Abstract:
A 3D rendered image of a radar-scanned terrain surface is provided from a radar return signal from the surface, wherein the return signal includes data indicative of azimuth, elevation, and range of a radar-illuminated area of the surface. The data are processed for transformation into X, Y, and Z coordinates. The X and Y coordinates corresponding to each illuminated area are triangulated so as to create a mesh of triangles representing the terrain surface, each of the triangles in the mesh being defined by a vertex triplet. 3D imaging information (grey scale shading and/or coloring information) is added to each triangle in the mesh, based on the amplitude of the radar return signal from the coordinates represented by each vertex in the triplet and the value of the Z coordinate at each vertex, so as to form the 3D rendered image.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    Not Applicable 
       FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT 
       [0002]    Not Applicable 
       BACKGROUND OF THE INVENTION 
       [0003]    The present invention relates to the field of radar imaging methods. More specifically, the present invention relates to a system and method for radar image rendering. 
         [0004]    Enhanced vision systems are vital in the control of aircraft, especially during take off approach, and landing in adverse conditions. Radar and Electro-Optical Infra-Red (EO/IR) systems are frequently relied upon to provide these capabilities. The effectiveness of these systems greatly depends on the quality of their imaging technology. 
         [0005]    Imaging techniques are well known and widely used in the art. Certain imaging technologies are better suited for certain applications. For example, radar imagery is widely used for navigation, surveillance, and reconnaissance, as well as target tracking and identification. 
         [0006]    Radar imagery is conventionally accomplished by a two-dimensional scan (range and azimuth). An image is rendered from the amplitude of the reflected signals from each resolution cell (azimuth beam width, or step by range resolution length or range step) by assuming all returns are from a flat plane, which allows transforming from range/azimuth coordinates into a level X, Y Cartesian frame. The resulting image is a plan view with image intensity, grey scale shading, color or some combination thereof, in each basic resolution cell related to the radar return level. These images created from a top down perspective are useful in many applications, but suffer from several shortcomings when a view from a different perspective is required, such as, for example, from a pilot&#39;s perspective. 
         [0007]    Conventional radar imaging systems do not provide all three coordinate dimensions (there is no elevation angle measurement) of the location of the basic resolution cell to enable the transformation of data (i.e., the image) to another perspective. Thus, they do not present objects at the proper height in the image, from the pilot&#39;s perspective. 
         [0008]    Some of the current state of the art radar image rendering systems use databases for vertical information. In such systems, the radar sensor location is determined by a precise navigation system, and the two-dimensional image generated, as described above, is registered in absolute coordinates, enabling the use of height data from the database. This approach suffers primarily in two respects: First, there is no capability of detecting objects with a vertical dimension not stored in the database, such as construction towers erected since the database was last updated. Second, the required resolution for some applications is not available, such as is the case when a helicopter is landing in a dust cloud or fog, where a resolution on the order of one foot (30 cm) is required to assure the pilot&#39;s situational awareness. 
         [0009]    Another shortcoming in the current state of the art in radar imaging is the irregular amplitude of returns from visually uniform surfaces due to a phenomenon known as “specular reflection.” Radar imagery traditionally employs relatively long wavelengths of reflected energy (no radiated waves), causing unnatural bright and dim areas in an image of a surface that would appear uniform to the human eye. Since the human eye is accustomed to receiving both radiated and reflected energy from detected surfaces, the reconstructed radar image seems unnatural. 
         [0010]    The current state of the art in radar imaging is unable to provide angular resolution comparable with EO/IR sensors. This lack of resolution causes a very grainy image in the azimuth dimension which, when coupled with the specular reflection characteristics, makes human interpretation of most radar images difficult. 
         [0011]    There is thus a need in the art for an improved system or method to provide images with better resolution, and to present them from a pilot&#39;s perspective rather than the radar location. 
