Telecommunications – Carrier wave repeater or relay system – Portable or mobile repeater
Reexamination Certificate
1998-06-03
2001-08-07
To, Doris H. (Department: 2682)
Telecommunications
Carrier wave repeater or relay system
Portable or mobile repeater
C455S012100, C455S427000
Reexamination Certificate
active
06272316
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to communications systems having mobile users and, in particular, to satellite-based communications systems wherein mobile user terminals are connected via at least one satellite to a terrestrial communications system.
BACKGROUND OF THE INVENTION
Mobile communications systems, and in particular Global Mobile Communications Systems employing a constellation of earth orbiting satellites, can exhibit blocking and shadowing of users which are attempting to connect or remain connected to one or more of the satellites. The satellites relay duplex user RF signals to terrestrial-based gateways for further connection to, by example, the Public Switched Telephone Network (PSTN). However, if the access of a given user is blocked to one or more satellites, the user may experience an inability to log onto the system, to initiate or receive calls, or to continue a call that is in progress. This problem is especially apparent when the user is a mobile user, and is in motion with respect to possible RF obstructions, such as foliage and buildings.
With known types of mobile user satellite systems, particularly geosynchronous satellite systems, generally only one satellite is in view of the user. Furthermore, the location of the satellite, on or about the equator, specifies the direction from the user to the satellite. In the northern hemisphere this direction is generally south.
However, recent advances in the communication, computer, and small satellite technology has enabled the concept of providing a constellation of satellites, wherein over large portions of the Earth's surface at least two satellites can be in view of any location. Furthermore, the development of hand-held user terminals using Code Division Multiple Access (CDMA) has made possible multiple satellite coverage employing diversity techniques as a means of mitigating shadowing and blocking of users. By example, a user terminal is enabled to maintain a connection simultaneously through two or more sat ellites of a constellation of LEO satellites that are simultaneously in view.
One problem that is presented when deploying a constellation of, by example, Low Earth Orbit (LEO) satellites is that the user generally cannot directly perceive the best direction to either face or move to in order to maximize his or her ability to establish or maintain communications through one or more of the satellites. This is because the satellites are in motion with respect to the user, and furthermore the satellites may not be, at any given time, located in some predetermined and essentially constant direction with respect to the user (i.e., south as in the geosynchronous satellite case).
For a constellation of earth orbit satellites (in non-polar orbits) it can be shown that the portion of the sky wherein a user will “see” the satellites is a function of latitude. At the equator (0° latitude) the sky is uniformly covered by satellite tracks, whereas at approximately 70° north latitude in only a relatively small portion of the southern sky will a user see a satellite. For intermediate latitudes the size of the region wherein no satellites are found, referred to herein as “obscura”, grows progressively small as the equator is approached. The southern latitude case is the mirror image of the northern latitude case for circular orbits.
Furthermore, in a typical case a variety of signal blocking obstructions are found, such as buildings, utility poles, trees, etc., in addition to the region wherein no satellites pass (obscura). It should be apparent that at any given time only a portion of the sky may be optimum for carrying a communication between a terrestrial user terminal and one or more of the satellites.
This problem is compounded if the user is located within a building, as the relatively high frequencies (e.g., S-band) that may be used on the forward link between the satellite and the user terminal will not normally penetrate to any great distance within the building. As a result, the user will typically experience better link quality if the user is located near to a window through which the satellite(s) can be “seen”.
It might be thought that if a user would always go to a south-facing window (in the northern hemisphere) that the best link quality will always be experienced. However this may not be the case. By example, assume that the user is being provided with two satellite diversity (i.e., a communication is being simultaneously conveyed through two satellites) and that three satellites are in view. Depending on which two satellites are being used, in one case a south-west facing window may be optimum, while for the second case a south-east facing window may be optimum. In either case a north-facing window would most likely provide for the poorest link quality (absent some fortuitous placement of signal reflecting surfaces).
It is an object of this invention to provide improved methods and apparatus to assist a user of a mobile communication satellite system to log on to, initiate and receive calls, and maintain calls.
SUMMARY OF THE INVENTION
The foregoing and other problems are overcome and the object of the invention is realized by methods and apparatus that enable a user to gain knowledge of a direction to one or more “best” satellites available to the user. This information enables the user to alter his or her position to take advantage of the one or more “best” satellites at any particular time in order to facilitate the ability to log-on, initiate and receive calls, and complete calls. The information can be supplied by the serving system, and can be employed by the user to select for a period of time, by example, a position on an east side of a building, to move to a south-east facing window, or to take some other action that enhances the user's connectivity potential to the satellite communications system.
This teaching of this invention exploits the fact that a gateway within whose service area a user terminal is located is aware of the location (latitude and longitude) of the user terminal, and can compute from satellite ephemerides data the locations and elevation angles of all satellites in view of the user terminal. Alternatively, the gateway can send satellite ephemerides data to the user terminal, which can then compute the locations and elevation angles of the satellite(s) in view.
The gateway has knowledge of the current state of the satellite paths that are co-visible (simultaneously visible to both the gateway and the user terminal), i.e., whether the paths are clear, impaired (by shadowing or specular reflection), or blocked. This knowledge is gained from the power required to close the forward and reverse communication links between the user terminal and gateway.
The link status information to the user can be automatically generated and sent by the gateway, or may be generated and sent by the gateway only in response to a request from the user. By example, if the user keys in a predetermined code into the keypad, the code is sent on the return link, via one or more satellites, to the gateway.
A method in accordance with this invention for operating a satellite communications system of a type that has a plurality of earth orbiting satellites, at least one terrestrial user terminal, and at least one terrestrial gateway, includes steps of determining a location and an azimuthal orientation of the user terminal; determining locations of one or more satellites co-visible to the user terminal and to the gateway; determining a signal quality of paths to each satellite co-visible with the gateway; displaying a representation to an operator of the user terminal, the representation depicting at least the orientation of the user terminal, the determined locations of the one or more satellites, and the determined signal qualities for satellites that are in use and those that are not in use; and changing the location of the user terminal in response to the displayed representation.
In the presently preferred embodiment the step of determining the locations of th
Monte Paul A
Penwarden Kent A
Wiedeman Robert A
Globalstar L.P.
Ohlandt Greeley Ruggiero & Perle L.L.P.
To Doris H.
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