GB2454939A

GB2454939A - Dual mode mobile position determining device

Info

Publication number: GB2454939A
Application number: GB0723126A
Authority: GB
Inventors: Surinder Singh Thind; Jung-Hwan Lee
Original assignee: Samsung Electronics Co Ltd
Current assignee: Samsung Electronics Co Ltd
Priority date: 2007-11-26
Filing date: 2007-11-26
Publication date: 2009-05-27
Anticipated expiration: 2027-11-26
Also published as: GB2454939B; GB0723126D0

Abstract

A location system is disclosed comprising, a transmitter 1 positioned at a terrestrial location adapted to transmit a data signal S1 indicative of the terrestrial location, and a mobile device 2 adapted to receive the data signal S1 and navigation signals N from satellites of at least one global navigation satellite system (GNSS). The mobile device 1 is further arranged to provide an indication of its current location as determined from either the data signal S1 or a plurality of GNSS signals N based on the signals received by the mobile device 1 at the current location L2a, L2b. The selection of either the data signal S1 or GNSS signals N for determining the location of the mobile device 1 may be based on the number of signals received, the presence or absence of one or more signals or the relative strengths of the received signals. The system may further comprise a GPS receiver (3,Fig.1) providing a location signal to the transmitter (1,Fig.1). The transmitter (1,Fig.1) may derive its data signal from the GPS receiver (3,Fig.3) and may transmit its data signal S1 into a space where navigation signals are not available V, so enabling the mobile device to provide an improved indication of location. The arrangement is particularly suited to allow a mobile device to determine its position within a building.

Description

LOCATION SYSTEM AND METHOD

Field of the Invention

The present invention relates to location systems comprising mobile devices adapted to provide an indication of current location to a user based on signals received. Particular embodiments of the invention are concerned with location systems incorporating mobile devices comprising GPS (global positioning system) receivers or receiver modules.

BackQround to the Invention Global navigation satellite systems (GNSS) and receivers operable to provide an indication of current location to a user based on signals received from the satellites of those systems at the current location are well known. These known systems include the GPS system, the Glonass system, and others. Typically, for a GNSS receiver to be able to determine its current location it needs to be able to receive signals from a plurality of the satellites (space vehicles) of that system at the current location. For example, with the OPS system, a receiver will typically need to be receiving signals from four different GPS satellites in order to determine its position with sufficient accuracy and provide an indication of that position (i.e. location) to a user, for example by means of a visual display.

In many locations (for example in rural locations) a GPS receiver (or other GNSS receiver) may easily be able to receive a sufficient number of navigation signals from space vehicles to enable it to determine its location. Equally, however, there are many locations where a GPS receiver is able to receive only a reduced number of navigation * signals, if any. For example, in urban environments the so-called problem of canyoning is well known. This is where a GPS receiver on the ground with relatively high buildings on either side is unable to receive navigation signals of sufficient strength from the : 30 requisite number of satellites to obtain a position fix. Another way of looking at this is that when the GPS receiver is located in a sufficiently built-up area, it may be unable to * see a sufficient number of the orbiting satellites. Similarly, when a GNSS receiver is carried into a building, it may no longer be able to receive any navigation signals (or at least any navigation signals of sufficient strength) to be able to continue to calculate its position. Thus, once a user carries the GNSS receiver device into a building it typically loses its location signals and is unable to provide the user with position updates as the user moves around inside the building. Clearly, in large buildings or building complexes a person may be able to move around over substantial distances and the positional information provided by the GNSS receiver device when the person entered the building or complex may be substantially different from the user's current location inside that building. Certain GNSS receivers operate in a manner such that when the satellite signals are lost (on entry to a building, for example) the device continues to display the last- calculated position. Other devices operate in a manner in which, once the navigation signals are lost, the device ceases to display positional information. Clearly, there are disadvantages associated with both of these options.

Thus, current GPS location accuracy is at least poor in buildings, and sometimes a fix on location cannot be obtained if the GISJSS satellites are not visible in a direct line of sight from the receiver. Another problem common in buildings or in built up areas is that reflections of satellite signals can result in multi-path signals arriving at the receiver, which are difficult to correlate. In some instances it may not be possible to determine a location of the receiver device from these multi-path signals, and in other instances although it may be possible to determine a location, this determined location may have a large error associated with it.

