GB2355365A

GB2355365A - Determining the location of a mobile terminal

Info

Publication number: GB2355365A
Application number: GB9919568A
Authority: GB
Inventors: Richard Wyrwas
Original assignee: ICO Services Ltd
Current assignee: ICO Services Ltd
Priority date: 1999-08-18
Filing date: 1999-08-18
Publication date: 2001-04-18
Also published as: GB9919568D0

Abstract

A method of determining the location of a mobile terminal of a mobile communications network relative to the network, comprising the steps of: transmitting a forward signal 61 to said mobile terminal from a network transmitter 4 of known position; transmitting a return signal 62 from the mobile terminal to a plurality of network receivers 4a-c of known positions in response to said forward signal, said plurality of network receivers being determined by said forward signal; calculating the signal propagation delay between the mobile terminal and the plurality of receivers to derive location information of the mobile station. Alternatively, location may be determined by the Doppler shift of the return signals.

Description

2355365 COMMUNICATION SYSTEM AND METHOD

FIELD OF THE INVENTION

The present invention relates to a mobile communication system, particularly but not exclusively a satellite mobile communication system, in which the geographic location of a user may be determined.

The reasons for wishing to establish the location of a mobile terminal, or handset, on the surface of the Earth are two fold. Firstly, in order to correctly route a radio signal to the mobile terminal, when this is required, it is necessary to know the approximate location of the mobile ten-ninal in order to correctly select an appropriate beam from of appropriate satellite which covers the portion of the Earth's surface where the user terminal is located. Secondly, in order to properly implement restrictions on the use of a mobile terminal based on the territory in which the mobile terminal may be operated, or to implement other territory based functions such as call barring or local billing in a satellite communication system, it is necessary to determine the location of the user terminal.

One approach to overcoming this problem is to provide a user terminal with a "Global Positioning by Satellite" (GPS) system with which to determine the position of the user terminal on the surface of the Earth. An example of such a system is disclosed in European Patent EP 0562 374. The user terminal transmits its exact position to the Earth station via a satellite link. This is then used by the Earth station to control the fiscal and mechanical aspects of the communication activity with the user terminal. However, such systems require multiple frequency mobile terminals in order that they may be capable of both communication and of GPS measurement. This therefore significantly increases the cost of the mobile terminals.

US 3,384,891 discloses a navigation system using satellites, or aircraft, to relay signals between a ground station and a mobile terminal. The method relies upon measuring propagation delays between two or more satellites or aircraft in known locations and the user terminal. The propagation delays are then used to calculate a locus of possible positions for the mobile terminal, through time-time triangulation.

EP 0856957 A discloses a satellite communication system for Earth based mobile users. This publication teaches a method of establishing the location of a mobile terminal on the surface of the Earth. This is achieved by measuring propagation delays and the Doppler shift in the transmitted signals between one or more satellites and the mobile terminal.

Thus both US 3,384891 and EP 0856957 A teach methods of determining the position of a mobile terminal without being reliant upon a separate GPS system. However, both of these methods require the mobile terminal to continually search for visible satellites. This is in order that the mobile terminal may receive relevant downlink signals which are required for the successful operation of the methods. This reduces the idle mode efficiency of the terminal and thus its battery life.

It would therefore be desirable to provide a mobile communication system and method in which the position of a mobile terminal can be determined with sufficient accuracy to allow correct routing of calls and the implementation of territory based ftinctions which overcomes the problems associate with the prior art systems.

Accordingly, the present invention provides a method of determining the location of a mobile terminal of mobile communications network relative to said network, comprising the steps of. transmitting a forward signal to said mobile terminal from a network transmitter of known position; transmitting a return signal from said mobile terminal to a plurality of network receivers of differing known positions in response to said forward signal, said plurality of network receivers be-Ing determined by said forward signal; calculating the signal propagation delay between said mobile terminal and a plurality of said receivers to derive location information relating to said mobile terminal.

The method of the present invention advantageously increases the idle mode efficiency of the mobile terminal. By arranging for a single satellite, which is being monitored by the mobile terminal, to transmit data to the mobile terminal defting all satellites which may be used to carry out a propagation delay location updating routine, the mobile terminal is not required to expend power continually searching for a sufficient number of visible satellites in order to successfully carry out the routine. Thus, power may be conserved whilst preserving the ability of the system to locate the mobile terminal.

