The Study Of Terrain Visibility Method For Secluded Sailing Task Of Underwater-vehicles

Terrain visibility is the quality of terrain to describe whether points on the surface of the terrain can be discovered or not. It has great significance for studying in the ocean, land and heavens. To land and heavens, terrain visibility analysis displays a significant application value in the fields of military operation, tourism, communication, urban planning and other. In these fields, the visibility of terrain is judged based on natural light and electromagnetic wave. But there is no application to analyses the terrain visibility in the ocean, and research schemes of designing are not into system. Actually, the secluded sailing of underwater vehicles has similarities with low-altitude penetration of airplanes both in geographical environment and navigation method. Low-altitude penetration of airplanes is extreme low-altitude flight depending on the blind zone of air defense system and penetrate by follow the terrain and avoid threaten. And the secluded sailing of underwater vehicles takes advantage of ocean-mountain for shielding to avoid discovery of enemy. But radar launches electromagnetic wave, which could not spread much in the water. In fact, the medium of signal in the ocean is sound, of which propagation path is various with electromagnetic wave. Therefore, this paper summarizes a method of terrain visibility analysis applying in underwater, which is based the visibility method of light and regards the principle of passive sonar as study background.First, the paper studies the basic influencing elements and sort of terrain visibility method, analyses and discusses the type of DEM as the material, and then studies the impact factors of analysis process. Then compare with the advantages and disadvantages of the methods considering light as medium, and point out the shortcoming in applying to the underwater environment. Second, aiming at the shortages of light spreading in the ocean, the paper defines the situation of visible from sonar, and studies the properties of acoustic spreading in the ocean, then analyses the acoustic energy the decay mode and the various influence degrees with different submarine surface. Third, combining with the above results and acoustic theory, consider the ray model as visibility analysis model, and deduce the formulas to calculate acoustic ray, then study the method to analysis the terrain visibility based on acoustic ray. At last, simulate the algorithm on both light and acoustic ray, then compare and analyze the results.The results make it clear that, the visibility analysis method on different spreading medium generates great varieties in cumulative visual matrix. And the algorithm on acoustic ray includes two situations that whether to consider the spreading restricting. In this paper, it is the first time to apply the acoustic theory to solve the issue of terrain visibility, which is worth planning the route of underwater vehicle in a certain degree, and the study of underwater terrain visibility has the significance in the field of terrain analysis in future.