Abstract:
A method and apparatus for determining the inclusion or exclusion of a current position of an aircraft within a satellite communication spot beam, which uses a technique of slightly moving the position being checked for inclusion, when it is determined that a ray drawn from the current position is coextensive with an endpoint of any line segment making up a boundary of the spot beam.

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to satellite communication, and more particularly relates to mobile satellite communication, and even more particularly relates to apparatuses and methods for controlling the operation of a mobile satellite transceiver. 
     BACKGROUND OF THE INVENTION 
     In the past, avionics engineers have endeavored to change the operation of a mobile transceiver depending upon the location of the transceiver with respect to the boundaries of geographic areas which have predetermined exposure to satellite signals for satellite communication. These predetermined geographic areas are often referred to as spot beams. If an aircraft is within the spot beam, the satellite communication transceiver will be operated differently than if it is outside of a spot beam. However, since aircraft travel at relatively high velocity, they may enter and exit several spot beams during a single flight. In the past, determining whether the aircraft is in a spot beam has often taken considerable time and consumed valuable computing resources on the aircraft. 
     One well-known approach has been to draw a line from the given point to any point that is known to be outside of the spot beam boundary (polygon), e.g. to infinity, and then counting the number of times the line intersects the polygon boundary. If the number of intersections is odd, then the given point is inside the polygon. If the number of intersections is even, then the point is outside the polygon. The same method can be applied to spherical coordinates; i.e. let the North Pole be the point outside of the polygon; this can be assumed for the case of a geo-stationary satellite spot beam. 
     However, some of these prior art methods do not always accurately determine the presence of the aircraft within a spot beam, especially with respect to intersections occurring at vertices of the line segments of the polygon. 
     Consequently, there exists a need for improvement in methods and apparatuses for determining whether a mobile transceiver is within a spot beam. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide enhanced quality of the determination of presence within a spot beam. 
     It is a feature of the present invention to use a slight latitudinal displacement of the current position in the process of determining location within a spot beam. 
     It is an advantage of the present invention to reduce ambiguity resulting from infinity lines drawn through vertices between adjacent line segments of the polygon. 
     It is another advantage of the present invention to reduce the computing resources necessary to make the spot beam determination with an adequate level of certainty. 
     The present invention is an apparatus and method for determining the location of a mobile satellite transceiver within a spot beam, which is designed to satisfy the aforementioned needs, provide the previously stated objects, include the above-listed features and achieve the already articulated advantages. The present invention is carried out in an “ambiguity-less manner” in a sense that the ambiguity associated with an intersection of a line drawn to infinity and through a vertex of two adjacent line segments of the spot beam polygon boundary, is greatly reduced. 
     Accordingly, the present invention is a system and method for determining presence within a spot beam by making a displacement of the current position and making a presence determination from the new position. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may be more fully understood by reading the following description of the preferred embodiments of the invention, in conjunction with the appended drawings wherein: 
     FIG. 1 is a simplified diagram of a method of the present invention, in which all of the dashed lines are drawn toward the North Pole. 
     FIG. 2 is a flowchart of the method of the present invention. 
    
    
     DETAILED DESCRIPTION 
     Now referring to the drawings, wherein like numerals refer to like matter throughout, and more particularly to FIG. 1, there is shown a simplified view of a plot of a representative spot beam, generally designated  100 , which is made up of a group of line segments having vertices  101 ,  102  and  103 . The line segments between the vertices may be great circle arc, as depicted in FIG. 1, for a spherical coordinate system, or they may be straight lines when a Cartesian coordinate system is used. The top of FIG. 1 is designated as north. Shown in FIG. 1 are four points to be checked for inclusion within the spot beam  100 . Point I is located within the spot beam  100  and a line drawn from it to the North Pole, the dashed line, intersects with only one segment of spot beam  100 . A line from point  2  to the North Pole intersects two segments. This even number of intersections indicates that point  2  is outside of the spot beam  100 . A line drawn from point  3  to the North Pole does not intersect with any of the segments of the spot beam  100 . Zero intersections indicate that the point is outside of the spot beam  100 . An ambiguity arises when a line is drawn from point  4  to the North Pole. This line intersects the polygon of the spot beam  100  at a vertex  103 . A determination of one intersection would normally lead to a conclusion of being inside of the spot beam  100 . This would be an incorrect conclusion. The present invention includes the step of slightly displacing the point  4  and then recalculating. Point  4 ′ is point  4  after being displaced. A line drawn from point  4 ′ intersects the spot beam  100  twice, indicating that it is outside. 
     The spot beam  100  is shown here for simplicity as having only three vertices and three line segments. The shape of the spot beams in a system varies considerably. Therefore, the number of vertices and line segments will also vary to accommodate for these variations. In a preferred embodiment of the present invention, these spot beams and their vertices and line segments are stored in memory as a series of line segments, defined by the vertices or endpoints. These vertices, as well as the current position of the aircraft, are preferably expressed in spherical coordinates and measured in float based radians. Floating point arithmetic allows the representation of very large or very small numbers with fewer bits, which is desirable for computer software applications. The present invention is believed to be best implemented with the aid of computer software, but if necessary or otherwise desirable, it could be implemented by mental steps or manually as well. 
     Now referring to FIG. 2, there is shown a flowchart of a method of the present invention, generally designated  200 . The method herein is described as a method for determining location of a mobile transceiver within a satellite spot beam, but it should be understood that the method is applicable for determining the inclusion or exclusion of any point in any polygon. 
     Step  202  starts the innovated method by determining a position to be checked for inclusion in the spot beam  100 . This is typically the current position of the aircraft, which is done by known means, such as GPS etc. Depending upon the coordinate system of the GPS, a conversion of the current position to a common coordinate system may be necessary. Next, step  204  involves selecting a line segment from the list of line segments which make up the spot beam  100 . These segments are preferably characterized by their vertices. Step  206  involves determining whether the selected segment has both of its vertices either east, west or south of the position being checked. If the answer is yes, then a line drawn from the point to the North Pole would not intersect the segment. Step  208  involves not incrementing the tally of intersections based upon this segment. After step  208 , step  220  is accomplished as described below. If step  206  results in a determination of no, then the line segment must have both of its vertices north of the current position, its vertices on each side of the current position. Step  210  determines if the current position is in the same longitude as one of the vertices of the segment; if yes, then step  212  suggests moving the position slightly, preferably to the west by a small amount, approximately 0.001 radians or 0.0573 degrees. Then the tally of intersections and segments checked is reset and the process is started anew with the new, slightly moved location. If the current position is not the same as the longitude of one of the vertices of the segment, per step  210 , then in accordance with step  214 , the intersection point is determined between the line segment being checked and the line extending to the North Pole. In accordance with step  216 , if the point of intersection is further north than the current position, then, in accordance with step  218 , the tally of intersections is incremented. After completion of steps  208  or  218 , the next step is step  220 , which determines if all segments of the spot beam  100  have been checked. If the answer is no, then the step  204  is repeated until the answer to step  220  is yes. Then step  222  determines if the tally total is even or odd. If even, then, in accordance with step  224 , a report that the current position is outside of the spot beam  100  is generated. If the result of step  222  is a determination that the tally total is odd, then, in accordance with step  226 , a report that the current position is inside of the spot beam is generated. 
     It is thought that the method and apparatus of the present invention will be understood from the foregoing description and that it will be apparent that various changes may be made in the form, construct steps and arrangement of the parts and steps thereof, without departing from the spirit and scope of the invention or sacrificing all of their material advantages. The form herein described is merely a preferred exemplary embodiment thereof.