Abstract:
Navigational systems and methods are disclosed herein to provide accurate processing of two-dimensional sensor images for navigation and determination of position in three-dimensions. Landmark features are extracted from two-dimensional sensor images and correlated to landmark features data stored in a database to estimate latitude, longitude, and altitude of a vehicle or missile.

Description:
TECHNICAL FIELD  
       [0001]     The present invention relates generally to guidance systems and methods and, more particularly, to guidance systems and methods for missiles, maneuvering re-entry vehicles (MaRVs), cruise missiles, and unmanned aerial vehicles (UAVs).  
       BACKGROUND  
       [0002]     Increased accuracy for vehicle navigation, missile guidance, and precise targeting, is in high demand by manufacturers and governments alike.  
         [0003]     Radar devices, such as a precision terrain-aided navigation (PTAN) radar system, in conjunction with digital terrain elevation maps which provide stored latitude, longitude, and elevation data for a given area or terrain, have been previously utilized to determine a vehicle position and assist navigation. Similar sensors for missile guidance systems, such as and lasar radars (ladars), that are capable of mapping in three dimensions, may be used in determining a missile position.  
         [0004]     However, certain terminal sensors, such as visible (optical) and infrared imagers, provide only two-dimensional sensor images. These sensors are incapable of determining terrain elevation data and thus cannot be employed in a scheme, such as PTAN, that matches stored terrain elevation data to sensor data.  
         [0005]     As a result, there is a need for systems and methods for processing two-dimensional sensor data to accurately determine position and aid in navigation.  
       SUMMARY  
       [0006]     Systems and methods are disclosed herein to provide accurate processing of two-dimensional (2-D) sensor images for navigation and determination of position in three-dimensions (3-D) for flying vehicles such as MaRVs, cruise missiles, and unmanned aerial vehicles (UAVs). For example, in accordance with an embodiment of the present invention, landmark features, such as roads, rivers, coastlines, and buildings, are extracted from two-dimensional sensor images and correlated to landmark features data stored in a database via pattern recognition techniques to thereby estimate latitude, longitude, and altitude of the vehicle.  
         [0007]     More specifically, in accordance with one embodiment of the present invention, a method of navigation is provided, comprising processing imagery to extract a first set of landmark features with a first extraction algorithm, processing imagery from an image sensor with a second extraction algorithm to extract a second set of landmark features, and correlating the second set of landmark features to the first set of landmark features to estimate a location of the image sensor.  
         [0008]     In accordance with another embodiment of the present invention, a method of navigation is provided, comprising selecting a target aimpoint, processing pre-existing imagery to extract a first set of landmark features with a first extraction algorithm, providing a landmark feature database including the first set of landmark features, and processing imagery from an in-flight image sensor with a second extraction algorithm to extract a second set of landmark features. The method further includes correlating the second set of landmark features to the landmark feature database to estimate a latitude, longitude, and altitude of the image sensor.  
         [0009]     In accordance with yet another embodiment of the present invention, a method of navigation is disclosed, comprising providing a database, processing two-dimensional image data from an airborne sensor, comparing the two-dimensional image data to the database, and determining a three-dimensional location corresponding to the two-dimensional image data.  
         [0010]     In accordance with yet another embodiment of the present invention, a navigation system is provided, comprising a database including a first set of landmark features extracted from pre-existing imagery, an image sensor to provide in-flight imagery, and a processor operably coupled to the database and the image sensor. The processor is operable to process imagery from the image sensor to extract a second set of landmark features, and to correlate the second set of landmark features to the first set of landmark features to estimate a location of the image sensor.  
         [0011]     The scope of the invention is defined by the claims, which are incorporated into this section by reference. A more complete understanding of embodiments of the present invention will be afforded to those skilled in the art, as well as a realization of additional advantages thereof, by a consideration of the following detailed description of one or more embodiments. Reference will be made to the appended sheets of drawings that will first be described briefly.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1  shows a block diagram illustrating a navigation system for processing two-dimensional sensor images in accordance with an embodiment of the present invention.  
         [0013]      FIG. 2  shows a flowchart illustrating a method of navigation by processing two-dimensional sensor images in accordance with another embodiment of the present invention. 
     
    
       [0014]     Embodiments of the present invention and their advantages are best understood by referring to the detailed description that follows. It should be appreciated that like reference numerals are used to identify like elements illustrated in one or more of the figures. It is also noted that the figures are not necessarily drawn to scale.  
