Gravity Gradient For Submarine Threat Object Detection

Most of the time submarine has to navigate underwater in the need of military operations. Safety and concealment of underwater navigation is the precondition of submarine to play its extraordinary combat power. Factors affecting the safety of submarine include anti-submarine plane from the air, anti-submarine ship on the surface, underwater enemy submarines, as well as wrecks in the shallow sea and the reef uncharted on charts and other obstacles on the submarine channel.Currently submarine underwater mainly use sonar for surrounding environment perception. For surface ships, underwater submarines and other threatening objects, we can use the passive sonar in detection; for wrecks, reef and other obstacles on channel, we need to use active sonar and emit acoustic signal in detection; as for aerial objects, submarine has to float to the surface to make detection.When using active sonar in detecting aerial and surface objects, the concealment of submarine will be damaged. Passive sonar can guarantee the concealment of submarine, but it cannot detect reefs, wrecks and other obstacles that are unable to emit signal. So for submarines, looking for a kind of real-time, passive, accurate object-detecting method is of vital importance. Inspired by the application of gravity exploration in search for oil, gas and solid mineral, this dissertation, in order to meet the demand of underwater submarine object detection and considering the advantage of gravity gradient compared with gravity in object detection, launched a study on the application of the gravity gradient in object detection. Specific content is as follows:In the chapter of gravity gradient reference map preparation, the preparation method of reference map based on Digital Elevation Model (DEM) is firstly introduced. When there are only terrain data, this method can rapidly prepare the local area gravity gradient reference map needed for object detection. Considering that the method ignores the influence of the terrain density, this dissertation introduced the isostasy model and proposed a new gravity gradient reference map preparation method, which is suitable for wide-range and large-scale reference map preparation. Because isostasy model is a rough estimate of large-scale terrain density, in order to meet the demand of high precision and high resolution in underwater object detection, another reference map preparation method based on adaptive gravity gradient anomaly ratio was proposed. This method overcomes the shortcoming of inaccurate estimation of terrain density in previous reference maps, thus making reference map prepared closer to the real situation.When using gravity gradient for underwater object detection, the result will be inevitably affected by external environment, and the wave of the sea is the biggest interference factors. This chapter first introduced the simulation method of ocean waves, then simulated the waves of different sea condition, and carried on the quantitative calculation of the disturbing gravity gradient produced by the waves in different measurement point. The results will provide effective data support to object detection and navigation.When submarine navigates in waters where gravity gradient or terrain is known, because the gravity gradient reference map of the area can be advanced prepared, we can directly use the gravity gradient anomaly caused by object to detect the object. In this chapter, the traditional algorithm named full tensor gravity gradient inversion algorithm was firstly introduced, then according to the defect of this algorithm in the acquisition of the mass of object, a new algorithm of joint gravity and gravity gradient anomaly was put forward. This algorithm solved the problem that there existed serious error in detection results when using the traditional algorithm to inverse the mass of object due to the uncertain integral range. Using in the known area, this algorithm can detect underwater and aerial objects real-time and accurately, providing powerful guarantee for the safety of the submarine’s navigation.If the submarine has to enter the unknown waters due to military or other reasons, this chapter put forward an object detection method based on gravity gradient difference ratio. The method uses the value differences between the adjacent gravity gradient record points in the submarine route, cleverly getting the location and mass of the object by inversion, bypassing the rigid demand for the reference map.