Abstract:
A method and apparatus are disclosed for a system where a user terminal  11  can communicate with satellites  9 A-C, the satellites  9 A-C can communicate with earth stations  7 A-E which in turn can communicate with the terrestrial telephone system  1 . The user terminal can register with one of the earth stations  5 C which has an associated mobile satellite switching center  5 C which acts as a gateway into the terrestrial system  1 . When attempting to contact the user terminal  11  from the terrestrial system  1  the mobile satellite switching center  5 C of visitor registration, if it cannot make the contact itself, instructs other earth stations  5, 7 , in turn, according to different methods of alternative earth station  7  selection, until contact is made, and then routes all voice messages  21  through itself  5 C to retain the single and original gateway into the terrestrial system  1.

Description:
FIELD OF THE INVENTION 
     This invention relates to a telecommunication system where remote user terminals are in receipt of access to a larger telecommunication system by means of direct access to a satellite which, in turn, is in communication with a ground station. It particularly relates to a system where, potentially, a user terminal can access more than one satellite and, via a satellite or satellites, can access more than one ground station. 
     BACKGROUND 
     First, it is necessary to describe the general terrestrial telephone system. This is the “larger telecommunication system” mentioned above. 
     The terrestrial telephone system comprises the sum of all of the wire, optical, point-to-point, microwave and any other means of conducting signals from one point to another via the surface of the earth, together with their associated switches, amplifiers, encoders, decoders, amplifiers and repeaters. On a national level, the network links all access points in a country. Country is linked to country via international trunk lines. Oceans are spanned by submarine cables. The terrestrial network is a transparent carrier where a signal may be introduced from an access point and routed to another, selected access point elsewhere on the surface of the earth. A subscriber can access the terrestrial system via various gateways. For example, the terrestrial system can be accessed from the public switched telephone network, a public switched data network, an integrated switched digital network, a cellular telephone system, or a satellite telephone system. Each gateway provides a plurality of different access points, which, depending upon the gateway, can range from a simple telephone to a complex data or image source. The subscriber&#39;s gateway, at the subscriber&#39;s request, commands the terrestrial system to route and carry the message from the subscriber to a selected destination. The message comes in through the subscriber&#39;s gateway, passes through the terrestrial system, and exits through the selected destination gateway to reach the access point required. Once the message path is established, two-way communication can commence. This is the way the terrestrial system works. Each gateway is connected to another single gateway. The terrestrial system is not adapted, readily, to switch, during the course of a message, between different source gateways or between different destination gateways. 
     Each gateway, in what is generally understood to be a telephone system, provides a plurality of access points whereby a subscriber may make or receive calls. Each access point is unique in its identity or location and definitely is unique to its particular gateway. In other words, one access point means one gateway. 
     Now, satellite systems provide a gateway to the terrestrial system via earth stations, capable of establishing two way contact with a satellite or satellites. In turn, the satellites can communicate with user terminals (portable handsets). The user terminals are thus put in contact with the earth stations which can act as a gateway to the terrestrial system. 
     Sadly, each earth station is a separate gateway. Generally, the earth stations are widely spread apart. There is envisaged an earth station density of one, two or just a few earth stations on each continent. Thus, in general, each user terminal will see satellites which are visible only to one earth station. This satisfies the terrestrial network requirement “one access point means just one gateway.” This situation is not a problem. However, there are circumstances, for example, in the middle of the Pacific ocean, in the middle of the Sahara desert, high atop the Andes, where a user terminal may be able to see a satellite or satellites which, in turn, are visible to more than one earth station. This now breaks the rule “one access point means just one gateway”. The user terminal is an access point. Each earth station is a gateway. Now there is the situation “one access point-many gateways”. The terrestrial network is not adapted to respond to multiple gateways. Further, the signals to and from the earth station, satellites and user terminals are forever changing. What was a perfectly good satellite link a moment ago is no longer functional because the satellite has moved, the user terminal has been shielded by a building or other structure, and a host of other reasons. However, one of the other user terminal to satellite to ground station links might still work. How can the switch between ground stations be effected without confusing the operation of the terrestrial network? The present invention seeks to solve the twin problem of multiple gateway access and operational gateway switching at minimal cost and with least disruption to the general functionality of the terrestrial network. 