One approach to try to overcome these problems would be to attempt to improve GPS receiver sensitivity such that location could be determined from navigation messages received at weaker strength. However, there will clearly still be some locations in which no satellite signals of sufficient strength may be received. Another approach would be to try to develop algorithms able to determine location from multi-path signals, but location accuracy is still likely to be poor with such techniques as the multi-path characteristics typically do not remain constant and are likely to change at any given time in a random * fashion. S...

An additional problem with the prior art is that after a GNSS receiver has been taken into :" 30 an environment where it loses its satellite signals and can no longer provide an updated position, when it later emerges from that environment it may have lost synchronisation with the satellite system and may therefore take time to re-synchronise and acquire the ::::* appropriate number of satellite signals to resume location calculations. S. * S

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It is therefore an aim of embodiments of the invention to ameliorate or mitigate at least

one of the problems associated with the prior art.

Summary of the Invention

According to a first aspect of the invention there is provided a location system comprising: a transmitter arranged at a terrestrial location and adapted to transmit a data signal indicative of a terrestrial location; and a mobile device adapted to receive said data signal and to receive navigation signals from space vehicles of at least one global navigation satellite system, the mobile device being further adapted to provide an indication of a current location of the mobile device, the mobile device being arranged to derive said indication of current location from either said data signal or a plurality of said navigation signals, according to the signals received by the mobile device at the current location.

The mobile device can therefore be regarded as dual mode. Whether it derives (i.e. calculates, determines, generates) the location indication from a received data signal or from a plurality of received navigation signals may be arranged to depend on the signals received at its current location in a variety of ways. For example, it may be based on the number of signals received, the presence or absence of one or more signals, on the relative strengths of received signals (in which case the mobile device is adapted to measure received signal strengths or powers), on some priority system (for example in which, if a data signal is received at a power above a certain threshold, the mobile device uses that, rather than any navigation signals received), or on some other criteria.

The present invention provides the advantage that the mobile device is not totally reliant on receiving navigation signals to provide a location indication to a user; it can also provide a location indication based on a received data signal, and the transmitter may * advantageously be arranged to transmit the data signal into an area, volume, space or S..

region in which navigation signal coverage is poor, or even non-existent. S...

:" 30 The data signal may, for example, comprise data indicative of a terrestrial indication. In : certain embodiments a mobile device receiving the data signal merely has to read the data in the data signal in order to determine the location indicated by the data signal. In certain embodiments this data may have been pre-programmed into the transmitter, or may be provided to the transmitter from a GNSS receiver, for example. In certain embodiments, the data signal may comprise data which has to be processed by the mobile device in order to determine the location indicated by the data signal. In yet further embodiments, the data contained in the data signal is not, on its own, indicative of a terrestrial location, but the data signal comprises additional information which, together with the data is indicative of a terrestrial location (i.e. it can be processed to determine that terrestrial location). In certain embodiments the transmitter is arranged to retransmit a GPS (or other GNSS) signal provided to it from a receiver. Thus, a GPS receiver may be arranged to receive a plurality of navigation signals, process those signals together to produce a carrier-stripped GPS signal (comprising data from the plurality of space vehicles (SVs) and timing information -in the form of the relative positions of the respective data messages in the combined signal) and provide that camer-stripped signal to the transmitter. The transmitter may then, in certain embodiments, be arranged simply to retransmit that GPS signal for receipt by the mobile device. The mobile device is then able to use its GPS processor to determine the location indicated by the retransmitted GPS signal using correlation techniques. The mobile device does not, however, have to perform the front-end processing it would need to do to derive a location indication from a plurality of directly received navigation signals; that has already been done by the GPS receiver providing the signal to the transmitter (repeater).

The terrestrial location of the transmitter may be fixed, for example on a wall, ceiling, roof, on the ground, underground, or on some other support structure.

In certain embodiments the transmitter is arranged to transmit the data signal in an ISM (Industrial, Scientific, Medical) band.

In certain embodiments the data signal (transmitted I broadcast wirelessly from the transmitter) is indicative of the location of the transmitter. It may, for example, be * indicative of the location of a transmitting antenna of the transmitter. It may, for *:::: example, comprise data indicative of the location of the transmitter S...

: 30 Alternatively, the data signal may be indicative of a location different from the location of the transmitter, for example a location at which the mobile device is able to receive the *5*SS * data signal (e.g. at a strength above a predetermined threshold), or a location of a GNSS receiver arranged to provide a location signal to the transmitter. . S.

