Navigation method for a missile

NAVIGATION METHOD FOR A MISSILE

Pearl Cohen Zeoek Latzer

P-74903-IL 211883/2

Navjgajwnaetlwdforjumilile

Specification

The invention relates to a navigation method for a missile, which comprises an infrared seeker head, an inertial navigation system (INS) and a device for the comparative evaluation of images just recorded with stored reference images, wherein in the evaluation milicUy a multiplicity of Three Dimensional (3D) fines are produced for the three-dirnc nssonal reconstruction of the detected scene from the recorded images, and these liner are compared, taking into consideration the perspective of the reference available, and the current location and position of the missile is determined therefrom.

Various methods are known for the navigation of a missile. This includes first of all the use of a satellite navigation system (GPS ~ Global Positioning System), which renders possible a precise navigation only as long as the communication with the corresponding sjolti !- < curbed.

Furthermore, inertial navigation systems are known, the precision of which decreases with inc--: -muz night distance, however. The combination of the two referenced systems does not rest:it in the desired uninterrupted guidance along a specific flight route in every exemplary con figuration, either.

It. Is therefore necessary to use a further system which processes image material recorded before the respective mission and provides it as a reference in the navigation computer of the missile Reference structures of this type can. be obtained from maps, satellite images or aerial photographs.

During tbi 'Hussion, for example, the seeker head takes over, which is directed from above at die flight path of the missile and continuously generates images, which are processed in the navigation computer of the missile and compared to the data of the aforementioned systems, it is customary io use infrared cameras for this purpose. 211883/2

From DE .:0 2007 054 950 Al a method is known for supporting the independent navigation of a missile having a forward looking camera and an inertial navigation system (INS), which compares the data from the current scene recorded by the camera to the reference Images available in the missile, in particular, from the recorded images by means of a 3D reconstruction a multiplicity of 3D lines are determined, which are projected in the perspective of the available reference images. Subsequently, by aligning with the reference, the current position and location of the missile is determined and the navigation ’bus supported, The alignment with an optical image is thus carried out. It is therefore sufficient to project the 3D image generated from the camera image into a reference image. According to the object, this specification is specifically related to the use of a camera designed for the infrared range or for fhe visible range and the processing of the date thereof. No reference is made to the use of systems that operate in other frequency ranges or to the special processing of the data of systems of this type.

To an increasing extent reconnaissance systems are used that operate with a radar with synthetic reporUr.-c; (SAR). SAR sensors are powerful systems for long-distance reconnaissance. They are all-weather capable and provide a high spatial resolution even for objeeU a: a great distance. However, due to the special characteristics of this imaging principle, the interpretation of SAR images is difficult and very different from convcntioi ai image material, DE 69306069 T2 describes a method that aligns a reference image to the SAR image of the miokw vn ordev to thus support the navigation of the missile. This is achieved essentially torough the correlation of fhe SAR image with the reference image. Ultimately, with this method only a Two Dimensional - Two Dimensional (2D-2D) alignment is carried out. With respect to the treatment of the known SAR artifacts, it is mentioned only that they lead to a slight deformation of the geometry of the image. In an oversiinpidfealion it ic assumed that, despite this deformation, a correlation between the SAR imago and the reference image leads to a correct localization of the missile and any errors in .'dtifvve that occur can subsequently be treated with methods of interference calc Ή . .ever, as soon as fhe overflown scene has raised objects, such as high building’.. Uc. die deformation of the geometry is no longer slight and a 2D-2D alignment -UH lead to imprecise results. 2

The object of the invention is therefore to propose an expansion of a known navigation system for operation with reference images generated on an SAR basis, which is able to correctly resolve scenes with large differences in height.

This object is attained according to the invention in that SAR (Synthetic Aperture Radar) images are used as a reference, which originate from an SAR reconnaissance system which at an earlier time and with known trajectory detected and saved the data of the overflown terrain and from which reference images composed of 3D line segments are generated, wherein together with the trajectory data associated with the respective image they are provided as a reference for the navigation method of the missile, and that from the images of the infrared seeker head homogeneous areas bounded by 3D lines and the spatial position thereof are obtained, wherein, taking into consideration the 3D structure of the overflown scene for each detected 3D object the corresponding azimuth position and range position arc calculated, and wherein the layover effects of the SAR images are modeled and, by means of analysis of the masking of 3D areas, the SAR shadow edges are generated in the virtual reference edge image, and that the virtual SAR images originating from the images of the infrared seeker head and generated from the calculations are compared to the reference edge images available as a reference and the trajectory data by means of a comparative evaluation, and continuously updated position data and location data of the missile are calculated therefrom.