       SUMMARY OF THE INVENTION 
       [0012]    The aforementioned need in the art is addressed by a novel three-dimensional (3D) radar image rendering system and method in accordance with the present invention. (Rendering is the process of generating an image from a model, by means of a software program. In the present application, the model is the description of three-dimensional objects, while the generated image is displayed on 2D computer graphics terminal). The invention provides significant improvement in the usability of airborne radar imaging systems. The illustrated embodiment optimally blends the data acquisition method and rendering techniques to provide pilot-centered, easily interpretable radar images. 
         [0013]    Broadly, the radar imaging system of the present invention employs a 3D radar scan (range, azimuth, and elevation) for the direct measurement of the location of a surface cell (the range of the return for each step in the angle scan), and for the direct measurement of the amplitude of the return from each cell. The availability of all three dimensions for each point allows the transformation of all data into a Cartesian frame (an X, Y horizontal plane and a Z vertical dimension). The X, Y coordinates of all cells causing returns are connected by lines forming triangles by a known triangulation algorithm, thereby creating a 3D “mesh” of triangles describing the detected surface. 
         [0014]    Grey scale shading and/or coloring of the triangular surfaces is then added, based on the radar-determined geometry of the three vertices of each triangle (Z coordinate value or range from a selected a point, for example). The intensity of the grey scale shading or coloring is based on radar return signal amplitude. The result is a simulated or “rendered” 3D surface (on a 2D display) comprising an arrangement of colored and/or shaded triangles approximating the detected surface, with each color or grey scale shading value being a function of radar return amplitude. 
         [0015]    In some applications, it may be desired to weight the triangle color or shading based on the distance from a desired perspective, thereby to enhance depth perception and ridgeline detection. A commercially available software package is then used to transform the 3D surface to the desired perspective (e.g., the pilot&#39;s seat, looking in the direction of the fuselage reference line). 
         [0016]    The data acquisition relies on real time scanning and measurement of the terrain to get accurate information of the topology. This scanning and measurement technique combines radar and navigational data to locate and map vertical obstacles in the target area. The employed graphic animation process allows the presenting of the reconstructed image from a desired perspective, which, in the most cases, is the pilot&#39;s perspective. Viewing a terrain from the pilot&#39;s perspective allows the easiest interpretation of the presented images. The rendering technique employed by the system further enhances usability of the system by providing life-like 3D images. The enhanced image quality is partially derived from more detailed and accurate vertical information. The result is a more natural image, thereby facilitating human interpretation. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIGS. 1A and 1B  depict an aircraft equipped with a radar which scans a field of view to collect terrain data; 
           [0018]      FIGS. 2A and 2B  show exploded views of the region illuminated by a single step in the radar scan (one beam position); 
           [0019]      FIG. 3  is an architectural block diagram of an airborne of 3D radar image rendering system in accordance with the present invention; 
           [0020]      FIG. 4  is a flow diagram showing major functional/processing blocks of an airborne 3D radar image rendering system in accordance with the present invention, 
           [0021]      FIG. 5  shows an X, Y “mesh” of triangles formed by lines describing a detected surface; 
           [0022]      FIGS. 6 and 7  show two 3D meshes of triangles of the same surface from different perspectives; 
           [0023]      FIG. 8A  shows a wireframe image of a surface on a video display; 
           [0024]      FIG. 8B  shows a rendered image of the same surface as  FIG. 7  with triangles filled in, based on geometry only; 
           [0025]      FIG. 8C  shows a rendered image of the same surface as  FIG. 8B  with triangles filled in, based on geometry and radar amplitude; and 
           [0026]      FIG. 8D  shows a rendered image of same surface as  FIG. 8C  in monochrome 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0027]    The radar imaging system described herein employs a three-dimensional (3D) radar scan having range, azimuth, and elevation data components. Data are obtained by direct measurement of a location of a surface cell, range of return for each step in the angle scan, and amplitude of the return from the cell. The availability of all three dimensions for each point allows the transforming of all data into a Cartesian frame (X, Y horizontal plane coordinates, Z vertical coordinate). The X, Y coordinates of all cells causing returns are connected by lines to form triangles, thereby creating a 3D “mesh” of triangles (using prior art techniques) that describe the detected surface, as shown in  FIGS. 5-7 . 