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In certain embodiments the system further comprises a receiver arranged at a terrestrial location, the receiver being adapted to receive navigation signals from space vehicles of at least one global navigation satellite system and to generate from said received signals a location signal indicative of the location of the receiver, the receiver being further arranged to provide said location signal to the transmitter, and wherein said data signal is indicative of the location of the receiver.

The location signal may, for example, comprise location data calculated from the received navigation signals. Alternatively, the location signal may comprise raw GPS data from the plurality of SVs, from which a mobile device can itself calculate a terrestrial position.

In certain embodiments the transmitter comprises means for setting the data signal (e.g. setting data within it). For example, the transmitter may be programmable. It may be programmable to determine the transmitted data entirely, or may be programmable to incorporate an offset in the data, another portion of the data being determined by other means.

In certain embodiments the transmitter comprises input means for inputting a location signal indicative of a terrestrial location, the transmitter being arranged to determine the data signal according to a location signal input via the input means. For example, the input means may comprise connection means for connecting the transmitter to a global navigation satellite system receiver to receive the location signal from the receiver. The connection means may be adapted to provide a wired connection, or in certain embodiments may be wireless. For example, in certain embodiments the system comprises a GNSS receiver adapted to transmit a wireless location signal, and the transmitter is adapted to receive that wireless location signal. The wireless signal may, for example, be a Bluetooth TM signal, or some other format. The wireless signal may * comprise a location calculated by the receiver. In alternative embodiments, the wireless signal may be a GPS (or other GNSS) signal, carrying navigation data from a plurality of SVs. The GNSS receiver may then be left connected (by wire, or wirelessly) to the transmitter to continue to supply a location signal, or in other embodiments the GNSS receiver may be used to set up the transmitter, being then disconnected once the * .*** * transmitter had received a location signal from which to derive its data signal to be wirelessly transmitted to a user device. * ** * *

In certain embodiments the transmitter further comprises offset means for setting the data signal (e.g. the data in it) such that it is indicative of a location offset from the location indicated by the location signal.

Certain embodiments further comprise a receiver arranged at a terrestrial location, the receiver being adapted to receive navigation signals from space vehicles of at least one global navigation satellite system and to generate from the received signals a location signal indicative of the location of the receiver, the receiver being further arranged to provide its location signal to the transmitter, and the transmitter being arranged to determine the data signal according to the location signal. The data signal may be indicative of the location of the receiver. Again, the location signal may comprise data corresponding to a position calculated by the receiver from the SV signals, or may comprise data which still needs to be processed in order to determine position.

The receiver and transmitter in certain embodiments are comprised in a single, common module. This may be described as a repeater module.

In alternative embodiments the locations of the transmitter and receiver are different.

The transmitter may further comprise offset means for setting (or adjusting) the data signal (e.g. the data carried by it) such that it is indicative of a location offset from the location of the receiver.

In certain embodiments the location signal further comprises timing information determined by the received navigation signals and indicative of timing in at least one GNSS. Then, the data signal also comprises timing information, determined by (e.g. * derived from or the same as) the timing information in the location signal, and the mobile device is arranged to use the timing information in a received data signal to maintain synchronisation with space vehicles of at least one GNSS. Advantageously, :ui4 30 synchronisation may thus be maintained, even though the mobile device may be unable *...: to receive any navigation signals in its current location. For example, the transmitter * may be arranged to retransmit a GPS (or other GNSS) signal, provided by a GPS receiver and carrying navigation message data from a plurality of SVs. From the retransmitted GPS signal the mobile device may be arranged to determine the location indication by processing the GPS with its data processing part (which can be described as the back-end of its GPS engine). Front end processing may not be required.

In certain embodiments the receiver used to provide a location signal to the transmitter is a GPS receiver, although in alternative embodiments other GNSS receivers may be employed.

In certain embodiments the transmitter is arranged to transmit said data signal into an interior space of a building, or some other space where satellite navigation signals are strongly attenuated or even not available.

In certain embodiments the transmitter comprises a transmitting antenna, and the transmitter and mobile device are adapted such that the mobile device is able to receive the data signal and derive said indication of position from the received data signal only when the mobile device is within a predetermined range of the transmitting antenna.

In certain embodiments the mobile device is operable to derive said indication of current location from the data contained in a received data signal. Generally, this requires significantly less processing than to derive the location indication from a plurality of GNSS navigation signals and so consumes less power.

