polarimeter prove its validity.

Common polarization imaging models are mostly based on an ideal polarizer assumption. This paper proposes a polarization imaging non-ideal model considering the non-ideality of polarizers. The corresponding correction formulas for degree of linear polarization (DoLP) and angle of polarization are also provided. Experiments on linearly polarized light and partially polarized light reflected by a glass plate suggest that when the extinction ratio of polarizers is 100:1, the DoLP relative error of linearly polarized light with the non-ideal model is reduced by 1.87% compared to that with the ideal model; the DoLP relative error of partially polarized light with the non-ideal model is reduced by 1.69% compared to that with the ideal model. Application of the non-ideal model can effectively improve the precision of polarization measurement. In particular, this improvement is more obvious with a low-extinction-ratio (less than 100:1) analyzer.

Submarine detection is a key technology of coastline defense, the direct and indirect imaging to detect submarines both involves solving the problem of detecting the water surface ripple. 3D surface measurement method based on polarization detection can effectively detect the water corrugated surface, in which the detection and calculation of surface polarization characteristics is an important part of surface reconstruction. The model of water surface polarization was established. Visible and infrared waveband of the water polarization characteristic under different water temperature, different meteorological conditions were simulated and analyzed. The results show that the surface polarization is s polarization in the visible waveband and p polarization in the middle and long infrared waveband, degree of polarization first increases with the increase of incident angle and then decreases, moreover the degree of water surface polarization increases with the rising of temperature. The degree of water surface polarization in visible, middle infrared and long infrared waveband was measured by polarization measurement system based on Stokes vector. The results of the simulation were consistent with the measured results, which proved the accuracy of the polarization model used. The characteristics of polarization imaging detection in common wavebands were analyzed, which can provide theoretical simulation and experimental methods for the analysis and calculation of polarization characteristics of water surface.