     SUMMARY OF THE INVENTION 
     According to a first aspect, the present invention consists in a communication system wherein a user terminal can communicate with a plurality of satellites; wherein each of said plurality of satellites can communicate with a plurality of earth stations; wherein each of said plurality of earth stations is operable to provide a gateway into a terrestrial system; and wherein said user terminal is registered at a selected one of said plurality of earth stations as its gateway station; said system being characterised by said gateway station, in the event of being unable to establish contact with said user terminal, being operative to select and instruct another earth station to establish contact with said user terminal to act as an alternative earth station and being further operative to pass messages to and from said user terminal through said alternative earth station while acting as said gateway into said terrestrial system. 
     The present invention further provides a communication system wherein said gateway earth station is operative to employ a low capacity data link to instruct said alternative earth station to attempt to establish contact with said user terminal and, in the event of said alternative earth station establishing contact with said user terminal, is operative thereafter to employ a high capacity voice link to pass messages to and to receive messages from said alternative earth station. 
     The present invention further provides a communication system wherein said gateway earth station is operative to recall the last known geographical location of said user terminal and is further operative to select, as said alternative earth station, that earth station most likely to establish contact with a user terminal at said last known geographical location. 
     The present invention further provides a communication system wherein said gateway earth station is operative to recall and employ a plurality of timed previous geographical locations for said user terminal to estimate the instant position of said user terminal and is further operative to select, as said alternative earth station, that earth station most likely to establish contact with said user terminal at said estimated instant position. 
     The present invention still further provides a communication system wherein said gateway earth station, in the event of being incapable of establishing contact with said user terminal, is operative to select in turn, as said alternative earth station, each of those earth stations which are geographically most proximate to said gateway earth station until an alternative earth station is found capable of establishing contact with said user terminal. 
     The present invention still further provides a communication system wherein said gateway earth station is operative to execute a calculation to determine whether or not said gateway earth station is capable of establishing contact with said user terminal and to select said another earth station as said alternative earth station if and only if said calculation indicates that said gateway earth station is incapable of establishing contact with said user terminal. 
     According to a second aspect, the present invention consists in a method for establishing communication with a user terminal for use in a communication system wherein a user terminal can communicate with a plurality of satellites; wherein each of said plurality of satellites can communicate with a plurality of earth stations; wherein each of said plurality of earth stations is operable to provide a gateway into a terrestrial system; and wherein said user terminal is registered at a selected one of said plurality of earth stations as its gateway station; said method comprising the steps of said gateway station, in the event of being unable to establish contact with said user terminal, selecting and instructing another earth station to establish contact with said user terminal to act as an alternative earth station and said gateway station passing messages to and from said user terminal through said alternative earth station while acting as said gateway into said terrestrial system. 
     The second aspect of the present invention further provides a method including the steps of said gateway earth station employing a low capacity data link to instruct said alternative earth station to attempt to establish contact with said user terminal and, in the event of said alternative earth station establishing contact with said user terminal, employing thereafter a high capacity voice link to pass messages to and to receive messages from said alternative earth station. 
     The second aspect of the present invention still further provides a method including the steps of said gateway earth station recalling the last known geographical location of said user terminal and selecting, as said alternative earth station, that earth station most likely to establish contact with a user terminal at said last known geographical location. 
     The second aspect of the present invention yet further provides a method including the steps of said gateway earth station recalling and employing a plurality of timed previous geographical locations for said user terminal to estimate the instant position of said user terminal and, thereafter, selecting, as said alternative earth station, that earth station most likely to establish contact with said user terminal at said estimated instant position. 
     The second aspect of the present invention yet further provides a method including the steps of said gateway earth station, in the event of being incapable of establishing contact with said user terminal, selecting, in turn, as said alternative earth station, each of those earth stations which are geographically most proximate to said gateway earth station until an alternative earth station is found capable of establishing contact with said user terminal. 
     The second aspect of the present invention further provides a method including the steps of said gateway earth station executing a calculation to determine whether or not said gateway earth station is capable of establishing contact with said user terminal, and selecting said another earth station as said alternative earth station if and only if said calculation indicates that said gateway earth station is incapable of establishing contact with said user terminal. 