4 - The method of the present invention also provides for improved accuracy of mobile terminal location. This is because satellites which would be undetectable by the mobile terminal, and hence not used in prior art systems to provide positional information about a given mobile terminal, may be used in the system of the present invention. This is because in certain instances, a satellite may be able to detect a mobile terminal uplink signal although the mobile terminal may not be able to detect the satellite downlink signal. This may arise in practice due to the differences in -antenna size and gain, together with the differences in signal processing capability between the mobile terminal and the satellite.

Furthermore, the method of the present invention allows for mobile terminal location without the requirement for complex and expensive handset modifications.

Other aspects and preferred embodiments of the invention are as described or claimed hereafter, with advantages which will be apparent from the following.

is BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure I is a block diagr am showing schematically the elements of a first communications system embodying the present invention; Figure 2a is an illustrative is a block diagram showing schematically the elements of mobile terminal equipment suitable for use with the present invention; and, Figure 2b is a corresponding block diagram; Figure 3 is a block diagram showing schematically the elements of an Earth station node forming part of the embodiment of Figure 1; Figure 4 illustrates schematically the beams produced by a satellite in the embodiment of Figure 1; Figure 5 illustrates schematically the disposition of satellites forming part of Figure 1 in orbits around the Earth; Figure 6 illustrates schematically the transmission of signals between the Earth node stations, satellites and user terminal in implementing the position determination procedure according to the preferred embodiment; Fig-Lire 7 is a diagram showing schematically the spectral distribution of frequency carriers on a user downlink beam; Figure 8 is a diagram showing schematically the structure of a TDNIA frame on one of the carriers of Figure 7; Preferred Embodiment Various mobile voice communications systems are known. The invention is envisaged for use in satellite mobile digital communications systems such as the TM IDmarsat-M satellite telephone system, the IridiUm satellite cellular system (described in, for example, EP-A-0365885), the COT. satellite cellular system (described in, for example, GB-A-2295296) or the OdysseyTM satellite cellular system (described in, for example EP-A-0510789). Although in the preferred embodiment a satellite mobile digital communications system is envisaged, the invention is also envisaged for use in corresponding terrestrial digital mobile communication systems such as GSM, or other mobile communication services.

Referring to Figure 1, a satellite communications network according to this embodiment comprises mobile user terminal equipment 2a, 2b (e.g. handsets 2a and 2b); orbiting relay satellites 4a, 4b; satellite Earth station nodes 6a, 6b; satellite system gateway stations 8a, 8b; terrestrial (e.g. public switched) telecommunications networks 1 Oa, I Ob; and fixed telecommunications terminal equipment 12a, 12b.

Interconnecting the satellite system gateways 8a, 8b with the Earth station nodes 6a, 6b, and interconnecting the nodes 6a, 6b with each other, is a dedicated ground-based network comprising channels 14a, 14b, 14c. The satellites 4, Earth station nodes 6 and lines 14 make up the infrastructure of the satellite communications network, for communication with the user terminals 2, and accessible through the gateway stations 8.

A terminal location database station 15 (equivalent to a GSM HLR) is connected, via a signalling link 60 (e.g. within the channels 14 of the dedicated network) to the gateway station and Earth stations 6.

The PSTNs 10a, 10b comprise, typically, local exchanges 16a, 16b to which the fixed terminal equipment 12a, 12b is connected via local loops 18a, l8b; and international switching centres 20a, 20b connectable one to another via transitional links 21 (for example, satellite links or subsea optical fibre cable links). The PSTNs I Oa, I Ob and fixed terminal equipment 12a, 12b (e.g. telephone instruments) are well known and almost universally available today.

For voice communications, each mobile terminal apparatus is in communication with a satellite 4 via a full duplex channel (in this embodiment) comprising a downlink channel and an uplink channel, for example (in each case) a TDMA time slot on a particular frequency allocated on initiation of a call, as disclosed in UK patent applications GB 2288913 and GB 2293725. The satellites 4 in this embodiment are non geostationary, and thus, periodically, there is handover from one satellite 4 to another.