In certain embodiments the mobile device is operable to derive said indication of current location from the data contained in, and from a received strength of a received data signal. For example, the transmitter may comprise a transmitting antenna and the mobile device may be adapted to determine a distance between itself and the transmitting antenna according to a strength of a received data signal, and to derive said indication of current location from the data in the received data signal and from said distance. The transmitter may be further adapted to transmit additional data in said data * signal for reception by the mobile device, the mobile device being arranged to determine a path loss for a received data signal from the strength of the received data signal and said additional data, and to determine said distance from the path loss. This additional *: 30 data may, for example, comprise data indicative of the power of the signal transmitted * from the transmitter, of the gain of a transmitter antenna, or of other parameters, * enabling the mobile device to make a determination of distance from the transmitter. S...

In certain embodiments the mobile device is arranged to monitor signal strengths of the data and navigation signals it receives and to derive said indication of current location from either said data signal or a plurality of said navigation signals according to the strengths of the signals it receives at the current location.

In certain embodiments the mobile device is arranged to monitor signal to noise (S/N) ratios of the data and navigation signals it receives and to derive said indication of current location from either said data signal or a plurality of said navigation signals according to the S/N ratios of the signals it receives at the current location. Thus, if a S/N ratio is too poor, then that signal may be ignored and/or the mobile device may revert to a default condition, for example one in which the location indication it provides simple corresponds to the location of a transmitter or a GNSS receiver providing the transmitter with a signal.

Certain embodiments comprise a plurality of said transmitters, each arranged at a respective terrestrial location and adapted to transmit a respective data signal indicative of a respective terrestrial location, and wherein the mobile device is adapted to receive said data signals and to derive said indication of current location from either at least one data signal or a plurality of said navigation signals, according to the signals received by the mobile device at the current location.

The mobile device may then be operable to derive said indication of current location from the data contained in, and from the relative strengths of a plurality of received data signals. The mobile device may, for example, be arranged to derive its indication of location from a plurality of received data signals using triangulation.

In certain embodiments the mobile device is operable to derive said indication of current location from the data contained in the strongest data signal received at a current * location. S...

Certain embodiments further comprise a plurality of receivers, each receiver being :" 30 arranged at a respective terrestrial location and adapted to receive navigation signals :" from space vehicles of at least one global navigation satellite system and to generate from said received signals a respective location signal indicative of the respective location of the receiver, each receiver being further arranged to provide said respective location signal to a respective one of said plurality of transmitters, and each transmitter being arranged to determine its respective data signal (for example by determining the data within it) according to the respective location signal. Such systems may then further comprise a receiver arranged at a terrestrial location and adapted to receive navigation signals from space vehicles of at feast one global navigation satellite system and to generate from said received signals a location signal indicative of the location of the receiver, the receiver being further arranged to provide said location signal to each of said plurality of transmitters, and each transmitter being arranged to determine its respective data signal according to the location signal. Each transmitter may then be arranged such that its data signal is indicative of a location offset from the location of the receiver by a respective amount. In other words, each transmitter is provided With/programmed with a respective offset.

The mobile device may take a variety of forms. For example, it may be a mobile phone with GPS capability, a PDA, or a dedicated GNSS (e.g. GPS) receiver.

Another aspect of the invention provides a transmitter for a location system in accordance with the first aspect.

Another aspect of the invention provides a transmitter-receiver module comprising a transmitter for a location system in accordance with the first aspect, and a receiver adapted to receive navigation signals from space vehicles of at least one global navigation satellite system and to generate from said received signals a location signal indicative of the location of the receiver, the receiver being further arranged to provide said location signal to the transmitter.

Yet another aspect of the invention provides a mobile device for a location system in accordance with the first aspect. Thus, the mobile device may be operable in a first mode in which it generates said indication of current location from a plurality of said navigation signals, and a second mode in which it generates said indication of current location from at least one said data signal. The mobile device may then be adapted to switch between said modes according to the signals it receives at a current location.

Another aspect of the invention provides a method of providing an indication of location, the method comprising: arranging a transmitter at a terrestrial location and transmitting a data signal from the transmitter, the data signal being indicative of a terrestrial location; providing a mobile device adapted to receive said data signal and to receive navigation signals from space vehicles of at least one global navigation satellite system;

F

operating the mobile device to derive an indication of said current location of the mobile device from either said data signal or a plurality of said navigation signals, according to the signals received by the mobile device at its current location; and providing said indication of current location with the mobile device.