     In a preferred embodiment, a plurality of earth stations are each uniquely connected to and controlled by a respective mobile satellite switching centre, the mobile satellite switching centre acting as a gateway into the terrestrial telephone system. The user terminal is preferably provided in the form of a portable telephone handset, capable of sending radio signals to and receiving radio signals from any one of a plurality of satellites, but equally could be a dedicated ship, aircraft, land vehicle or manually portable device. The satellites are preferably in a constellation comprising ten satellites in each of two orthogonal orbits, each orbit being at 45 degrees to the equator of the earth, and each at a height of 10,500 km above the surface of the earth, but equally could be in any configuration capable of presenting access to one or more satellites at a time to the user terminal and of presenting one or more satellites at a time to an earth station for contact to be made between at least one satellite and one earth station. Each mobile satellite switching centre preferably contains a visitor register whereat the user terminal can register as a visitor so that the terrestrial system, when originating a call to the user terminal, can consult a home register which holds details of at which mobile satellite switching centre the user terminal is registered as a visitor, so to route telephone calls directly to that mobile satellite switching centre whereat the user terminal is registered as a visitor. 
     Each earth station is capable of passing commands to all the other earth stations and of receiving responses back, preferably by means of a low capacity data link, but equally by any means exhibiting the necessary characteristics. The mobile satellite switching centres are provided with means to send voice messages between each other, preferably by means of a dedicated voice grade link, but alternatively by any other means capable of so sending and receiving voice messages. 
     In its operation, the overall system keeps the mobile satellite switching centre whereat the user terminal is listed as a visitor as the gateway into and out of the terrestrial system and, if it becomes necessary to employ a different earth station to communicate with the user terminal, signals between earth stations and receives voice messages from their associated mobile satellite switching centres so that, even though a different earth station is conducting the communication, the mobile satellite switching centre whereat the user terminal is registered as a visitor remains the sole gateway into the terrestrial system. 
     When seeking to contact the user terminal, the mobile satellite switching centre whereat the user terminal is registered as a visitor uses various techniques, including geographical location, estimation of geographical location, and surrounding area coverage to select, if required, an alternative earth station to contact the user terminal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention is further explained, by way of an example, by the following description, read in conjunction with the appended drawings, in which: 
     FIG. 1 is a highly schematised representation of the global communications system within which the present invention is comprised; 
     FIG. 2 is a flow chart of the activities of the user terminal when interacting with the system of FIG. 1; and 
     FIG. 3 is a flow chart of the activities of the main Mobile Satellite Switching Centre of FIG.  1 . 
    
    
     DESCRIPTION OF EMBODIMENTS 
     Attention is drawn to FIG.  1 . The terrestrial system  1  comprises a plurality of gateways  3  which may be individual national public or cellular telephone systems, public data switched data networks, public land mobile systems, other satellite systems, and so on. Included in the gateways are a number of mobile satellite switching centres  5 A  5 B  5 C  5 D  5 E. Each mobile satellite switching centre  5 A-E is uniquely connected to one, and only one, earth station  7 A  7 B  7 C  7 D. The earth stations  7 A-E are scattered about the world. They are designed to interact with a constellation of satellites  9 A  9 B  9 C, in two orthogonal orbits, with ten operational satellites in each orbit with two spares, each orbit at 45 degrees to the equator and at a height of 10,500 km from the surface of the earth. This is only an example of one satellite system with which the present invention would work. As will become apparent to those skilled in the art upon reading the following description and claims, the invention is applicable to virtually any satellite system where a multiple gateway problem arises. 
     Each of the earth stations  7 A-E can communicate with any of the satellites  9 A-C which may be within its line of sight. At any one time, different satellites  9 A-C may be in the line of sight of different earth stations  7 A-E. Some satellites  9 A-E may be visible to more than one earth station  7 A-E. 
