Abstract:
An avionic weather radar system tracks aircraft orientation with respect to acquired scan radar data to correct the display of the weather radar data for range distortion and orientation changes of the aircraft between radar acquisition and display, reducing image artifacts.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
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     CROSS REFERENCE TO RELATED APPLICATION 
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     BACKGROUND OF THE INVENTION 
     The present invention relates to aircraft electronics and in particular to aircraft weather radar systems. 
     Weather radar uses echo returns from transmitted radio signals (typically in the megahertz and gigahertz spectrum) to locate and characterize precipitation and its motion. Precipitation may be characterized by the intensity of the echo return and motion characterized by Doppler principles in which moving particles provide a frequency shift to the echo return. 
     Aircraft weather radar uses these principles to provide guidance to pilots with respect to storm cells and the like. In such systems, a radar antenna may be fixed to the nose or wing of the aircraft and scanned either mechanically or using phased array techniques. Typically the radar beam is collimated to a focused ray which is scanned in a horizontal plane directed outward along the flight path of the aircraft to provide information about weather in the path of the aircraft, while reducing reflections from the ground (ground clutter). The angle of the radar beam with respect to the horizon may be adjusted, however, so that the pilot may direct the radar to regions of interest not necessarily at the current altitude of the aircraft. 
     The weather data derived from the echo returns may be displayed in real-time on a display in the cockpit. The image on the display is normally displayed in 2 dimensions and oriented with respect to a small fixed aircraft icon so that weather systems directly in front of the aircraft are positioned directly above the icon while those to the left and right of the aircraft are depicted to the left and right of the icon respectively. This approach provides the pilot a constant indication of weather patterns in front of the aircraft flight path. 
     While mechanisms for moving radar antenna or adjusting the angle of a phased array can theoretically scan the field of view of the radar at high speed, as a practical matter the need to wait for echo returns and sufficient data sampling, limit the scan rate of the antenna to on the order of seven seconds per scan acquisition. 
     Unlike a ground-based radar, the antenna used in aircraft weather radar is mounted to a moving object (e.g. the aircraft) which can be subject to constant changes in orientation particularly if there is buffeting in storm regions. It is known that some weather radar systems stabilize the angle of the scanning plane of the radar using a vertical gyroscope on the aircraft. 
     SUMMARY OF THE INVENTION 
     The present inventors have recognized that the relatively slow scan rate of weather radar combined with the relative mobility of the aircraft can create significant artifacts in the display of weather data when the heading of the aircraft has changed significantly between the acquisition of the data and its display. Specifically, weather radar systems may fail to accurately display the bearing to large weather cells for many seconds during the banking of the aircraft, after which the weather cell will appear to jump abruptly on the display. The range (distance) to weather patterns and their shape is also subject to artifacts caused by changing pitch of the aircraft. Such artifacts may be misleading to the pilot and may complicate efforts to moderate fuel consumption when steering around a storm cell which may seem to jump, vary in distance or change in shape as the plane banks or changes pitch. 
     The present invention addresses this problem by correcting displayed weather data according to the changing heading and pitch of the aircraft. In this way, although the weather pattern will shift dimensionally in minor aspects between each scan, based on evolution of the weather system, the general bearing and shape of the weather pattern with respect to the aircraft will be largely accurate and free from sudden jumps or artifacts. 
     In one embodiment, the invention provides an electronic display processor for an avionic weather radar system having inputs for receiving electronic aircraft data providing aircraft heading and pitch, and for receiving radar scan data providing for echo information at a plurality of scan angles and ranges. The electronic display processor may output a graphical representation of at least a portion of the radar scan data mapped to a display angle and display range and may include an electronic computer communicating with input and output devices and executing a stored program to correct a display angle of echo information in the graphical representation according to any change in heading of the aircraft between an acquisition of the radar scan data and the outputting of the graphical representation. 
     It is thus an object of at least one embodiment of the invention to provide a display of weather data that better assists the pilot in making real time decisions about course adjustments with respect to weather conditions. 