         [0028]    As described above briefly, the detected surface model comprises an arrangement or “mesh” of contiguous triangular areas, the edges of which are straight lines. By using straight lines on the display device to connect the vertices of the triangles for which the position uncertainty is derived primarily from the radar beam width, there is an apparent improvement in resolution as compared to, for example, a mosaic of overlapping circles corresponding to the radar beam width. This effect primarily influences the horizontal dimension, since range resolution effects provide improved vertical resolution. 
         [0029]    During a rendering process, grey-scale shading or coloring of the triangular surfaces is added, based on information extracted from the 3D location of each vertex (such as height, range, slope, etc.). A conventional 2D (X, Y or horizontal image plane) radar image is also formed by projecting all return amplitudes into the image plane. The 2D return amplitude data are used to create a texture map that modulates the intensity of the triangular surface grey scale or color, thereby capturing the additional information from the multiple range resolution cells within a single triangle. The resulting image has three key characteristics: an overall (outline) shape generated from the triangles based on the radar-measured terrain geometry; the color or grey scale shading of the individual triangular areas based on parameters extracted from the position of the three vertices defining each triangle; and the intensity of the grey scale shading or the coloring of each triangle based on return radar signal amplitude. In some applications, weighting the triangle (grey-scale or color) based on distance from a desired perspective point can be used to enhance depth perception and ridgeline detection. A commercially available software package is then used to transform the 3D image to the desired perspective (e.g., from the pilot&#39;s seat, looking in the direction of the fuselage reference line). 
         [0030]    The foregoing 3D radar image rendering method provides imagery based on real time measured elevation angles from the pilot&#39;s perspective, thereby assuring that the elevation angle appears correct in the recently created image. The vertical resolution of the radar imagery is determined by radar range resolution, as opposed to radar elevation angle resolution, with the resulting vertical positioning being based on real time direct angle measurement. The use of geometry variables from the image perspective point helps to minimize the effect of the reflected power from uniform surfaces, thus facilitating the mimicking of human visualizations, which rely on radiated power as well as reflected power. 
         [0031]      FIGS. 1A. 1B ,  2 A, and  2 B show an operating environment of a 3D radar image rendering system. An aircraft  10  is equipped with a radar transceiver system R ( FIG. 1B ) that transmits a radar beam  12  that scans a field of view relative to an image plane  16  having a U-dimension  14  and a V-dimension  15 . The field of view is defined by an azimuth scan angle  11  and an elevation scan angle  21 . The transmitted radar beam  12  has a beam width that is narrower than both the azimuth scan angle  11  and the elevation scan angle  21 . The transceiver R receives radar return signals in a range that is proportional to the radar reflectivity of objects (such as a vertical obstacle  13  and terrain surface  22 ) illuminated within the beam  12  for each range resolution cell  26  ( FIG. 2A ) defined within an illuminated terrain area  17  having a terrain return  23  ( FIGS. 1B ,  2 B). In  FIG. 1A , the regions potentially illuminated by the beam  12  are indicated by the numeral  18 , with the region shadowed by the vertical obstacle  13  being indicated by the numeral  19 . In general few range resolution cells will contain non-zero returns because there are no objects immediately below the aircraft  10 , and no returns are received at ranges greater than the range to the terrain surface  22  (no reflections from below the terrain surface  22 ). 
         [0032]      FIGS. 2A and 2B  show exploded views of the area  17  illuminated by a single beam position, illustrating the multiple amplitude samples at several ranges. For each antenna position, a single range to the terrain or obstacle is estimated, or a single point is determined. A single point is defined by an elevation angle with an elevation resolution  24 , an azimuth angle with an azimuth resolution  25 , and a range estimate  27  consisting of a single selected range resolution cell  26 . 