     A user terminal  11 , otherwise unconnected to the terrestrial system  1  because it is remote or does not have the ability so to connect, is able to exchange radio signals with any of the satellites  9 A-C which may be in its line of sight and sufficiently elevated above the horizon for communication to be established and maintained. The user terminal  11  is generally in the form of a portable radio telephone handset, not unlike a cellular radio telephone handset, but adapted to function with satellites  9 A-C. Now, a satellite  9 A-C, visible to the user terminal  11 , may, in turn, be visible to more than one earth station  7 A-E. Likewise, more than one satellite  9 A-C may simultaneously be visible to the user terminal  11  so that a user terminal  11 , potentially, may be in simultaneous contact with multiple earth stations  5 A-E. 
     When a subscriber  13  wishes to call the user terminal  11  the terrestrial system  1 , recognising the unique identifier of the user terminal  11 , is referred to a home location register  15 , located within the terrestrial system  1 . The home location register  15  is the normal register whereat the details of the user terminals  11  are stored, and may, physically, be located proximate to the point where the user terminal  11  is generally located. For the purpose of this example, it is assumed that the user terminal  11  has moved, and is no longer in its usual location. 
     At some point, the user terminal  11  has logged on to a satellite and has thus engaged one of the Mobile Satellite Switching Centres  5 A  5 B  5 C  5 D. This is its visitor location. Each Mobile satellite Switching Centre  5 A-E comprises a corresponding Visitor Location Register  17 A-E. When the User terminal  11  is logged on to the terrestrial system  1  via the earth station  7 A-E to which it selected access and, in turn, onto its corresponding gateway, which, in this instance, is the related Mobile Satellite Switching Centre  5 A-E, a message is sent through the terrestrial system  1  so that the identity of the particular gateway  5 A-E wherethrough the user terminal  11  might be found, is stored in the home location register  15 . When the subscriber  13  attempts to call the user terminal  11 , reference is first made to the home location register  15 . The home location register  15  now holds the identity of the gateway  5 A-E wherethrough the user terminal  11  is to be found. The audio signal, from the subscriber and via the subscriber&#39;s gateway  3 , is routed directly to the visitor location (let us say  5 C) whereat the user terminal  11  is registered. 
     As a result of the operation of the terrestrial system  1 , the subscriber  13  is directly connected, via the subscriber&#39;s gateway  3 , for two way voice communication, as indicated by the arrow  17 , to the relevant Mobile Satellite Switching Centre  5 C, whereat the user terminal  11  happens to be registered as a “Visitor Location.” 
     Now, so far, everything, as described, is perfectly normal, so far as the terrestrial system  1  is concerned. A subscriber  13  has gone through their related gateway  3  to access another gateway  5 C for establishment of two way communications. From now on, things become a little more complicated because the user terminal  11  happens to be connected via an earth station  5 C whereat the connection via the relevant satellite  9 A-C is a transient possibility and whereat the possibility of any form of satellite connection between the user terminal  11  and the earth station  5 C whereat the visitor location information  17 C happens to be stored is, at best, uncertain, At any moment, the earth station  5 A-C could change from the visitor location earth station  5 C to one of the others  5 A  5 B  5 D  5 E. This is because satellites are constantly changing their position with respect to the user terminal  11  and with respect to the earth stations  5 A-E. Conditions can change, making the location of the best positioned earth station  7 A-E a matter of some uncertainty. 
     Each of the earth stations  7 A-E is linked to every other earth station  7 A-E by a low capacity digital link  19  capable of passing instructions and responses between the earth stations  7 A-E but of insufficient capacity to carry voice signals. 
     Each Mobile Satellite Switching Centre  5 A-E is linked, via the terrestrial network  1 , to every other Mobile Satellite Switching Centre  5 A-E by a voice link  21  capable of two way voice grade communication. 
     The digital link  19  is a simple packet network and it is envisaged that this can be a permanent connection. Alternatively, it can consist in leased or borrowed time slots in systems comprising part of the terrestrial system. 