     The graphical representation may be oriented with respect to an axis of the aircraft and the correction of display angle of the echo information in the output graphical representation may change a display angle of the echo information from a scan angle of the echo information by the difference between a heading of the aircraft at the time of acquisition of the radar scan data and a heading of the aircraft at the time of the display of the radar scan data. 
     It is thus an object of at least one embodiment of the invention to provide a simple correction system that minimizes heading artifacts. 
     The electronic aircraft data may further provide aircraft pitch and the electronic computer may execute the stored program to correct a display range of the echo information in the graphical representation according to a pitch of the aircraft at a time of acquisition of the radar scan data. 
     It is thus an object of at least one embodiment of the invention to ensure proper display of range data despite changes in the pitch of the aircraft, particularly when aircraft pitch may be changing abruptly. 
     The aircraft data may further provide for radar angle indicating an elevation angle (tilt) of the radar scan data with respect to an axis of the aircraft and the electronic computer may further execute the stored program to correct a display range of the echo information in the graphical representation according to the radar tilt angle at the time of acquisition of the radar scan data independent of the current tilt angle and aircraft pitch. 
     It is thus an object of at least one embodiment of the invention to accommodate changes in radar tilt angle independently of changes in aircraft pitch. 
     The correction may change the display range of the echo information according to a cosine of the sum of the radar tilt angle and aircraft pitch at the time of acquisition. This correction, when applied, will alter the displayed shape of weather patterns to more accurately represent their actual shape in relation to ground-based landmarks. 
     It is thus an object of at least one embodiment of the invention to provide a consistent reference of weather range according to ground distances independent of aircraft pitch or radar beam tilt angle. 
     The radar scan data for each scan angle may be linked to aircraft heading information. 
     It is thus an object of at least one embodiment of the invention to record conditions of radar acquisition for correction on as little as a single radar scan line. 
     The electronic computer may output repeated graphical representation of the portion of the echo information at a first frequency higher than a second frequency at which radar scan data for the echo information at the plurality of scan angles is obtained. 
     It is thus an object of at least one embodiment of the invention to provide artifact-reduced data to the pilot on a timescale substantially shorter than the scan time of the radar. 
     The graphical representation may be formed of rectilinear rows and columns of pixels and the correction may interpolate between one or more echo information samples and/or one or more pixels. 
     It is thus an object of at least one embodiment of the invention to provide for reduced image artifacts in the mapping between one or both of range and angle corrected data. 
     These particular features and advantages may apply to only some embodiments falling within the claims and thus do not define the scope of the invention. The following description and figures illustrate a preferred embodiment of the invention. Such an embodiment does not necessarily represent the full scope of the invention, however. Furthermore, some embodiments may include only parts of a preferred embodiment. Therefore, reference must be made to the claims for interpreting the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  is a perspective diagram showing heading angle and pitch angle of an aircraft and array angle and scan angle of the weather radar affixed to the aircraft such as will be referred to in the discussion of the present invention; 
         FIG. 2  is a front elevational view of a weather radar display such as may be generated from data collected by the radar of  FIG. 1  showing a weather cell with respect to the current orientation of the aircraft; 
         FIG. 3  is a functional block diagram of the present invention showing a radar system, navigational avionics, and an electronic computer for processing the same according to a stored program to be described herein for display of the data on a display screen; and 
         FIG. 4  is a flowchart of the stored program with accompanying diagrams of data structures and calculations implemented by the present invention in correcting for heading and range related weather radar artifacts. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to  FIG. 1 , an airframe  10  of an aircraft  12  extends along an axis  14  directed between the nose  11  and tail  13  of the aircraft  12 . The orientation of the axis  14  with respect to a fixed reference direction  16  (for example geographic North) in a generally horizontal plane will be termed herein “heading angle” β whereas the orientation of the axis  14  with respect to the horizon  18  in a vertical plane will be termed herein “pitch angle” γ. 
