Communications: directive radio wave systems and devices (e.g. – Directive – Including a satellite
Reexamination Certificate
1999-07-13
2001-06-12
Blum, Theodore M. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Directive
Including a satellite
C455S012100, C701S226000
Reexamination Certificate
active
06246360
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to a method for determining satellite coverage over a planet's surface, and more particularly, to the determination of satellite coverage over the planets surface by a constellation of satellites. In particular, the present invention relates to a system for determining statistics of satellite coverage using a satellite-centric point of view instead of a planet-centric view.
BACKGROUND OF THE INVENTION
Determining how well a group of satellites cover a planet is a critical component of constellation design. Usually some minimum number of satellites must be visible to every point in the region of interest in order to satisfy the mission. A prime example is that of a cellular telephone satellite system. In order to provide worldwide coverage at every instant of time, at least one satellite must be visible to every point of the earth with an elevation greater than some minimum limit. From a constellation designer's point of view, the goal is to determine the minimum number of satellites required to satisfy the mission constraints. The fewer the number of satellites, the lower the cost of the entire constellation.
Classically, this optimization problem was solved using a brute force, planetary frame of reference approach. For example, if the goal is to provide worldwide single satellite coverage with n satellites, such that every ground point on the surface of the planet saw at least one satellite at an elevation of 15° or higher the method would be as follows.
The planet's surface is divided into a grid of latitude/longitude points at some degree of resolution (say 10°). The process is started at one latitude/longitude point. The orbit of one of n satellites is propagated over the period of interest checking (and retaining) the visibility of the one satellite over time. This is repeated for all n satellites. The next adjacent latitude/longitude point is advanced and the process is repeated for all n satellites until the entire grid has been evaluated. Assuming, for example, n=24 and a 24 hour time period of interest at 1 minute time steps then a total of [1440 time steps*24 satellites*(36*18) latitude/longitude points=2.24e07] visibility calculations have to be performed. Obviously, as more satellites are added, grid density is increased, and constraints are added, the computational burden grows geometrically.
This classic approach has the advantage of being simple to implement and test, but the disadvantage of being extremely slow as more satellites are added. With the advent of satellite systems such as Iridium (66 satellites) it has become necessary to develop a coverage algorithm that gives results in a reasonable amount of time.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a new method for calculating visibility that does not suffer from the computational burden of the prior art.
It is another object of the present invention to provide a simulation for determining satellite coverage over the earth's surface.
It is yet another object of the present invention to provide a method of determining satellite coverage by determining which grid points are in a satellite's line of view.
Another object of the present invention is to provide a method and apparatus wherein adding constraints reduces the number of calculations to perform.
Yet another object of the present invention is to store and output as much information about visibility as possible so that the resulting data can be analyzed for further information without having to repeat the analysis process.
Yet a further object of the present invention is to provide a system for determining statistics of satellite coverage using a satellite-centric point of view instead of a planet-centric view.
The new technique described in this patent application now solves the coverage problem at least 1000 times faster than the previously described prior art technique. Rather than calculate the amount of satellites that can be seen by a point on the ground, the new technique calculates the ground points which can be seen by the satellite. Consider a satellite that is orbiting the earth at a fairly low altitude at a point in time when it just crosses the equator at 0° longitude. From the satellite's point of view, only a small circle of earth is visible at this time. For example, say one point on the horizon is at 20° north and −30° east. Its complimentary point would be at 20° north and +30° east. Once these two points are known all other points between −30° east and +30° at 20° north can be marked as visible. This is repeated over the entire horizon filling in the visible region. This is done for every time step, in the orbit until the end time is reached. Whereas the old method calculated thousands of visibilities at each time point, the new method only calculates a few, and taking advantage of symmetry, fills in the rest.
These and other objects of the present invention are achieved by a method of determining satellite coverage over the surface of the earth by determining ground points visible to each of m satellites in a constellation orbiting the earth including dividing the surface of the earth into a matrix of grid points. A satellite ground footprint longitude width is determined at a given latitude for each satellite in the constellation for each time step in the orbit. The width of this line determines which grid points satisfy line-of-sight geometric constraints for each satellite in the constellation for each time step in the orbit. Coverage statistics for each satellite are generated.
The foregoing objects of the present invention are achieved by an article including at least one sequence of machine executable instructions on a medium bearing the executable instructions in machine readable form wherein execution of the instructions by one or more processors causes the one or more processors to divide the surface of the earth into a grid of grid points. A satellite ground footprint longitude width is determined at a given latitude for each satellite in the constellation for each time step in the orbit.
The foregoing objects of the present invention are achieved by a computer architecture including means for dividing the surface of the earth into a grid of grid points. First determining means are provided for determining a satellite ground footprint longitude width at a given latitude for each satellite in the constellation for each time step in the orbit. Second determining means are provided for determining which of the grid points satisfy line-of-sight geometric constraints for each satellite in the constellation for each time step in the orbit. Generating means are provided for generating coverage statistics.
The foregoing objects of the present invention are achieved by a computer system a processor and a memory coupled to the processor, the memory having stored therein sequences of instructions, which, when executed by the processor, causes the processor to perform the steps of dividing the surface of the earth into a grid of grid points. A satellite ground footprint longitude width is determined at a given latitude for each satellite in the constellation for each time step in the orbit. It is determined which of the grid points satisfy line-of-sight geometric constraints for each satellite in the constellation for each time step in the orbit. Coverage statistics for each satellite are generated.
Still other objects and advantages of the present invention will become readily apparent to those skilled in the art from the following detailed description, wherein the preferred embodiments of the invention are shown and described, simply by way of illustration of the best mode contemplated of carrying out the invention. As will be realized, the invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the invention. Accordingly, the drawings and de
Brand, Jr. Donald D.
Kalke, Jr. Jerome Joseph
Kling, Jr. Ronald Lee
Blum Theodore M.
Lowe Hauptman & Gilman & Berner LLP
TASC, Inc.
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