     The voice link  21  is an expensive item to create and to run. It is to be recalled that the earth stations  7 A-E may be many thousands of kilometres apart and more than likely separated by oceans. Thus, the associated Mobile Satellite Switching Centres  5 A- 5 E are similarly separated. While it is within the scope of the present invention that the voice link  21  can be a permanent, dedicated voice grade channel or channels, it is preferred that the voice link  21  is a line, already existing within the terrestrial network and which can be called upon when required. For example, when one Mobile Satellite Switching Centre  5 C wishes to call another  5 E, being itself a gateway, the one Mobile Satellite Switching Centre  5 C simply places a call through the terrestrial system  1  to the other  5 E and establishes the voice link  21  that way. This example is not intended to be restrictive, and those, skilled in the art, will be aware of numerous other ways in which the voice link  21  can be created. 
     FIG. 2 is a flowchart of the behaviour of the user terminal  11 . 
     Even when not functional in the ordinary sense, the user terminal  11  is active and monitoring incoming messages. Each of the satellites  9 A-C puts out constant general messages for the information and instruction of whatever user terminals  11  can hear them. In a first activity  23 , the user terminal  11  hears the messages, broadcast from the satellites  9 A-C. These messages contain information concerning with which earth station  7 A-E the particular satellite is in contact. If a first test  25  detects a general message from a satellite  9 A-C, control is passed to a second activity  27  where the information is extracted concerning from which earth station  7 A-E the message originated. Control then passes to a third activity which updates a running log of the number of general messages received from each earth station. 
     The log keeps a record of the number of messages, received in a predetermined time, from each earth station  7 A-C which can be heard. The log thus represents a measure of the relative accessibility of each earth station  7 A-C. The predetermined period can be as long as a day or as short as a few minutes, depending upon the service required. For example, if the user terminal is in a slow moving vehicle such as a sailing boat or a land vehicle in a desert, the rate of change of position is relatively slow so that a log, kept over a day, will contain valid information about the most easily accessible earth station  7 A-C. On the other hand, if the user terminal is in a fast vehicle, such as an aircraft or a low orbiting space vehicle, the rate of change of position is very fast so that a log lasting over just a few minutes will give the best information. 
     Control is passed from the third activity  29  back to the first activity  23  where the user terminal  11  continues to monitor messages from the satellites  9 A-C. 
     If the first test  25  detects no message, a second test  31  looks for any necessary call making activity which may be required. If no call making activity is required, control is passed back to the first activity  23  where the user terminal  11  continues to monitor messages from the satellites  9 A-C. 
     If the second test  31  detects a call making requirement, control is passed to a 4th Activity  33 . 
     The call making requirement may come from various sources. The user terminal  11  is required, after a predetermined lapse of time, to call into the satellite system to “log on” to confirm its presence and, by means outside the boundaries of the present invention, to indicate its approximate location on the surface of the earth. Equally, the owner of the user terminal  11  may wish to make a telephone call. Similarly, an earth station  7 A-C may have paged the user terminal  11  and require a response. This list is merely representative and not exhaustive of the various reasons the user terminal  11  may wish to originate a call. 
     The fourth activity  33  looks up the log, selects that earth station  7 A-E which has the most log entries, and causes the user terminal  11  to page that earth station  7 A-E. A third test  35  monitors whether or not the called earth station responds. If no response is received, control is passed to a fifth activity  37  which causes the user terminal  11  to attempt to page that earth station with the second largest number of entries in the log. If a fourth test  39  does not detect that contact has been made, control can be passed back to the fourth activity  33  to attempt, once more, to page the most likely earth station  7 A-E. Not shown in the flow chart, but implicit from this example, is the possibility of going right down the list of earth stations  7 A-E in the log until one is found that responds. Also not shown, but also implicit, is the possibility that the fourth test  39 , or its equivalent depending on how may earth stations  7 A-E on the log are paged, simply terminates the attempt to raise an earth station  7 A-E and passes control back to the first activity  23 , or passes control back to the first activity  23  after a predetermined number of attempts to raise different earth stations  7 A-E. 
     In the above manner, the user terminal  11  gains access to the terrestrial system  1  via the satellites  9 A-C and the earth stations  7 A-E in the most expeditious manner possible and with the highest probability of a successful connection. 
     When contact is made, a sixth activity  41  sends, among other information which is of no concern to this invention, details of the log to that earth station  7 A-E with which contact is made. If no voice contact is required (i.e. the user terminal  11  is not required to make or receive a call), a fifth test  43  passes control back to the first activity  23 . If voice contact is required, a seventh activity  45  makes or receives the call until a sixth test  47  detects that the call is over and passes control back to the first activity  23 . 