Terminal 2 Referring to Figures 2a and 2b, a user i-erminal equipment 2 of Figure I is shown.

The terminals 2a, 2b may be similar to those presently available for use with the GSM system, comprising a digital low rate coder/decoder 30, together is with conventional microphone 36, loudspeaker 34, battery 40 and keypad components 38, and a radio frequency (RF) interface 32, capable of tuning the frequency at which signals are transmitted and received to specific bands in a broad, usable RF spectrum, and antenna 31 suitable for satellite communications.

A display 39 (for example a liquid crystal display) and a 'smart card' reader 33 receiving a smart card (subscriber identity module or SIM) 35 storing user information are also provided. Specifically, the SIM 35 includes a processor 35a and permanent memory 35b.

8 - Also provided is a terminal control circuit 37 (which may in practice be integrated with the coder 30) consisting of a suitably programmed microprocessor, microcontroller or digital signal processor (DSP) chip. The control circuit 37 performs various ftinctions including framing speech and data into TDMA time frames for transmission (and likewise demultiplexing received TDMA frames); and performing encryption or enciphering.

The coder/decoder (codec) 30 in this embodiment comprises a low bit rate coder 30a, generating a speech bit stream at around 3.6 kilobits per second, together with a channel coder 30b applying error correcting encoding, to generate an encoded bit stream at a rate of 4.8 kilobits per second.

The SIM memory 35b stores various subscriber identity data including the international mobile subscriber identity (IMSI), which is a unique number associated with that SIM (and hence the subscriber to whom it belongs).

Unlike conventional mobile terminals, mobile terminals according to the is present invention are suitably programmed to identify received signals concerned with location updating procedures and to respond to such signals as described below. This programming may be stored on and implemented by the SIM 35 or the terminal control circuit 37, or similar memory and processing device.

Earth Station Node 6 The Earth station nodes 6 are arranged for communication with the satellites.

Each Earth station node 6 comprises, as shown in Figure 3, a conventional satellite Earth station 22 (functioning somewhat equivalently to the Base Station of a cellular system) consisting of at least one satellite tracking antenna 24 arranged to track at least one moving satellite 4, R-F power amplifiers 26a for supplying a S signal to the antenna 24, and 26b for receiving a signal from the antenna 24; and a control unit 28 for storing the satellite ephemera data, controlling the steering of the antenna 24, and effecting any control of the satellite 4 which may be required (by signalling via the antenna 24 to the satellite 4).

The Earth station node 6 ftirther comprises a mobile satellite switching centre 42 comprising a network switch 44 connected to the trunk links 14 forming part of the dedicated network. It may be, for example, a commercially available mobile switch centre (MSC) of the type used in digital mobile cellular radio systems such as GSM systems. A multiplexer 46 is arranged to receive switched calls from the switch 44 and multiplex them into a composite signal for supply to the amplifier 26 via a low bit-rate voice codec 50. Finally, the Earth station node 6 comprises a local store 48 storing details of each mobile terminal equipment 2 within the area served by the satellite 4 with which the node 6 is in communication.

The local store 48 acts to fulfil the functions of a visited location register (VLR) of a GSM system, and may be based on commercially available GSM products.

Alternatively, satellite control may be provided from a separate control station.

The gateway stations 8a, 8b comprise, in this embodiment, commercially available mobile switch centres (MSCs) of the type used in digital mobile cellular radio systems such as GSM systems. They could alternatively comprise a part of an international or other exchange forming one of the PSTNs 10a, l0b operating under software control to interconnect the networks 10 with the satellite system trunk lines 14.

The gateway stations 8 comprise a switch arranged to interconnect incoming PSTN lines from the PSTN 10 with dedicated service lines 14 connected to one or more Earth station nodes 6.

Also provided in the gateway stations 8 is a store for billing, service and other information relating to those mobile terminals 2 for which the gateway station 8 is the home gateway station.

The database station 15 comprises a digital data store which contains, foi every subscriber terminal apparatus 2, a record showing the identity (e.g. the International Mobile Subscriber Identity or IMSI); the service provider station 8 with which the apparatus is registered (to enable billing and other data to be collected at a single point) and the currently active Earth station node 6 with which the apparatus 2 is in communication via the satellite 4.

