Telecommunications – Radiotelephone system – Zoned or cellular telephone system
Reexamination Certificate
1998-09-09
2001-10-02
Nguyen, Lee (Department: 2683)
Telecommunications
Radiotelephone system
Zoned or cellular telephone system
C455S012100, C455S456500, C342S357490, C342S357490
Reexamination Certificate
active
06298238
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to satellite communication systems and networks. More particularly, the present invention relates to quickly and accurately determining the position of a user terminal in a satellite communications system.
2. Related Art
A typical satellite-based communications system comprises at least one terrestrial base station (hereinafter referred to as a gateway), at least one user terminal (for example, a mobile telephone), and at least one satellite for relaying communications signals between the gateway and the user terminal. The gateway provides links from a user terminal to other user terminals or communications systems, such as a terrestrial telephone system.
While the advent of the wireless user terminal has freed users to take their phones wherever they go, it has also imposed upon the satellite communications carrier the burden of determining the location of the user terminal. The need for user terminal position information stems from several considerations. One consideration is that the system should select an appropriate gateway for providing the communications link. One aspect of this consideration is allocation of a communications link to the proper service provider (for example, a telephone company). A service provider is typically assigned a particular geographic territory, and handles all communication links or calls with users in that territory. When communications are required with a particular user terminal, the communications system can allocate the call to a service provider based on the territory within which the user terminal is located, at the time. In order to determine the appropriate territory, the communications system requires the position of the user terminal. A similar consideration arises when calls must be allocated to service providers based on political boundaries or contractual relationships.
Several methods of determining the location of the user terminal involve measuring the distance (range) between the user terminal and the satellite and the rate of change of that distance (range rate). When these measurements are combined with other data, such as the position of the satellite, the location of the user terminal can be determined with great precision. Techniques for determining user terminal location using satellite-user terminal range and range rate are disclosed in commonly-owned, copending U.S. patent applications entitled “Position Determination Using One Low-Earth Orbit Satellite,” Ser. No. 08/723,751; and “Unambiguous Position Determination Using Two Low-Earth Orbit Satellites,” Ser. No. 08/723,725, which are incorporated by reference herein in their entirety.
Once the range and range rate quantities are known, they can be combined with the known position and velocity of the satellite to generate a set of non-linear equations that cannot be solved exactly. According to conventional methods, a solution to these equations is obtained by using a numerical approach, such as the well-known iterative Newton-Raphson method.
One disadvantage of this approach is that such iterative numerical approaches require significant time and computational resources to execute. A crucial requirement in position determination for a satellite-based communications system is speed. When communications are required with a particular user terminal, the position of the user terminal should be selected rapidly. Since user terminal position is used as part of the process of establishing service, delays in determining position result in undesirable delays in establishing service. A mobile terminal user is not likely to tolerate a delay of more than a few seconds when placing a call or setting up a communications link.
SUMMARY OF THE INVENTION
The present invention comprises an apparatus and method for rapidly determining the position of a user terminal in a satellite communications system based on the distances (ranges) between the user terminal and one or more satellites, the rate of change of those distances (range rates), and the positions and velocities of the satellites.
In one embodiment, a single satellite is used to determine the position of the user terminal. In this embodiment, the method includes the steps of estimating the radius of the Earth at a user terminal initial position estimate to produce a coarse radius estimate; determining the distance between the user terminal and a satellite and the rate of change of that distance; and determining a coarse position estimate based on the coarse radius estimate, the distance, and the rate of change of the distance.
In this embodiment, the coarse position estimate can be improved using either of two methods. One method includes the steps of updating the coarse radius estimate using the coarse position estimate to produce a fine radius estimate, and then updating the coarse position estimate based on the fine radius estimate, the distance, and the rate of change of the distance to produce a fine position estimate for the user terminal. Another method is to iteratively improve the coarse position estimate using a numerical root-finding method, such as the Newton-Raphson method.
In another embodiment, two satellites are used to determine the position of the user terminal. In this embodiment, the method includes the steps of estimating the radius of the earth at a user terminal initial position estimate to produce a coarse radius estimate; determining the distances between the user terminal and the satellites; and determining a coarse position estimate based on the coarse radius estimate and the distances.
In this embodiment, the coarse position estimate can be improved using either of two methods. One method includes the steps of updating the coarse radius estimate using the coarse position estimate to produce a fine radius estimate, and then updating the coarse position estimate based on the fine radius estimate and the distances to produce a fine position estimate for the user terminal. Another method is to iteratively improve the coarse position estimate using a numerical root-finding method, such as the Newton-Raphson method.
The primary advantage of the present invention is that it permits the position of a user terminal to be determined accurately and rapidly.
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Nadav Levanon; Quick Position Determination Using 1 or 2 LEO Satellites; Jul. 1998; IEEE Transactions on Aerospace and Electronic Systems; vol. 34; No. 3; pp. 736-754.
Nguyen Lee
Nguyen Simon
Ogrod Gregory D.
Qualcomm Incorporated
Wadsworth Phillip R.
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