     Now, if the call was originated from the user terminal  11 , the Mobile Satellite Switching Centre  5 A-E associated with the earth station  7 A-E which received the contact from the user terminal becomes the visitor location of that user terminal and details of the particular user terminal  11  are transferred from the home location register  15  in the terrestrial system  1  into the visitor location register  17 A-E in the appropriate Mobile Satellite Switching Centre  5 A-E. Under the terms of the present invention, the log details, transferred in the sixth activity  41  to the earth station  7 A-E, are also stored in association with the details held in the visitor location register  17 A-E. 
     FIG. 3 is a flowchart of the activities of the Mobile Satellite Switching Centre (chosen in this instance, per FIG. 1, to be  5 C) whereat the user terminal  11  has been registered as a visitor location ( 17 C). 
     An eighth activity  49  awaits a request from the terrestrial system  1  until a seventh test  51  detects such a request, whereupon control is passed to a ninth activity  53  which recalls the details of the particular user terminal it is desired to access from the visitor location register  17 C and its associated memories. 
     An eighth test  55  checks the associated data to see if a geographical location is stored. This data, as was earlier described, is derived from a prior knowledge of the position of the satellites  9 A-C, doppler frequency shift measurements and measurement of signal path delays to determine the location of the user terminal to within some 50 meters on the surface of the earth. If such a location is stored, control is passed to a tenth activity  57  where it is calculated whether or not, given the instant disposition of the satellites  9 A-E, the user terminal  11  can contact the earth station  7 C whereat it is registered as a visitor from its last recorded location. If a ninth test  59  confirms that the user terminal  11  can, at that instant, be reached, control is passed to an eleventh activity  61  which causes the user terminal  11  to be paged. If the user terminal  11  is successfully paged, a tenth test  63  passes control to a twelfth activity  65  which allows the call to the user terminal  11  to proceed and passes control back to the eighth activity  49  on call completion. 
     If the eighth test  55  detected no geographical details, it is still a fair bet that the particular earth station  7 C whereat the user terminal  11  is listed as a visitor is the most likely earth station  7 C to reach it. Accordingly, the eighth test  55 , if no geographical data is available, passes control to the eleventh activity  61  which pages the user terminal in any event and proceeds with the call if successful. 
     If the attempt to page the user terminal  11  has failed, or if the user terminal  11  has been deemed unreachable by the ninth test  59 , an eleventh test  65  checks to see if the details of the log, generated by the user terminal in the third activity  29  and sent, at last access, by the sixth activity  41 , to the earth station  17 C, are present. If no log data is available, the eleventh test  65  passes control to a twelfth activity  67  which terminates the attempted contact with the user terminal  11  and passes control back to the eighth activity  49  where the Mobile Satellite Switching Centre  17 C awaits a further approach from the terrestrial system  1 . 
     If the eleventh test  65  detects the presence of the log data from the user terminal  11 , it passes control to a thirteenth activity  69  which causes the user terminal  11  to be addressed from another earth station  7 A-E. 
     Let us say, for the sake of example, that the user terminal had heard  25  calls from a first earth station  7 B,  50  calls from a second earth station  7 C and  10  calls from a third earth station  7 D. Now the earth station  7 C  5 C whereat the user terminal  11  is registered as a visitor is clearly unable to make contact. Accordingly, the thirteenth activity  69  selects the next most likely earth station  7 B, with  25  calls in the log, to address the user terminal  11 . 
     The Mobile Satellite Switching Centre  5 C, whereat the user terminal is registered as a visitor, becomes a hub and maintains access to the terrestrial network  1  as the gateway for the user terminal  11  despite the user terminal  11  being addressed by a different earth station  7 B. 
     Firstly, the second earth station  7 C sends a message via the data link  19  to the first earth station  7 B commanding the first earth station  7 B to page the user terminal  11 . The first earth station  7 B attempts the page and reports back to the second earth station  7 C whether or not the paging attempt was successful. If a twelfth test  71  detects that the paging attempt was unsuccessful, and a thirteenth test  73  detects that there are still other earth stations audible to the user terminal  11 , control is passed back to the thirteenth activity  69  where the next most likely earth station  7 D is selected. 