Thus, in this embodiment the database station 15 acts to fulfil the functions of a home location register (HLR) of a GSM system, and may be based on commercially available GSM products.

Periodically, the Earth station nodes measure the delay and Doppler shift of communications from the terminals 2 and transmit these to the database station 15, which calculates the rough terrestrial position of the mobile terminal apparatus 2 using the differential arrival times and/or Doppler shifts in the received signal, and knowledge of which beams of which satellites 4 the signal was received through.

The position is then stored in the database 48.

Satellites 4 The satellites 4a, 4b comprise generally conventional communications satellite bus such as is used in the HS601 available from Hughes Aerospace Corp, California, US with a communications payload, and may be as disclosed in GB 2288913. Each satellite 4 is arranged to generate an array (typically hexagonal) of beams each beam being generated by a separate antenna, as described in EP 421722A. The beams cover a footprint beneath the satellite, each beam including a number of different frequency channels and time slots, as described in GB 2293725 and illustrated in Figure 4.

On each beam, the satellite therefore transmits a set of user downlink frequencies as shown in Figure 7. The frequencies are separated by a frequency guard band. The downlink frequencies on adjacent beams are different, so as to permit frequency re-use between beams. Each beam therefore acts somewhat in the manner of a cell of a conventional terrestrial cellular system. For example, there may be 19, 37, 61, 121 or 163 beams. The frequencies are allocated between satellites such that within each plane, neighbouring satellites use different frequencies at those of their beams which overlap and satellites from one plane use different frequencies than those used by satellites of the other plane.

12 - Similarly, each satellite is arranged to receive radiation in an array of beams, which in this embodiment cover the same footprints beneath the satellites, to provide a plurality of user uplink beams each carrying different frequencies.

In this embodiment, referring to Figure 8, each uplink and downlink frequency carries a plurality (e.g. 6) of time division channels, so that each mobile terminal 2 communicates on a channel comprising a given time slot in a given uplink and downlink frequency. The time slots are separated by a short guard interval.

Within each beam there is also provided a downlink common broadcast control channel (equivalent to the broadcast common control channel or BCCH of the GSM system) which occupies at least one of the frequencies for each beam; the frequencies used by the broadcast control channels of the beams are stored within each mobile terminal 2 which is arranged to scan these frequencies.

The satellites of this embodiment function as repeaters. Each satellite acts as a "bent pipe", amplifying and relaying signals from the user terminals 2 on the user terminal uplink, to the Earth station nodes 4 on a feeder downlink. Also (although it is not gen-nane to this invention) signals from the Earth stations 4 on a feeder uplink are relayed down to the user terminals 2 on a user downlink.

Every frequency/channel in the user uplink therefore has an equivalent channel in the feeder downlink, and the satellite payload operates in accordance with a predetermined routing table, to translate a user link frequency from one of the user uplink beams (e.g. at around 2 GHz) to an equivalent frequency channel in - 13 the feeder downlink (e.g. at 5 or 7 GHz). The satellite performs amplification of the user uplink signal, in this embodiment at an intermediate ftequency. The user uplink signals are not, however, digitally decoded and then remodulated. Thus, RF information such as delay and Doppler shift is preserved in the feeder downlink signal reaching the Earth station node 6.

The position of each satellite is known to a high degree of accuracy, using Earth based observations and/or by the use of a global positioning system (GPS) receiver in each satellite 4. The movement of each satellite, defined by the parameters of its orbit (the ephemeris) therefore enables each Earth station node 6 to know where the satellite is, and where it will be in future.

From a knowledge of th-- shape of the Earth (the polar and equatorial radii, for example) and the satellite orbits, the satellite Earth station node 6 can calculate position data of a transmitting source from its signal propagation delay and its Doppler shift in the uplink signal.

The satellites 4a are arranged in a constellation in sufficient numbers and suitable orbits to cover a substantial area of the globe (preferably to give global coverage).

For example 10 (or more) satellites may be provided in two mutually orthogonal intermediate circular orbits (or more) at an altitude of, for example, about 10,500 kilometres (6 hour orbits) and equatorial inclinations of 450, as shown in Figure 5. Equally, however, larger numbers of lower satellites may be used, as disclosed in EP 0365885, or other publications relating to the Iridium system, for example.

