Data processing: vehicles – navigation – and relative location – Navigation – Employing position determining equipment
Reexamination Certificate
1998-03-26
2001-04-17
Cuchlinski, Jr., William A. (Department: 3614)
Data processing: vehicles, navigation, and relative location
Navigation
Employing position determining equipment
C701S207000, C701S214000, C342S357490, C342S357490
Reexamination Certificate
active
06219615
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to satellite communication systems, and a method of operating such systems. The invention particularly relates to satellite communication systems and related methods where an earth based user terminal communicates with a satellite which in turn communicates with an earth station, and vice versa. Most particularly, the present invention relates to such a system and its related methods where there is a requirement for the location of the user terminal, on the surface of the earth, to be known to the earth station.
BACKGROUND OF THE INVENTION
It is known, in a satellite communications, for a user terminal, perhaps in the form of a radio telephone handset, not dissimilar to a cellular radio telephone handset, to communicate with an earth station, via a satellite, to establish a telephone call or to receive a telephone call, by means of the earth station linking into the terrestrial wire and cable system or into another radio telephone system.
There are twin purposes relating to establishing the exact position of the user terminal on the surface of the earth. Firstly, in order to know how to direct a radio signal to or from a user terminal, when required, from particular satellite at a particular time, it is necessary to know the approximate location of the user terminal so that the appropriate beam from the appropriate satellite can be selected to cover the portion of the Earth's surface where the user terminal is located. Secondly, in a satellite communication system, in order that call barring, local billing or other restrictions based on the territory wherein the user terminal may be operated can be observed, it is necessary to determine the location of the user terminal with sufficient accuracy for the necessary restrictions to be imposed.
It is known to provide a user terminal where the individual terminal employs “Global Positioning by Satellite” (GPS) to determine, with some great accuracy, the position of the user terminal on the surface of the earth. The user terminal then transmits, to the earth station, via the satellite or satellites involved in communications, its exact position which is then used by the earth station, in subsequent interactions with the user terminal, to control the fiscal and mechanical aspects of the communication activity with the user terminal. An example of such a system is to be found in European Patent EP 0562 374 by Motorola Corporation filed 27th March 1993.
Such systems require multiple frequency capability from the handset, together with enhanced complication of the handset, in order that the handset may be capable both of communications and of GPS measurements.
The present invention seeks to provide a solution to the problem of determining the location of a user terminal or handset, on the surface of the earth, when interacting with a satellite communication system, without the necessity to resort to a solution involving GPS and with sufficient accuracy for the operational and fiscal requirements of a satellite communication system.
A GPS system requires very accurate measured characterisation of the orbits of each satellite so that the position of each satellite can be made known, to a terminal, on the surface of the earth. The terminal itself then calculates its own position. Because the GPS system is one where the satellites alone transmit, information flow is one-way. Measurement of satellite position, which is directly related to the accuracy with which the position of the terminal can be found, is achieved only intermittently by a dedicated facility, and drifting can occur between measurements. The dedicated facility may not be optimally placed on the surface of the earth when the position of the satellite is measured, introducing unwanted errors.
The present invention seeks to provide a continuous updating of the satellite position by exploiting the two-way characteristics of a satellite communications system.
SUMMARY OF THE INVENTION
According to a first aspect, the present invention consists in a system wherein a satellite is operative to cooperate with an earth station to measure and record the position of a terminal on the surface of the earth by reference to an estimated, instant position of said satellite, said earth station being operative to estimated said instant position of said satellite with a first accuracy, said system being characterised by being operative to incorporate the measured position of said terminal to make a re-estimation of said instant position of said satellite with a second accuracy, said second accuracy being greater than said first accuracy.
According to a second aspect, the present invention consists is a method for use in a system wherein a satellite is operative to cooperate with an earth station to measure and record the position of a terminal on the surface of the earth by reference to an estimated, instant position of said satellite, said earth station being operative to estimated said instant position of said satellite with a first accuracy, said method including the steps incorporate the measured position of said terminal in a re-estimation of said instant position of said satellite with a second accuracy, said second accuracy being greater than said first accuracy.
The invention also provides a system and method employing the result of said re-estimation as the new estimated instant position of said satellite when next measuring the position of said terminal. In the preferred embodiment, this provides an improved estimation of the orbit of the satellite and thus contributes to a better measurement of the position of the terminal at the next instance of measurement thereof.
The invention further provides a system and method where the terminal is one of a plurality of terminals, and including measuring and recording the position of each of the plurality of terminals and incorporating the measured position of each of the plurality of terminals to execute the re-estimation in each instance. In the preferred embodiment, this means that the position of the satellite is three-dimensionally triangulated from a vast number of points, possibly numbering thousands, instead of from just a few earth stations.
The invention, still further, provides a system and method where said plurality of terminals can incorporate one or more other earth stations. Since earth stations have their position very accurately known, their contribution to overall accuracy of estimation of the satellite is significant.
The invention further provides a system and method where the re-estimation of the position of the satellite incorporates a weighting function relating to the significance of the accuracy and geometry of the measurement of the position a terminal, the weighting function being employed to give greater favour to those measurements of position of a terminal which contribute most to improving the accuracy of estimation of the instant position of said satellite. In the preferred embodiment, this feature provides that terminal position measurements where there is a great error in measurement of the terminal position, or where there is a great disadvantage in the geometry because the terminal is badly placed relative to the satellite, are given a small weighting and significance in working out the position of the satellite, whereas terminal position measurements with the opposite property are given a greater significance. By down-playing poor results, and by emphasising good results, the estimation of the position of the satellite is better improved.
The invention, still further, provides a system and method wherein the weighting is incorporated in the re-estimation by providing the accuracy of measurement and the quality of the geometry as state variable inputs to a Kalman filter algorithm.
Finally, the invention provides a system and method where the earth station is one of a plurality of earth stations, each of the plurality of earth stations sharing information concerning the re-estimation. This means that, should the satellite pass from one earth station to
Arthur Gertrude
Cuchlinski Jr. William A.
ICO Services Ltd.
McDermott & Will & Emery
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