         [0033]      FIG. 3  shows the major functional/processing modules of an image rendering system  100  in accordance with a preferred embodiment of the present invention. The rendering system  100  includes four functional subsystems: a radar signal processing subsystem  101 ; a terrain geometry processing subsystem  102 ; a terrain image processing subsystem  103 ; and a graphics library subsystem  104 . 
         [0034]    The radar signal processing subsystem  101  performs radar signal processing and data reduction. This processing involves frequency diversity employed to reduce the variation in return signal amplitude from a uniform surface, the selection of large amplitude return signals, and range decimation to reduce the quantity of data. The raw radar return signal amplitudes from transmissions made at different frequencies are first averaged over a time period equal to the time the antenna beam position dwells in one angular step, and the resultant averaged return in each range resolution cell is processed to ignore those range bins containing negligible return amplitudes. The data are further compressed by combining amplitudes over multiple range bins under low grazing angle conditions (long range and low elevation angles). This operation is referred to as range decimation. The decimated averaged range samples are applied to the terrain geometry processing subsystem  102  and the terrain image processing subsystem  103 . 
         [0035]    The terrain geometry processing subsystem  102  calculates terrain geometry. The terrain geometry processing subsystem  102  generates two basic outputs: a 3D mesh of triangles characterizing the terrain surface topology within the radar field of view; and a list of obstacles (points) detected above the terrain surface. 
         [0036]    The radar data is input as amplitude versus time for each angle step (azimuth and elevation) in the raster scan of the field of view. Ignoring noise and returns through antenna side lobes, for each angle step, where the terrain is illuminated by the radar beam, non-zero returns will be received over a range interval. For beam directions not pointing towards the ground (i.e., the terrain surface  22 ), no ground return is received, but returns from obstacles (e.g. the vertical obstacle  13 ) above the terrain surface may be received. The coordinates of each terrain surface point detected during the scan period are defined in a level plane to enable the graphics tool to apply texturing, and a “vertex list,” comprising the coordinates of each detected point and an associated ID number, is constructed. Each resolution cell within the level plane can have only one terrain point (overhanging cliffs and the like are ignored). The lowest point above the level plane detected is declared a terrain point, and any other points with higher elevation values are declared obstacle points and entered on an “Obstacle list”. Entries in the vertex list (terrain points) are input to a triangulation algorithm, which operates on the horizontal coordinates of the points by forming triangles using the well-known principles of “Delaunay” or “Delauney” triangulation. The output of the triangulation algorithm is a list of triples from the vertex list identifying the three vertices of each triangle. 
         [0037]    The terrain image processing subsystem  103  generates terrain image intensity maps. The terrain image path generates a 3D radar image based on the amplitude of radar returns. 
         [0038]    The graphics library subsystem  104  is a commercially available graphics library software package, such as, for example, the OpenGL interactive 3D and 3D graphics application programming interface available from Seaweed Systems, Inc. of Burlington, Mass. (www.seaweed.com). It accepts a 3D mesh of triangles defining a surface geometry (the triangle and vertex lists), color and/or grey scale values for the vertices (the information extracted from the 3D location of each vertex), a texture map defining intensities for the triangular areas (the 3D amplitude-based texture map), and a position and look direction, defining an image perspective. The graphics library  104  takes these inputs typically at the rate of one per second. Then it colors or shades the terrain geometry model surfaces by blending or smoothing the vertex values across the triangular surface area, and it adjusts display intensity per the intensity map (variations in both color or grey scale and intensity within each triangular facet). Intensity is based on radar return signal amplitude. In addition, points and/or other shapes can be added anywhere in the image, a feature that is used to add obstacles (radar returns above the terrain surface) to the terrain image. The created terrain model image is presented from the desired perspective, typically the pilot reference position. The output image is updated at typically, tens of Hz to provide a current image from the moving platform. 