Mode of Operation When a communication session is initiated between the user terminal 2 and the network, a registration process as is well known in the art of mobile communications is carried out. This allows, amongst other functions, the identity of the user terminal 2 to be established.

On carrying out the registration process, the identity of the user terminal 2 is recorded at the local store 48 of the Earth station 6 serving the satellite 4 in communication with the user terminal 2. As discussed above, the local store 48 holds the satellite epheinera data which defines the instantaneous position and orientation of each satellite 4. Thus, as local store 48 is aware of the spot beam with which the user terminal 2 is in communication and the area of the Earth's surface covered by the beam, the approximate location of the user terminal 2 is known for the duration of the session.

The identity of the Earth station node 6 which is in communication with the user terminal 2 is copied to the terminal location database 15 as part of the location management of the system, in a similar manner to the interaction of the home location register and the visitor location registers in the GSM system.

When the communication session is terminated, the user terminal 2 returns to "Idle" mode; i.e. the user tenninal 2 is no longer in full circuit communication with the network, but the user terminal 2 continues to monitor signalling - information which may be transmitted to it on the broadcast control channel of one or more satellites from which the user terminal 2 receives the strongest broadcast control channel signal. When the user terminal 2 is in "idle" mode, only the last known position of the user terminal 2 is known to the network. Since this information is retained by the local store 48 and the terminal location database 15 when the communication session is terminated.

Periodically, after the termination of the last communication from the user terminal 2, the Earth station node 6 may instigate the location update process of the present invention in order to accurately establish the position of the user terminal 2, for the reasons described above.

The frequency with which the location update process is implemented depends upon the operational parameters of the system. However, in general it is desirable to obtain updated locations for each user terminal 2 at a significantly higher frequency than the frequency with which spotbeams pass over a stationary user terminal 2. In this manner a given user terminal 2 may progress through the moving coverage areas of adjacent spotbeams of each arriving satellite 4 in a predictable way. Thus, the possibility of the user terminal 2 becoming lost to the network is greatly reduced.

Although in this embodiment it is the Earth station controller 28 which implements the location update process, it will be understood that alternatively, or additionally, the process could be instigated by the database station 15. This may be achieved by the database station 15 signalling the Earth station node 6a with which the user terminal 2 in question was last registered, via signalling link 60 shown in Figure 1, to implement the location update process.

The location update process will now be described with reference to Figure 6.

The controller 28 of the Earth station node 6a calculates which spotbearns of which satellites currently overlap the footprint area of the spotbeam, 5 through which the user terminal 2 was last in communication, at the moment that the previous session was terminated. This is done using the satellite ephemera data at its disposal, as explained above. The exact number of such satellites will depend upon various factors, such as the location of the user terminal 2 and the instantaneous positions of the two orbits of satellites 4 with respect to user terminal 2. These satellites may include satellites 4a and 4b, which at that moment communicate with Earth station node 6a and satellites 4c, which at that moment communicates with Earth station node 6b. i.e. the user terminal 2 may be visible to one or more satellites, such as satellite 4c, which are at that moment not visible to the Earth station node 6 carrying out the location update process.

VvIen all potential satellites 4 and relevant spotbeams 5 have been identified by the Earth station controller 28, it transmits a location update instruction 61 (analogous to a paging message) to the user terminal 2. The location update instruction 61 is transmitted by the Earth station node 6a on a feeder uplink to satellite 4b from where it is broadcast on the downlink broadcast control channel to the area in which the user terminal 2 was last located.

17 - This location update instruction 61 instructs the user terminal 2 to transmit a series of location determination signals 62a-c at accurate, predetermined time intervals, at set frequencies defined in the signal 6 1. Each of the transmitted signals is at the correct frequency to be received by one of the satellites 4a-c previously calculated by the Earth station controller 28 as having a footprint overlapping the footprint of the spotbearn which the user terminal 2 is expected by the network to be monitoring.

