Geometrical instruments – Indicator of direction of force traversing natural media – Combined
Patent
1997-01-28
1999-04-20
Fulton, Christopher W.
Geometrical instruments
Indicator of direction of force traversing natural media
Combined
33 1CC, 33352, 33431, G01C 2120
Patent
active
058946748
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a method and an apparatus for determining the direction of a defined position from a variable position. The present invention is particularly directed towards determination of the direction that an antenna for a portable communications terminal (the variable position) must be pointed to find a communications satellite (the defined position).
In telecommunications, a signal may be transmitted using microwaves or over a cable. As microwaves are easily absorbed by solid objects, the use of microwaves in telecommunications requires that the transmitter and receiver be in line of sight. Communications satellites provide a method of achieving line of sight communication over long distances. Communication between the satellite and terrestrial communication networks is provided via a land earth station which lies within the coverage area of the satellite.
In remote areas, where telephone services are either inadequate or unavailable, portable telephones and data terminals which directly access a communications satellite may be used, by means of a satellite communications service such as the InmarsatM.sup.SM service. Such satellite telephones, also known as Mobile Earth Stations (MES) are being used by news agencies, U.N. relief agencies and other groups which operate in remote areas. New models of MES may be as small as a large briefcase.
One particular feature of an MES, as compared with cellular telephones, is that the antenna is not small and omni-directional but, instead, must be pointed towards the satellite before the MES can be used. The antenna is relatively large, about 30 cm in diameter and typically has a beamwidth of 15.degree. to 20.degree.. Thus, a user of an MES must be able to determine the direction of the satellite from his location sufficiently accurately to pick up a signal from which the antenna can be directed more accurately by optimising the signal--he must be able to determine both the compass direction to the satellite (the azimuthal component) and the elevation of the satellite in the sky. It is also useful to be able to make at least a rough determination, before the MES is set up, of the direction of the satellite from the user's general location to ensure that there are no solid objects, such as buildings or trees, blocking the line of sight between the satellite and the user's location.
BACKGROUND ART
Various methods have been used for determining the direction to a satellite from a variable location. It is known to use a computer or processor to calculate the required elevation and azimuth to a selected satellite from data regarding the location of the user. Such data may be entered into the computer as the user's latitude and longitude read from a world map or by indicating the user's location using a cursor in an electronic display showing a world map. Alternatively, the user can make use of a map, such as that shown in FIG. 1. The map is of a standard projection but the map grid does not show the normal lines of latitude and longitude. Instead, the map grid is a curvilinear grid showing lines of azimuth and elevation for a particular satellite so that the user can determine the correct azimuth and elevation for his location by finding the location on the map. In all these methods, the user then employs a compass to point the antenna in the required azimuthal direction and adjusts the angle of the antenna to set the elevational component of the direction to the satellite.
Disclosure of the Invention
According to a first aspect of the present invention, there is provided a method of determining the direction of a defined position from a variable position within a specified area, the specified area including positions which are separated from said defined position by a number of degrees of latitude or longitude greater than a predetermined limit, using an arrangement comprising a map of said specified area and a map defined point positioned relative to said map to represent said defined position relative to said specified area,
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Fulton Christopher W.
International Mobile Satellite Organization
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