     The nose  11  of the aircraft  12  may house a weather radar transceiver  22  having an antenna  24  directing a radar beam  20  forward and generally along the axis  14  in the direction of the flight path of the aircraft  12 . The radar beam  20  will generally subtend an acquisition angle  28  within a horizontal plane  30  to provide an image of a horizontal section of a weather system  32 . Generally, the radar beam  20  will take echo measurements along a set of radar rays (radials)  34  within the horizontal plane  30  at different radar scan angles φ with respect to axis  14 , either by mechanical scanning of the antenna  24  or by phase array techniques. The angle of the horizontal plane  30  with respect to the axis  14  of the airframe  10  may be adjusted in elevation by a radar tilt angle α to be directed generally upward or downward with respect to the axis  14 . 
     Referring now to  FIG. 2 , a radar image  40  may be displayed on a graphics display  42  in the cockpit of the aircraft providing a mapping of the echo signals obtained along each of the radar radials  34  mapped to image pixels within a sector  44  defined by acquisition angle  28  of the radar beam to provide a top plan view of a cross-section of the weather system  32 . The graphics display  42  depicting radar image  40  may also provide navigational aid markers  46  providing a context for the location of the weather system  32  as well as range markers  48  providing a distance (e.g. 20 and 40 nautical miles) from an aircraft icon  50 . The aircraft icon  50  is typically fixed in orientation with respect to the radar image  40  such that the axis  14  of the aircraft icon  50  extends vertically bisecting the radar image  40 . In this way, weather systems  32  to the starboard of the aircraft  12  will be displayed on the right side of the radar image  40  and weather systems  32  to the port of the aircraft  12  will be displayed on the left side of the radar image  40 . 
     The display  42  may be a conventional CRT or LCD display and may provide a refresh rate on the order of 60 frames per second during which refresh new calculated radar image  40  data may be received. The display  42  may provide for color renditions of the weather system  32 , for example, indicating precipitation velocities through the use of Doppler techniques of type well known in the art. Generally, each element of the weather cell  32  in the radar image  40  will have a display angle  49  and display range  53  with respect to the aircraft icon  50  mirroring an angle and distance between the actual weather cell  32  and the aircraft  12 . As noted, typically the aircraft icon  50  has a fixed orientation however the invention also contemplates systems allowing movement of the aircraft icon  50  in addition to or instead of movement of the weather cells  32  upon a change in heading and/or range. 
     Referring now to  FIG. 3 , the display  42  and display control buttons  51  may communicate with electronic display processor unit  52  providing, for example, an internal processor for executing a stored program  62 , as will be described and as is stored in memory  56 . The display processor unit  52  may receive navigational data  57  from navigational avionics  58 , for example, heading and pitch information obtained through devices well known in the art, for example, compasses, gyroscopes, and GPS receivers. The display processor unit  52  may also receive radar signals  60  providing echo data as well as radar ray angle φ and radar elevation angle α described above. 
     Referring now to  FIG. 4 , the program  62  executed by the display processor unit  52 , may contemporaneously execute a first and second threads  64  and  66 . The first thread  64 , as indicated, by process block  68  acquires echo data values  70  organized by the sum of current heading β′ and the radar scan angle φ, depicted as data columns  34  and stored in memory  56 . Multiple columns  34  collectively provide echo data value  70  for the entire acquisition angle  28  within the horizontal plane  30 . 
     The echo data values  70  comprise raw echo data received from the weather radar transceiver (shown in  FIG. 3 ) processed and sampled by techniques known in the art including frequency filtration and the like to provide quantitative range delineated values indicating the characteristic of the atmosphere at particular ranges from the aircraft  12  along the corresponding radar scan angle φ. 
     Each of the echo data values  70  in a column  34  may be linked to a column-specific value of the aircraft pitch γ′, and the antenna array tilt angle α′ at the time of acquisition. Each echo data value  70  may also be linked to a calculated range value r′ being generally a function of the propagation speed of the radio waves in the atmosphere and time. These orientation and range variables at the time of acquisition are distinguished from current values of the same variables by the addition of the prime mark. 