         [0039]    The functions of the above-described functional subsystems  101 - 104  are implemented by several sub-function modules. The algorithms to implement each sub-function are not described, as they may be conventional algorithms that would be easily determined or devised by those of ordinary skill in the art. Accordingly only the sub-functions provided by the algorithms are identified by their respective modules. Each of the sub-functions is performed with a predetermined repetition rate. These rates are:
       r 1  where the rate is defined by c/δR for pulsed radar (where “c” is speed of light and “δR” is the range resolution of the radar);   r 2  where the rate is determined by the repetition period of the pulsed radar or the dwell time for frequency modulated continuous wave radar;   r 3  where the rate is determined by the averaging update interval (several r 2  periods);   r 4  where the rate is the volume scan rate; and   r 5  where the rate is determined the display update rate.       
 
         [0045]    Referring to  FIG. 4 , the radar signal processing function  101  includes an analog-to-digital conversion module  105 , a data averaging module  106 , and a signal selection module  107 . The conversion module  105  converts the analog radar data to digital form for further processing. The processing rate is r 1 . The data averaging module  106  receives the digitized radar data from the conversion module  105 , and it performs data averaging of each angle step period. Averaging returns from individual resolution cells allows use of returns from differing radar frequencies (frequency diversity), which has the effect of reducing the variation in return amplitude from different areas of a uniform object. This method more closely replicates the passive energy collected by the human eye in forming an image. Averaging also compresses the number of data samples that must be processed, thereby reducing data rates and processor loading. Range decimation, described above, further reduces data rates and processor loading. The processing rate of this calculation is r 2.  The signal selection module  107  selects the strongest return signal for further processing. The processing rate of this calculation is r 3.    
         [0046]    The terrain geometry processing function  102  includes several sub-function modules, the first of which is a range estimation module  108  that performs range-to-terrain estimate calculations from the radar data received from the radar signal processing function  101  for each angle step, where the terrain is illuminated by the radar beam. The processing rate of this calculation is r 3.    
         [0047]    A range filter module  109  combines estimated range values and predicted range values to arrive at the range, azimuth, and elevation coordinates of the point on the terrain surface toward which the radar beam is currently pointed. The estimated range data are obtained from the range estimation module  108 , while the predicted range values are obtained from a predicted range module  113 , described below. The processing rate of the range filter module  109  is r 3.    
         [0048]    A transform to terrain model coordinates module  110  takes the radar data (azimuth, elevation, range), the aircraft position (latitude, longitude, altitude), and the aircraft attitude (heading, pitch, roll), and computes the three dimension X, Y, Z position in a terrain model coordinate system. This also includes projecting the X, Y, Z position coordinates onto the image plane to compute the texture map U, V “coordinates”. (The U, V “coordinates” are standard, normalized values allowing the graphics library  104  to register correctly the texture map onto the terrain surface.) The processing rate of this calculation is r 3.    
         [0049]    A terrain test module  111  scans the set of 3D points to segregate a subset of points defining a geometric terrain (no points overhanging any other points)—the vertex list—from other points—the obstacle list—. The obstacle list is converted into a set of points and/or shapes for input to the graphics library  104 . The terrain test module  111  outputs the list of vertices and the obstacle data to the graphics library  104 . 
         [0050]    Referring again to  FIG. 2A , the expanded view of the region illuminated by a single beam position illustrates that, for practical radar parameters, the range resolution provides much better resolution than does the resolution imposed by either the azimuth or elevation antenna beams. Modulating the intensity of the terrain geometry model by mapping the texture map derived from radar amplitude data onto the surfaces provides a computationally efficient mechanism to retain the significant improvement in vertical resolution provided by the range resolution, as opposed to that provided by the elevation beam width. The multiple range cells within each beam position can be thought of as a means to interpolate within a single elevation beam sample.  FIG. 2B  shows the region where the main beam intercepts the ground. The fine lines within the antenna beam width represent the improvement in vertical resolution. The processing rate of this calculation is r 4 . 
