The development of space-based air target detection against sea surface is conducive to improving the country's ocean sensing capability and providing an effective way for the refined detection and identification of distant sea targets. The traditional infrared detection method is greatly affected by environmental factors and is prone to problems such as target submersion in the background. In the case of low contrast between target and background infrared radiation intensity, the combination of polarization characteristic difference with infrared intensity detection can significantly improve the detection and identification capability of the system. With the continuous development of infrared multi-band imaging and guidance technology, how to use the polarization characteristics and select the band to be used to improve the target detection capability has become a difficult issue in space-based air target detection against sea background. In order to improve the infrared target detection performance, in this study, we model the infrared band polarization detection link of airborne targets in the sea surface background, combine the polarization calculation principle, compare the difference of polarization between target and sea surface in different bands, and analyze the effect of target temperature, height, and detection pitch angle on improving the contrast of target/sea surface polarization. The research results can be helpful to improve the infrared polarization detection capability of space-based sea surface air targets.

In this study, simulations are performed to compare and analyze the target and background in the band of 0.7-12.0 μm in combination with polarization. Firstly, the radiation transmission link of the space/sea-based airborne target detection system is established by combining the reflection model and spontaneous radiation model, and the relevant environmental parameters are obtained by using MODTRAN software simulation. Then, the Sellmeier formula is combined with the reflectivity calculation method to calculate the polarization of the target and the sea surface and explore the relationship among the polarization and wavelength, temperature, detection pitch angle, and height. The difference between the polarization of the target and the sea surface is further calculated to evaluate the detectability performance of the air targets against sea surface. The optimal detection band is clarified by simulation experiments on aircraft target and sea surface. Finally, based on the calculation model of the infrared polarization characteristics, the sensitivity of the wide-band polarization degree of the target and the sea surface to the target temperature, height, and detection pitch angle is explored at a solar altitude angle of 30°, a detection azimuth angle of 0°, and a flight speed of 272.24 m/s. The relationship among the vertical polarization component, the horizontal polarization component, and wavelength is analyzed by combining the variation law of polarization of the target and the sea surface. In addition, the parameter settings to improve the detection performance at the best wavelength are clarified by considering the atmospheric path, solar azimuth, and other factors.

According to the infrared characteristics of the target and the background change, it is found that there are two peak intervals of differential polarization between the target and the sea surface. By considering the influence of the atmospheric window on the radiation intensity, the atmospheric transmittance at 5-8 μm is low, which is not conducive to detection. By using the 2/2 peak position as the detection band selection criteria, the detection band of 8.56-10.08 μm is conducive to improving the contrast between the polarization of the target and the background and enhancing the polarization detection performance (Fig. 2). The effects of the target temperature, target height, and detection pitch angle on the detection performance are explored in different wavebands. Firstly, as the temperature difference between the target and the sea surface gets larger, the target/sea surface differential polarization degree becomes greater. In addition, the sensitivity of target/sea surface differential polarization degree to temperature is different in different wavelength bands (Fig. 4). Secondly, as the target height gets higher, the polarization gets smaller (Fig. 5). The sensitivity of target/sea surface differential polarization degree to target height varies in different wavelength bands (Fig. 6). Thirdly, the polarization degree of the target and the sea surface decreases when the detection pitch angle decreases, although the space-based detection platform is closer to the target and the sea surface in terms of distance (Fig. 7). The detection pitch angle increases in the narrow band of 8.56-10.08 μm, which makes the detector close to the ground-skimming state and is conducive to improving the polarization detection effect. In the actual detection, the atmospheric path, solar azimuth, and other factors should also be taken into account to ensure that the selection of the appropriate large angle in the narrow band of 8.56-10.08 μm can achieve good detection results (Fig. 8).

For the problem that the best detection band of air target infrared polarization detection against sea surface is unknown, the influence of wavelength on polarization degree is theoretically verified and analyzed, and the simulation experiment of wide-band space-based polarization detection imaging against sea surface is carried out. The influence law of target temperature, target height, and detection pitch angle on target and background polarization characteristics is explored in combination with the polarization degree calculation method. On the basis of the significant variation of the infrared polarization characteristics of the target and the background in each band, the best detection band can be determined to be 8.56-10.08 μm, and the vertical polarization component dominates. In the best detection band, increasing the temperature difference between the air target and the sea surface, reducing the flight height of the target, and selecting a larger angle of detection pitch are conducive to enhancing the polarization detection capability of space-based air targets against sea surface. This study provides a useful reference for studying the geometric sensitivity of infrared polarization radiation characteristics of air targets against sea surface and promoting the development of multi-band target stealth and identification technology.