The method may further comprise arranging a GNSS receiver at a terrestrial location, using the receiver to generate a location signal from a plurality of navigation signals received by the receiver at its terrestrial location, providing the location signal to the transmitter, and deriving the data signal from the location signal.

Transmitting the data signal in certain embodiments comprises transmitting the data signal in an ISM band1 although other bands may be used in alternative embodiments.

In certain embodiments deriving the indication of current location comprises determining a path loss for a data signal received by the mobile device from the transmitter.

The method may further comprise arranging the transmitter to transmit said data signal into a blind spot of at least one of said GNSS systems, and/or arranging the transmitter to transmit said data signal into an interior space of a building, structure1 tunnel, or space in which navigation signals are unavailable or greatly attenuated.

In certain embodiments the method further comprises: providing a plurality of said transmitters; arranging each transmitter at a respective terrestrial location; transmitting from each transmitter a respective data signal indicative of a :..::: respective terrestrial location; and operating the mobile device to derive said indication of current location from either at least one data signal or a plurality of said navigation signals, according to the signals received by the mobile device at the current location.

The method may further comprise operating the mobile device to derive said indication of current location from the data contained in, and from the relative strengths of a plurality of received data signals.

Brief Description of the Drawings

Embodiments of the invention will now be desc,ibed with reference to the accompanying drawings, of which: Fig. I is a schematic view of a location system embodying the invention; Fig. 2 is a schematic representation of another location system embodying the invention; Fig. 3 is a diagram illustrating the arrangement of receiver and transmitter antennas and the resultant signal coverage areas in an embodiment of the invention; Fig. 4 is a schematic representation of signal coverage in another embodiment of the invention; Fig. 5 is a schematic representation of signal coverage in yet another embodiment of the invention; Fig. 6 is a diagram illustrating the determination of current location from a signal received from a transmitting antenna in an embodiment of the invention; Fig. 7 is a diagram illustrating the determination of user location from data signals received from two transmitting antennas in an embodiment of the invention; Fig. 8 is a schematic representation of another location system embodying the invention; p * * S..

Fig. 9 is a representation of received signal powers in an embodiment of the invention;

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I..... * .

Fig. 10 is a schematic representation of a location system in accordance with another embodiment of the invention; S... p 5 *

Fig. 11 is a schematic representation of another embodiment of the invention; Fig. 12 is a schematic representation of another embodiment of the invention; and Fig. 13 is a schematic representation of yet another location system embodying the invention.

Detailed Description of Embodiments of the Invention Referring now to fig. 1, this illustrates a location system embodying the invention and which provides a solution to the problem of improving GPS location in the interior of buildings. This solution uses existing GPS and wireless technology available in a mobile handset. Extra processing of data is costly in terms of battery powered equipment, e.g. mobile phones. Thus, to avoid this problem the embodiment employs existing resources in the mobile device (which is a mobile phone incorporating a GPS receiver) to achieve this goal.

Generally, the embodiment of figure 1 employs a standard GPS receiver 3 mounted on a roof, or side of a building B or any position inside that can allow the receiver to provide a location fix. The location obtained by the GPS receiver is then passed to a transmitter 1 to be retransmitted. In this embodiment the location is retransmitted on a frequency band commonly used for wireless networking and not requiring an authority license e.g. an ISM band. Any receiver of a mobile device in the immediate vicinity of. this transmitter with the correct receiver configuration can then receive the location data. The format of the transmitted data in embodiments of the invention can, for example, be a standard data format as used in NMEA (National Marine Electronics Association) or can be any other format, proprietary or otherwise.

In this first embodiment it will be noted that the location received by users of mobile devices in the building B will be the location of the GPS antenna /receiver and not the precise location of the user. This introduces an error in location but if the range of transmitter is kept to a short range, say 10 meters then accuracy becomes reasonable.

* With limited range (1Dm or less) it is difficult to estimate location outside this range. The *::::* 30 range of the transmitter could be increased by increasing transmit power but this could degrade accuracy as path length increases giving a larger radius R of coverage.