     For the sake of this example, let us imagine that the paging attempt from the third earth station  7 D has succeeded. The twelfth test  71  passes control to a fourteenth activity  73  where the Mobile Satellite Switching Centre  5 C at the second earth station  7 C establishes the voice link  21  to the Mobile Satellite Switching Centre  5 D at the third earth station  7 D. This has earlier been described. Control is then passed to a fifteenth activity  75  where the third earth station  7 D actually communicates with the user terminal  11 , but the voice message and any other data for the terrestrial system  1  are passed via the voice link  21  to enter the terrestrial system  1  via the Mobile Satellite Switching Centre  5 C at the second earth station  7 C. 
     When a thirteenth test  77  detects that the call is over, control is passed to a sixteenth activity  79  where the third earth station  7 D is instructed to terminate its contact with the user terminal  11 , the mobile satellite switching centre  5 C at the second earth station  7 C disconnects the voice link  21  between itself and the mobile satellite switching centre  5 D at the third earth station  7 D, and the mobile satellite switching centre  5 C at the second earth station  7 C disconnects from the terrestrial network  1 . Control then passes back to the eighth activity  49  where the mobile satellite switching centre  5 C at the second earth station  7 C awaits requests from the terrestrial system  1 . 
     Had the log from the user terminal  11  been otherwise than as in the example given, the thirteenth activity  69  would simply have gone down the list of earth stations  7 A-E in descending order of their probability of accessibility to the user terminal  11  as reflected by the number of entries against each one in the log, until one was found that could execute a successful page of the user terminal  11 . 
     The twelfth activity  67  can also have another variant where log information is not available. Instead of simply terminating the call, in the twelfth activity  67  the mobile satellite switching centre  5 C at the second earth station  7 C uses the digital link  19  to instruct other earth stations  7 C, in the geographic vicinity of where the user terminal  11  was believed to be, or simply surrounding the second earth station  7 C, to see if a successful page could be executed. If a page succeeds, the contact is made via the other earth station  7 A-E as described above. If a successful page is not possible, the call is terminated. 
     As an example of this last variant, if a user terminal is believed to be somewhere in the Caribbean, but cannot be paged from a North American earth station  7  where its visitor location is stored, because of its location the user terminal might be accessible from earth stations  7  in South America or Africa. Accordingly, the North American earth station  7  causes the African and South American earth stations  7  to attempt to page the user terminal  11 . 
     If no information is available concerning the likely location of the user terminal  11 , its visitor location earth station  7  in North America commands not only the South American and African earth stations  7  to attempt to page the user terminal  11 , but also in Europe, the Pacific Rim and the polar region. In this way the earth station  7  whereat the user terminal has its visitor location does its best to complete the ring around itself so that, no matter in which direction the user terminal  11  may have escaped, it can be found. 
     A variant exists for the tenth activity  57 . Instead of simply taking the last known geographical location of the user terminal  11 , the times and locations of the user terminal  11  on a previous plurality of accesses are used to plot, for example, a speed and direction for the user terminal  11  which would be of great utility if the user terminal were on a rapidly moving transport such as a jet aircraft or a low orbiting spacecraft. An estimate of the present location can then be made when calculating whether or not the user terminal  11  is accessible and the appropriate earth station  7  closest to the estimated location instructed to page the user terminal  11 . 
     The user terminal  11  generally comprises a subscriber identity module (known as a SIM) which is provided in the form of a smartcard comprising memory and a microprocessor, all of which can be inserted into the body of the user terminal. In this embodiment, it is preferred that the activities of the user terminal are comprised within the overall action of the smartcard, being additions to the overall controlling software. Those, skilled in the art, will be aware that there are many other ways in which the user terminal  11  can be caused to behave as hereinbefore described. 
     Equally, in this embodiment of the invention, it is preferred that the overall activity of the mobile satellite switching centres be comprised as an addition to the data processing activity which already exists therein. Those, skilled in the art, will be aware of numerous other ways in which the mobile satellite switching centres can be caused to behave as hereinbefore described.