Not all of the satellites 4a-c which are selected in this manner will necessarily be able to receive a signal from the user terminal 2, despite the fact that their footprint overlaps the spotbearn which the user terminal 2 is expected by the network to be monitoring. This may be because the user terminal 2 is not within their footprint, or because the user terminal 2 may be shaded by buildings from the satellites, for example.

Included in the location update instruction 61 is the list of frequencies on is which the transmissions are to be made, each corresponding to a position update channel used a particular satellite of the group of selected satellites. In this embodiment each satellite shares no common frequencies with other satellites in the constellation which have footprints which may periodically or permanently overlap with that of another satellite. This is also the case with the position update channels for each satellite. However, the skilled reader will realise that the system could be adapted such that this is not the case.

If no response is obtained from the user terminal 2, then the location update instr-uction is re-attempted. The procedure for locating a "lost" user terminal will vary depending upon the size of the beam footprints, the power with which the location update instruction 61 is transmitted and upon other system variables.

However, initially the link margin of the location update instruction 61 may be increased in order to reach a user terminal 2, in the event that the direct line of sight between the satellite 4 and the user terminal 2 is blocked by clothing, or other objects, as described in EP 0856957 A.

If still no reply is received, then the "area of search" may be broadened by including further spotbeams and satellites whose current footprints cover areas adjacent to and then more distant from the area where the user terminal 2 was last recorded. If still no response is obtained from the user terminal 2, the Earth station controller 28 assumes that the user terminal 2 is temporarily uncontactable, due to being obstructed or turned off. The process of locating the user terminal 2 and other transmissions to be made to the user terminal are then temporarily suspended, to be re-attempted after a predetermined time period.

Once the location update instruction 61 has been received, the user terminal 2 proceeds to transmit the location determination signals 62a-c over a signalling channel at the frequencies defined by the received location update instruction 61.

The location determination signals 62a-c are transmitted sequentially on a random access channel (termed RACH in the GSM system).

The content and format of each of the location determination signals 62ac are broadly similar, differing only in their frequency of transmission and in that each includes a label indicating the number of thattransmission in the sequence of transmissions. It will be noted that the number of transmissions in the sequence corresponds to the number of satellites 4a-c selected by the Earth station controller 28 as described above. The number of each transmission allows the time of transmission for each to be determined, in order that the propagation delay between the user terminal 2 and each receiving satellite 4a-c may subsequently be calculated. This is explained further below.

The signals are formatted so as to be recognisable as position determination signals and include data identif,ing both the user terminal 2 and the Earth station node 6 which implemented the location update process.

In the present embodiment the user terminal 2 is identified in each location determination signal 62a-c by the international mobile subscriber identity number is (IMSI) of the SIM 35 in use with that user terminal 2. However, it will be appreciated that other methods of identification could be used, for example the repetition of a unique code generated by the Earth station node 6a and transmitted to the user terminal 2 in the location update instruction 6 1.

By identifying the user terminal 2 in question, the possibility of confusing location determination signals 62a-c from two or more user terminals 2 which are simultaneously implementing a location update process is avoided.

In the present embodiment the Earth station node 6 which implemented the location update process is identified by including a short data code, unique to that particular Earth station node 6, in each location determination signal 62a-c. It is necessary to indicate which Earth station node 6 implemented the location update process, so that the location determination signals 62a-c may be relayed back to that Earth station node 6, even by selected satellites 4c which are not visible to that Earth station node 6. In such cases the location determination signal 62c is relayed to the Earth station node 6 from a further Earth station node 6b, in communication with that satellite 4c, via a satellite system trunk line 14c.

It will be appreciated that since it is the propagation delay from the user terminal 2 to each respective satellite 4a-c which is to be measured in order to determine the position of the user terminal 2, the exact time at which each location determination signal 62a-c is transmitted and at which it is received must be known.

In the present embodiment, this is achieved by configuring the user terminal 2 to sequentially transmit each location determination signal 62a-c in the sequence of signals after a predetermined time delay from the moment of receiving the location update instruction 61. However, as an alternative, this could be coded for in the location update instruction 61. This approach avoids the need for an expensive real time clock in the user terminal 2 and relies merely upon the oscillator circuits already present in the user terminal 2.

Each location determination signal 62a-c which is received by its intended satellite 4a-c is relayed by that satellite 4a-c to the Earth station node 6 with which it is communicating at that time.