Referring to fig. 1 in more detail, this figure shows a location system embodying the invention in which GPS data is retransmitted to a mobile device in an ISM band. The system comprises a transmitter 1 arranged as a fixed terrestrial location (in this case it is fixed generally on the inside of a roof R of a building denoted generally by B). This transmitter I is arranged to transmit a data signal Si from its transmitting antenna 11, that data signal comprising data which is indicative of a terrestrial location. The system also comprises the mobile device 2 which is adapted to receive navigation signals from the space vehicles of at least the GPS system. The GPS mobile device 2 is dual-mode in that it is also adapted to receive the data signal Si from the inside transmitter 1. The mobile device 2 is adapted to provide an indication of its current location L2 in the form of a visual display on a display screen 22. The mobile device is arranged so that it can derive (i.e. generate, calculate, or otherwise produce) the indication of its current location from either the received data signal Si or from a plurality of received navigation signals N, according to the signals it receives at its current location.

In the arrangement illustrated in Fig. I the mobile device 2 is inside the building B and is unable to receive any of the satellite navigation signals N directly (or at least those signals are so weak at location L2 that the device 2 is unable to calculate position from them). However, at position L2 the mobile device 2 is within range of the signal Si from the transmitting antenna 11. The mobile device is adapted to respond to its receipt of signal Si and the absence of navigation signals N to derive an indication of current location from just the data signal Si and to provide that indication to a user by means of the display screen 22. In the figure the interior space or volume of the building is indicated generally by arrow V. In this first embodiment the location system also comprises a GPS receiver 3 having a GPS receiving antenna 31 and being located at a fixed terrestrial location such that the * 25 GPS antenna 31 is able to receive a plurality of navigation signals N from space vehicles of the GPS system. In this example the location of the GPS receiver 3 is on an exterior part of the building B, and in particular on the roof R. However, it will be appreciated that in other embodiments it is not essential for the GPS receiver to be located only on a roof; instead it could be provided at another location suitable for its antenna 31 to receive a sufficient number of navigation signals for the receiver 3 to calculate position. **** * S...

* Returning to the present embodiment, from the received plurality of navigation signals N the GPS receiver determines (calculates, derives etc) its location, and provides a location signal to the transmitter 1 via a suitable link 32 (which may also be referred to as a GPS data feed). The location signal is thus indicative of the location of the GPS receiver (or more particularly its antenna 31). In this embodiment the location of the GPS receiver arid antenna 31 although outside the building B is close to the location of the transmitting antenna 11 arranged inside the building to transmit the data signal Si into the interior space V. Thus, in this simple embodiment the transmitter 1 in effect is arranged just to retransmit the location data from the GPS receiver for receipt by the mobile device 2. In other words, the data signal Si comprises data which is indicative of the location of the GPS antenna 31.

It should be noted that the transmitter in this embodiment does not retransmit the navigation signals N received by the GPS antenna. Instead, it is transmitting a signal Si which comprises the data indicative of antenna location which has already been calculated from the received navigation signals N. An advantage of this is that the mobile device 2 can derive its indication of current location from the received signal Si with little or no processing; rather than having to process a plurality of navigation signals it merely has to read the data signal Si from the transmitter.

In this first embodiment the mobile device 2 is adapted so that when it receives signal SI and no navigation signals N it provides an indication of current location which simply corresponds to the location indicated by the location signal to the transmitter, i.e. the location of the GPS antenna 31.

Additionally in this embodiment, the GPs receiver in its location signal to the transmitter 1 provides timing information which the transmitter 1 in turn passes onto the mobile device 2 by means of the data signal Si. The timing information is arranged so that when the mobile device receives it it can use this information to maintain synchronisation with the satellites transmitting the navigation messages N even though in location L2 it is unable to receive any of those signals directly. Thus, the mobile device is able to maintain synchronisation with the external satellite system even when inside a building B so that when it emerges from the building it may quickly and easily resume determination of location from received navigation signals.

It will be appreciated that the example shown in fig. 1 incorporating a single transmitter provides a location data signal (for use by the mobile device) having a limited range. To increase coverage and maintain accuracy another transmitter could be added, and such an embodiment is shown in fig. 2. Here, each transmitter la, lb has its own location coordinates with respect to GPS receiver and identification that a mobile user could receive as part of down link data. To simplify interconnectivity between the GPS receiver 3 and the ISM band transmitters I for passing GPS location data to each ISM transmitter, in certain embodiments this is done through mains supply cabling as all transmitters will require power supply.

In the arrangement shown in fig.2, the mobile device (and mobile user) can now be in any coverage area of antenna 1 or antenna2 and still be able to provide a location indication. In certain embodiments the mobile device (which may also be described as a mobile receiver) is able to identify which TX antenna la or lb to use for location estimation from down link information received.