Typical SAR artifacts are the reason that SAR images cannot simply be processed with the methods of the evaluation of an infrared image. An SAR image differs from an infrared image in that for each pixel the direction and the distance to the pixel, not location coordinates, are measured. As long as a scene is flat, this leads to images of equal quality, since in this case there is a clear connection between aspect angle and distance.

However, this situation changes when the scene is no longer flat or, for example, contains higher buildings. In this case, the SAR image generates from the individual points of the building facade a mirror image folded onto the ground, because these points lie at the same distance as their mirror image on the ground. This is referred to as the layover effect. 3

Another artifact is the so-called SAR shadow. This occurs when due to the shielding of other objects in a certain distance range no reflections occur. These SAR shadows often form clearly visible edges in SAR images, but have no counterpart in the optical image. These shadows mean that a direct alignment of SAR image and optical image leads to errors for scenes with raised objects.

The methods cited in the above-referenced specifications, in which optical images and SAR images are compared directly with one another, consequently lead to a systematic positioning error.

The particular advantage of the invention can be seen in that for the first time a missile with infrared seeker head is using the image data of a high-precision SAR reconnaissance system as reference images, whereby it has been possible to clearly improve the precision of the navigation compared to previous systems.

One exemplary embodiment of the invention is shown diagrammatically simplified in the only figure of the drawing and is described in more detail below.

The drawing initially shows two functional switching circuits. The one with reference character SAR-A stands symbolically for an SAR reconnaissance system which, at an earlier time and with known trajectory, has detected and stored the data of the terrain over which the mission of the missile FK is to take place. The missile FKis shown with its components essential for the invention as a second functional switching circuit.

The reconnaissance system SAR-A records an SAR image and extracts therefrom a multiplicity of line segments, which together produce the reference edge image RKB. This reference edge image RKB is provided to the missile FK for the mission as a reference, as is indicated by an arrow.

Furthermore, in the reconnaissance system SAR-A, the data of the sensor trajectory ST are recorded and likewise transmitted to the missile FK. This is likewise shown by an arrow.

During the mission, the camera installed in the seeker head of the missile FK continuously records images of the scene below the missile, wherein the camera as a rule is directed obliquely forwards. The infrared images 1RB-FK generated therewith are 4 subsequently processed. The 3D structure of the scene below the missile FK is calculated thereby by means of a 3D reconstruction, as is described in DE 10 2007 054 950 Al. The result is a multiplicity of 3D line segments LS, which correspond to the edge images of the scene.

At the same time, in the infrared images IRB-KFK homogeneous areas, which are bordered by the found line segments, are extracted by means of a computing method. A method of this type is known under the name Region Growing. The 3D structure of the line segments that surround the areas is used to determine the 3D location of these areas. The result of this reconstruction process is a multiplicity of line segments LS and 3D areas l-IFL. A virtual SAR image VSAR-KB is then determined from this. The stored data of the sensor trajectory ST are taken into consideration hereby. This 3D structure makes it possible, together with the known trajectory of the reconnaissance system SAR-A and a hypothesis originating from the internal inertial navigation system for the current position of the missile FK, to determine a hypothesis for the 3D structure of the SAR image. To this end, the found 3D lines are projected into the SAR plane, taking into consideration the layover effect. The 3D areas HFL and their location are used to find the position of the relevant geometric effects such as shadow edges and corner reflectors. The result is then a virtual SAR image.

With the aid of a method for line matching, the virtual SAR image VSAR-B is then aligned in the next step A with the SAR edge image RKB stored as a reference. From the disparity of the two images a correction KORR of the position hypothesis PI-1 of the missile FK can then be calculated. The navigation of the missile is thus supported.

It is crucial with this method for converting an infrared image into an SAR image that the geometric characteristics typical of SAR are taken into consideration in the same manner as if an SAR image just recorded is compared to a stored SAR reference image. This is the only way that the advantage of the high resolution can be utilized in the SAR evaluation. 5 ©

List of reference characters A Evaluation method FK Missile HFL Homogenous area INS Inertial navigation system IRB-FK Infrared images of the FK seeker head KB Edge image with 3D lines KOR.R Correction LS Line segment PH Position hypothesis RKB Reference edge images SAR Synthetic Aperture Radar SAR-A SAR reconnaissance system SAR-B SAR images ST Trajectory data VSAR-B Virtual SAR image 3D-FL Line segments in 3D obtained from infrared image 3D-LS Areas in 3D obtained from infrared image 6