In normal circumstances, in the present embodiment, this may include both the Earth station node 6a which implemented the location update process and further Earth station nodes 6b, as discussed above. Irrespective of which Earth station node 6 receives the relayed location determination signals 62a-c, that Earth station node 6 logs the exact time at which it was received.

Each Earth station node 6, on receiving a relayed location determination signal 62a-c, identifies the implementing Earth station nodes 6a using the short data code in the location determination signal 62a-c, as described above. Any Earth station node 6b which did not implement the location update process then forwards any received transmissions, together with identity of the satellite 4 which relayed the message, and its logged time of arrival via a satellite system trunk line 14c to the Earth station nodes 6a which implemented the procedure.

After a predetermined period from sending the location update instruction 61, that Earth station node 6a may conclude that any location determination signals 62a-c which it has not received, were not received by a satellite 4, due to being blocked by an obstruction or due to the satellite 4 below the minimum elevation angle with respect to the user terminal 2.

The Earth station control unit 28 then proceeds to calculate the position of user terminal 2 in the following manner.

The Earth station control unit 28 knows, through the stored satellite ephemera data and an accurate real time clock, the exact position of each satellite 4 involved in relaying each location determination signal 62a-c at the time of relaying the signal. Since the exact position of each Earth station node 6 to which location determination signals 62a-c were relayed is also known, the propagation time for all location determination signals 62a-c between each satellite 4 and its respective Earth station node 6 can be calculated, since the signals propagate at the speed of light. For the same reason, the propagation delay between the Earth node station 6a and the satellite 4b of the location update instruction can be calculated.

Furthermore, for each successfully relayed location determination signal 62a-c, the predetennined time delay between the reception by the user terminal 2 of the location update instruction 61, and the transmission of the location determination signals 62a-c by the user terminal 2 is also known. This is dependent upon the order in which the messages were sent; this order and the length of each delay being coded for in the location determination signal 62a-c, as described above.

Thus, the Earth station control unit 28 can subtract these time periods from the total period from the instant that the Earth station node 6a began to transmit the location update instruction 61 to the time of receipt of each relayed location determination signal 62a-c by the respective Earth station nodes 6. The remaining propagation delay for each signal corresponds to the combined propagation delay (and hence distance) of the location update instruction 61 between satellite 4b and user terminal 2 and the location determination signal 62a-c between the user terminal 2 and each relaying satellite 4a-c.

In the specific case where the location update instruction 61 travels the same route as the location determination signal 62b (i.e. between satellite 4b and user terminal 2), the distance between the user terminal 2 and the satellite 4b may be easily calculated, since half of the combined propagation delay is the time taken to travel between the user terminal 2 and that satellite at the speed of light. This information may then be used to solve for the distance between the user terminal 2 and each of the satellites 4 involved in relaying location determination signals 62 a and c.

It will be understood that for a giver. satellite 4 this distance defines a spherical locus of points in which the user terminal 2 may be located. If the distances between four satellites 4 and the user terminal 2 can be established, then the position of the user terminal 2 in three dimensional space may be accurately established.

However, if less user terminal 2 to satellite 4 distances are obtained, the position of the user terminal 2 may nevertheless be determined. For example, if three distances were obtained, there would be only two (at most) possible locations in which the user terminal 2 could be. However, as knowledge of the altitude of the user terminal 2 is not normally essential, the Earth's surface can be approximated as a sphere on which it may be assumed that the user terminal 2 is located; thus again giving a precise location for the user terminal 2.

In the event that only two user ten-ninal 2 to satellite distances are obtained, the locus of possible position to the user terminal 2 is a circle. Again the Earth's surface can be approximated as a sphere on which it may be assumed that the user terminal 2 is located, to give rise to two possible locations for the user terminal 2.

If one of these locations does not fall within the spot beam of any one of the satellites 4 through which the location determination signals 62a-c were relayed, that location can be excluded. Thus, an accurate 3 dimensional position of the user terminal 2 will again be obtained.