       DETAILED DESCRIPTION  
       [0015]      FIG. 1  shows a block diagram illustrating a navigation system  100  that processes two-dimensional sensor images in accordance with an embodiment of the present invention. System  100  includes a central processing unit (CPU)  102 , a storage medium  104  including a database  106 , read-only memory (ROM)  108 , and random access memory (RAM)  110 , each element operably coupled to a data bus  112 . An image sensor  114  is also operably coupled to data bus  112  via a communication interface  118 . System  100  may further include a user interface  120  and an output communicator  122  operably coupled to data bus  112 .  
         [0016]     Image sensor  114  provides data, including but not limited to two-dimensional sensor images for navigation of a missile system, to other elements of system  100  through communication interface  118 . Image sensor  114  may include a digital camera, a visible (optical) imager, and/or an infrared imager. Image sensor  114  may also include other means for providing digital images and/or means for converting images to digital images.  
         [0017]     Optionally, an additional navigation system data receiver  116  may also provide data from other navigation devices to system  100  through communication interface  118 . Additional navigation system data receiver  116  may receive data from navigation devices such as a global positioning system (GPS) using a satellite-based navigation system, a direction sensor, and/or other sensors for providing navigational data. In one embodiment, communication interface  118  is also a communication link between system  100  and a remote base station (not shown).  
         [0018]     Data through communication interface  118  is transmitted to CPU  102 , which may perform multiple functions, including but not limited to calibration, signal processing, image processing, correlation, and/or guidance functions.  
         [0019]     Landmark feature database  106  stored in storage medium  104  includes extracted objects and features from pre-existing imagery, taken from a previous survey in one example. Database  106  comprises positional data, such as latitude and longitude coordinates, to describe landmark feature positions. Database  106  may further comprise other data representing identification and characteristics of the landmark features, including but not limited to range-dependent data such as size, length, width, and volume, and range-independent data such as color, perimeter squared over area, length over width, and shape. In a further example, database  106  may further include modeling information, map information, and image sensor information. Storage medium  104  may comprise non-volatile memory, a hard disk drive, CD-ROM, or an integrated circuit in which digitized landmark feature information has been stored. Other types of storage media are within the scope of the present invention.  
         [0020]     Software for directing the operation of system  100  may be stored in main memory  108  for execution by CPU  102 . Memory  108  may comprise any of a wide variety of non-volatile memory such as, for example, read-only memory (ROM), reprogrammable non-volatile memory such as flash memory or SRAM, CD ROM, optical disk, or PCMCIA cards. System random access memory (RAM)  110  permits reading and writing of the information necessary to execute such software programs.  
         [0021]     Input and output means of system  100  include a user interface  120  and an output communicator  122 , respectively. The user may input data, such as a target aimpoint, through user interface  120 . User interface  120  may be in a remote location from the vehicle or missile in one embodiment. Output communicator  122  receives data processed by CPU  102  and may transmit vehicle location data to vehicle navigation means and/or to the remote base station (not shown).  
         [0022]     A method of navigation in accordance with an embodiment of the present invention will now be discussed in greater detail with reference to both  FIGS. 1 and 2 .  FIG. 2  shows a flowchart illustrating a method of navigation  200  by processing two-dimensional sensor images.  
         [0023]     A target aimpoint is selected in step  210 , and in one embodiment may be entered via user interface  120 .  
         [0024]     Imagery, from various sources, of an intended area of operation (AOO) is then processed by a feature extraction algorithm or algorithms, as shown in step  212 . Various techniques, such as contrast enhancement or cellular automata for linear or circular features, can be used to identify and extract features, including but not limited to roads, rivers, coastlines, and buildings.  
         [0025]     Landmark features may be extracted from imagery through the use of various known techniques, alone or in combination, such as “cellular automata”, “contrast enhancement”, and/or the Modified Iterated Hough Transform (MIHT) useful in finding “linear” (one-dimensional) features, such as coastlines, rivers, roadways, canyon, or crater rims. “Areal” (two-dimensional) features such as buildings, lakes and reservoirs, and forests, are usually found by examining various thresholds on intensity or color (when color imagery is available) or by looking at linear features that form closed boundaries. Different extraction techniques may also be available for different landmarks.  