     This linking of the echo data value  70  to orientation and range variables at the time of acquisition may be done in a variety of fashions including a table structure as shown or using a linked list or database or other data structures known in the art. 
     The thread  64  executes repeatedly, typically on a period of about once every seven seconds, the time required to obtain a full scan of radar data  71  over the acquisition angle  28 . 
     Referring still to  FIG. 4 , the second thread  66  repeatedly loops to perform a display of the data stored in memory  56  on the display  42 . At first process block  78 , the thread  66  initiates a loop in which each pixel  72  of the display is updated in a scan defined by a loop consisting of process block  78  and process block  80 . Within this loop at process block  82 , for each given pixel  72 , a display angle θ and display range r of the pixel with respect to the aircraft icon  50  is determined providing essentially a polar coordinate value for the pixel  72 . 
     At process block  84 , the screen coordinates of the pixel  72  are mapped to an echo data value  70  of the radar scan data by adding to the display angle θ to the current heading β and using this sum to directly access the appropriate column  34  which represents the sum of the acquisition heading β′ and the acquisition angle φ. In this way, stale radar information (possibly as much as 7 seconds old) will be correctly positioned on the display  42 . 
     At next process block  86 , correct echo data values  70  within the radar radials  34  will be identified by determining the aircraft pitch γ′ and the radar angle α′ associated with the column  34  to correct for the foreshortening effect on displayed ground distance when the radar is not horizontal to the ground. In particular, the display range value r will be divided by the cosine of the sum of α′ and γ′ to determine the appropriate range value r′ to be applied to the particular column  34  to identify the appropriate echo data value(s)  70  closest to the pixel  72 . 
     At succeeding process block  90  an optional interpolation step may be provided allowing for interpolation between values of β′ and r′ with respect to the mapping of pixel  72  into the radar data  71 . In one embodiment, a two-dimensional interpolation or four point interpolation may be used according to techniques well known in the art wherein the interpolated value is a function of distance to each of the surrounding points. 
     At process block  80  the computer pixel value may be output to the screen and next pixel scanned and processed as described above. 
     It will be appreciated that this described process of program  62  serves to correct the displayed bearing of the radar data according to a difference between the heading of the aircraft at the time the data was acquired and the heading at the time the data is displayed. In addition range data is corrected according to the angle of the radar beam  20  at the time of its acquisition. 
     Certain terminology is used herein for purposes of reference only, and thus is not intended to be limiting. For example, terms such as “upper”, “lower”, “above”, and “below” refer to directions in the drawings to which reference is made. Terms such as “front”, “back”, “rear”, “bottom” and “side”, describe the orientation of portions of the component within a consistent but arbitrary frame of reference which is made clear by reference to the text and the associated drawings describing the component under discussion. Such terminology may include the words specifically mentioned above, derivatives thereof, and words of similar import. Similarly, the terms “first”, “second” and other such numerical terms referring to structures do not imply a sequence or order unless clearly indicated by the context. 
     When introducing elements or features of the present disclosure and the exemplary embodiments, the articles “a”, “an”, “the” and “said” are intended to mean that there are one or more of such elements or features. The terms “comprising”, “including” and “having” are intended to be inclusive and mean that there may be additional elements or features other than those specifically noted. It is further to be understood that the method steps, processes, and operations described herein are not to be construed as necessarily requiring their performance in the particular order discussed or illustrated, unless specifically identified as an order of performance. It is also to be understood that additional or alternative steps may be employed. 
     References to “a controller” and “a processor” can be understood to include one or more controllers or processors that can communicate in a stand-alone and/or a distributed environment(s), and can thus be configured to communicate via wired or wireless communications with other processors, where such one or more processor can be configured to operate on one or more processor-controlled devices that can be similar or different devices. Furthermore, references to memory, unless otherwise specified, can include one or more processor-readable and accessible memory elements and/or components that can be internal to the processor-controlled device, external to the processor-controlled device, and can be accessed via a wired or wireless network. 
     It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein and the claims should be understood to include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims. All of the publications described herein, including patents and non-patent publications, are hereby incorporated herein by reference in their entireties.