It will be appreciated that with a constellation of satellites as described herein, the distances between two or more satellites 4 and the user terminal 2 will be regularly achieved, by virtue of the technique of causing the user terminal 2 to transmit to all possible satellites 4 which may be in its direct light of sight. If 2D nevertheless insufficient satellite 2 to user terminal distances are obtained, the use of Doppler shifts in received signals, as described above, may additionally be used, in order to better locate the user terminal 2 as described in EP 0856957 A; which is hereby incorporated in its entirety by reference.

Once the position of the user terminal 2 has been established, any location based function, such as changes in tariff rates, or service, such as call barring, may then be implemented.

It will be appreciated by the skilled reader that many alternatives and adaptations may be made to the above described embodiment. For example, the Earth station nodes of the system may co-ordinate between themselves the times and frequencies at which the user terminals are instructed to transmit location determination signals, so as to minimise the risks interference between location determination signals emitted by different user terminals.

Additionally, an alternative access protocol may be used. For example Code Division in Multiple Access may be used instead of Time Division Multiple Access.

The method of the present invention may also be implemented while the user terminal is- engaged in full circuit communication with the network, in order to more accurately locate the user during these periods. This may be especially useful where the user is situated in a peripheral beam of a given satellite beam array, which due to &-e curvature of the Earth covers a larger area than that of a central spot beam.

Furthermore, the location updating procedure could be initiated at the request of the user of the user terminal, his position being subsequently transmitted is to him, in the form of a latitude and a longitude for example, in the form of a GSM style short message.

Claims

1. A method of determining the location of a mobile terminal of a mobile communications network relative to said network, comprising the steps of:

S transmitting a forward signal to said mobile terminal from a network transmitter of known position; transmitting a return signal from said mobile terminal to a plurality of network receivers of differing known positions in response to said forward signal, said plurality of network receivers being determined by said forward signal; calculating the signal propagation delay between said mobile terminal and a plurality of said receivers to derive location information relating to said mobile terminal.

2. A method according to claim 1, wherein one of the said plurality of network receivers is in substantially the same position as said network transmitter.

3. A method according to claims I or 2, wherein said forward signal determines the frequency of one or more of said return signals, to match the frequency of reception of a said network receiver.

4. A method according to any preceding claim, wherein said forward signal determines the order of transmission of a plurality of said return signals.

5. A method according to any preceding claim, wherein the said forward signal determines the time delay between receipt of said forward signal by said mobile terminal and transmission of a said return signal.

6. A method according to any one of claims I to 5, wherein the time delay between receipt of said forward signal by said mobile terminal and transmission of a said return signal is predetermined.

7. A method according to any preceding claim, wherein said forward channel is a duplex channel.

8. A method according to any one of claims 1 to 7, wherein said forward chainel is a simplex channel. 15

9. A method according to claim 9, wherein said forward channel is a paging channel.

10. A method according to any preceding claim wherein at least one return 20 channel is a simplex channel.

11. A method according to any preceding claim, wherein at least one of said network receivers is a satellite.

12. A method according to claim 11, wherein said satellite is non geostationary.

13. A method according to any preceding claim, wherein said network is a satellite communication network.

14. A method according to any one of claims I to 10, wherein at least one of said network receivers is a base station of a terrestrial cellular communications network.

15. A method according to any one of claims I to 10 or 14, wherein said 15 network is a terrestrial cellular network.

16. A method according to any preceding claim, wherein said mobile terminal is a mobile phone.

17. A mobile communication system arranged to implement the method of any preceding claim.

18. A communications network control system arranged to cause the transmission of a forward signal to a mobile terminal from a network transmitter of known position and to receive a return signal transmitted from said mobile terminal via a plurality of network receivers of differing known positions, in response to said forward signal, said plurality of network receivers being determined by said forward signal, the system being further arranged to calculate the signal propagation delay between said mobile terminal and a plurality of said receivers to derive location information relating to said mobile terminal.

19. A mobile terminal arranged to receive a forward signal from a transmitter of a communications network and being further arranged to transmit a return signal to a plurality of network receivers of differing known positions in response to said forward signal, said plurality of network receivers being determined by said forward signal.

20. A method substantially as herein described with reference to the accompanying drawings.

21. A communications network control system substantially as herein described with reference to the accompanying drawings.

22. A mobile terminal substantially as herein described with reference to the accompanying drawings.