         [0026]     To be useful, each extracted feature must have a “point” associated with it. For linear features, points can be extracted from intersections of various linear features (e.g., a river and a road) or from bends, loops, or bifurcations (e.g., a river with an island shows a bifurcation—the stream divides to go around the island) in the linear feature. For areal features the point assigned is usually the centroid of the area. Each point is then identified by its location, for example, its latitude, longitude, and altitude.  
         [0027]     Identified landmark features extracted from existing imagery are then used to create a database, as shown in step  214 . The identified features populate a database, such as database  106 , which is stored in a memory, such as storage medium  104 . Such a landmark feature database is then used for subsequent comparison with in-flight sensor data obtained during vehicle/missile flight (step  218 ).  
         [0028]     The landmark database, hereinafter also referred to as the landmark “correspondence point” data set, includes information to identify correspondences between existing imagery and imagery obtained from the vehicle&#39;s imaging sensor. In one example, the database can include but is not limited to a cataloging number or name, the correspondence point location (e.g., latitude, longitude, and altitude), and the feature type (feature “primitive”—an image element from which more complicated images can be constructed) or technique used to extract the feature. A set of vectors pointing to the nearest neighboring features may also be included in one embodiment.  
         [0029]     The two-dimensional (2-D) sensor images during vehicle flight are processed by the same or substantially similar algorithm(s) used to extract features used to populate the landmark feature database, as shown in step  216 . In operation, in-flight imagery from the vehicle&#39;s sensor is subjected to the same or substantially similar type of feature extraction as was performed to identify features in the imagery used for the correspondence point data set. However, the location of each correspondence point from the in-flight imagery is not known.  
         [0030]     The features extracted from the sensor images in step  216  are then compared with and correlated to the stored landmark feature database to determine the position of the vehicle and/or the vehicle&#39;s position relative to the intended target aimpoint, as shown in step  218 .  
         [0031]     By examining, correlating, and associating the correspondence points from the in-flight imagery with the correspondence points in the database, a registration of the two images may be made based on similarity measures, which include but are not limited to a maximum likelihood estimate considering identification items such as the feature primitives and vectors to the nearest neighbors, and/or a correlation function, which correlates pieces of the vehicle imagery with pieces of the original imagery near each correspondence point. Registration by correlation function requires that the imagery be transformed to a common viewpoint, scale, and orientation, or the correlation may be quite complex requiring either large processing power or processing time. However, the MIHT method reduces the need for image transformation to a common view. Once the registration is made, the location information (e.g., latitude, longitude, and altitude) from the correspondence point data set may be identified with the feature points extracted from the vehicle&#39;s in-flight imagery.  
         [0032]     Then, as shown in step  220 , the viewpoint from which the image was taken can be estimated using the ordinary trigonometric calculations of projective geometry (e.g., utilizing scale and orientation of the landmark features) knowing the locations of the various features extracted from the in-flight imagery. This viewpoint estimate may include the location (e.g., latitude, longitude, and altitude) and view angles (azimuth or compass angle and elevation or depression angle) of the image sensor. Accordingly, a 3-D projection based on 2-D images is provided by perspective transformation in accordance with an embodiment of the present invention.  
         [0033]     Next, a decision may be made to either refine the viewpoint estimate or not, as shown by decision block  224 . If no, the process ends at step  228 . If yes, an estimate of position, orientation, and velocity may be refined by taking a series of images as illustrated by loop  221 , and by fitting the data from the series of images to an assumed curve of location, as shown by step  226 . Normally this curve will be limited to a simple 3 rd  or 4 th  order polynomial with terms for initial position, velocity, acceleration, and perhaps jerk, so that a plurality of images (e.g., any set of more than four images) can be used to refine the estimates of the location curve terms (e.g., position, velocity, acceleration) through a least squares fit or other estimating procedure.  
         [0034]     Optionally, as shown by block  222 , data from an additional navigation device, such as a GPS, a direction sensor, an inertial sensor, and/or other sensor, may be included with steps  220  or  226  to increase the accuracy of the estimated location terms including position, velocity, and acceleration.  
         [0035]     Embodiments described above illustrate but do not limit the invention. It should also be understood that numerous modifications and variations are possible in accordance with the principles of the present invention. Accordingly, the scope of the invention is defined only by the following claims.