         [0051]    Referring again to  FIG. 4 , a 3D triangulation module  112  is a real-time implementation of an algorithm published in the open literature and well-known to those of ordinary skill in the pertinent arts. A suitable example is described in “A Sweepline Algorithm for Voronoi Diagrams,”  Algorithmica , Vol. 2, pp. 153-174 (1987), the disclosure of which is incorporated herein by reference. The triangulation module  112  accepts declared terrain points as input from the terrain test module  111 , and it operates on the horizontal coordinates of the points by connecting adjacent points to form triangles. The output of the processing is a list of triples from the vertex list identifying the three corners of each triangle. The processing rate of this calculation is r 4 . 
         [0052]    The predicted range module  113  calculates the predicted range by combining the current position, supplied by a navigation system (such as GPS/INS), with the last terrain model provided by a coordinate transformation module  114 . The processing rate of this calculation is r 3 . The coordinate transformation module  114  provides coordinate transformations on the terrain geometry model in earth-fixed coordinates received from the terrain test module  111  (vertex list) and the triangulation module  112  (vertex triples or triangle list). The predicted range module  113  accepts the earlier terrain model information in current coordinates and calculates a predicted range used to refine the current range estimate. The processing rate of this calculation is r 4 . 
         [0053]    A summing module  115  adds a perspective offset to position data supplied by the GPS/INS navigation system to represent actual position. 
         [0054]    The terrain image intensity function  103  includes an image transformation module  116 , an amplitude-summing module  117 , and an intensity-mapping module  118 . The image transformation module  116  transforms the location of each radar return from the moving radar coordinate system to the earth-fixed frame, and projects (via the summing module  117 ) all data into a horizontal area in the earth-fixed coordinate system (the image frame). The processing rate of this calculation is r 3.    
         [0055]    The intensity mapping module  118  forms the texture map used by the graphics. The illuminated area  17  of the image plane  16  ( FIGS. 1A and 1B ), illuminated by the current radar scan, is divided into 2″ by 2″ bins. The radar return signal amplitudes of all returns above each bin are summed and projected into a single bin. The processing rates for the amplitude-summing module  117  and the intensity mapping module  118  are r 3  and r 4 , respectively. 
         [0056]    The graphics module  104  performs the actual image rendering as described briefly above. The inputs defining the terrain include the 3D mesh of triangles (vertex and triangle lists) with associated values for color or grey scale for each vertex, and a texture map defining intensity. Obstacle inputs can be in the form of points or another independent pair of vertex/triangle lists, causing 3D surfaces to be drawn above the terrain surface (akin to hanging sheets of paper oriented perpendicular to the look direction). An image perspective is defined by a location and the look direction. During a rendering process, grey-scale shading or coloring of the triangular surfaces is added by blending the values assigned to the vertices across each triangular area with the intensity being derived from the radar return signal amplitude. The result is a 3D surface consisting of triangles approximating the detected surface, where the grey-scale or color within each triangle varies to match values calculated for the three vertices based on vertex height or another geometrical relationship (e.g., range from the image perspective), and the intensity varies with the amplitude of the radar return signal. In some applications, weighting the triangle (grey-scale or color) based on distance from a desired perspective point can be used to enhance depth perception and ridgeline detection. The graphics module  104  is used to transform the 3D image to the desired perspective (e.g., from the pilot&#39;s seat, looking in the direction of the fuselage reference line).  FIGS. 8A-8D  illustrate the results of the rendering process, as displayed on a video display screen  200  in the aircraft  10 . 
         [0057]    The foregoing 3D radar image rendering system provides imagery based on real time measured elevation angles from the pilot&#39;s perspective, thereby assuring that the elevation angle appears correct in the recently created image. The vertical resolution of the radar imagery is determined by radar range resolution, as opposed to radar elevation angle resolution, with the resulting vertical positioning being based on real time direct angle measurement. 
         [0058]    The use of geometry, elevation, and range from the image perspective point in addition to radar return helps to minimize the variation in the reflected power levels from uniform surfaces. This geometry data is used to make up for the missing radiated power from radar data and thus facilitates better human visualizations, which relies on both radiated power and reflected power. Furthermore, the present invention improves image resolution, realized by drawing sides of the triangles between points on the reflecting surface, as opposed to the use of fixed-size angular pixels related to radar antenna beam width.