Referring again to Fig. 2, in this embodiment a single GPS receiver 3 provides a location signal indicative of its own location (and derived from received navigation signals N) to both transmitters 1A, lB via a suitable link 32. In one variant of the embodiment of Fig. 2 each of the two transmitters, via its respective antenna, transmits the respective data signal S1A, SiB comprising data which is simply indicative of the position of the GPS receiver 3. In another variant, however, each transmitter 1 A, 18 is programmed with a respective offset, such that the data contained in signal S1A is indicative of the particular location of transmitter 1A, offset from the position of the GPS receiver, and data signal SIB similarly comprises data indicative of the location of transmitter IB, offset by a different amount from the position of receiver 3.

An advantage of these two transmitters 1 A, 18 continuing to be connected to the GPS receiver and receiving a location signal from it is that they are able to respond to any changes in the location signal from the GPS receiver. For example, based on received navigation signals N, the GPS receiver may be able to calculate its position with increased accuracy, or may for some reason revise its location. This improved or revised location may then also be reflected in the data signals S1A and SiB transmitted into the interior of the building 8. Also, the GPS receiver 3 may continue to provide synchronisation information to the transmitters IA, lB which they can then in turn pass on to a mobile device within range of their signals so that that device can maintain its synchronisation with the external satellite system. In such embodiments (where the transmitters transmit signals indicating different locations), the mobile device may be simply arranged to provide an indication of current location based on the location data contained in the strongest signal received (ignoring the other one). In alternative embodiments, the mobile device may be arranged to make a more sophisticated determination of its location based on the relative strengths (or powers) of the signals received from the plurality of transmitters.

Fig.3 illustrates a technique, in an embodiment of the invention, for estimating the location of mobile user (in other words, estimating mobile user coordinates). In fig. 3 the coordinates of antenna 1 (1 Ia) and antenna 2 (1 Ib) are relative to GPS coordinates (x,y,z) (i.e. the coordinates of the GPS antenna). For a user in the coverage area for antenna 1, the coordinates for antenna 1 can be used with antenna height adjustment; height in this case is the height of ceiling. A user location in this case with reference to GPS coordinates can be approximated to; (x -x1, y -Yi. z-h). This location is approximate and is limited by the range of transmitting antenna. The same case applies to a mobile device user under coverage area for antenna 2.

To improve coverage and reduce "dead spots" the antenna locations in certain embodiments are such that their coverage areas overlap. The overlapping coverage areas in one embodiment of the invention (aimed at covering dead spots) are shown in fig. 4. In the overlap region the coordinates of either antenna may be used for estimating user location. To cover very large areas, a network of antennae are employed in certain embodiments, with each antenna location specified with respect to the location of a GPS antenna, as shown in fig. 5. Fig 5 shows the coverage areas of a network consisting of four antennae. In this case a mobile device can receive transmitted data signals from more then one antenna, depending on location; this could lead to interference and incorrect estimation of location. Accordingly, certain embodiments use the technique of Time Division Multiple Access -TDMA, with only one antenna being allowed to transmit at a time, in a predetermined sequence of transmission. Fig. 5 thus illustrates the technique of extending location coverage area. * ***

Certain embodiments are able to provide enhanced location accuracy, as follows.

Location accuracy in the system as shown in figure 1 will depend on the range of the transmitter 1 and the sensitivity of the ISM band receiver in receiving mobile device.

Without any additional manipulation, accuracy is limited in that example to 10 meters in any direction (360 degrees in horizontal plane) with respect to transmitting antenna.

However in certain embodiments the receiving device 2 has the capability of measuring received power, and accuracy can then be enhanced by determining path loss (power loss) to indicate the distance between the transmitting antenna 11 and the user's (mobile device's) receiving antenna. Path loss in line of sight' is the difference between the transmitted power and the measured received power.

For a transmitting antenna the Effective Isotropic Radiated Power (EIRP) is determined by the product of transmitting power (Pa) into an antenna and the Isotropic gain (Gsa) of the antenna.

EIRP = Pta.G Path loss L5 = EIRP -Mobile Received Power Mobile received power Pr = 0ra Pmr where G, is receiving antenna gain and Pmr is the measured power by the mobile receiver.

L3ElRP-G. Pmr L5 = Pta.Ga -G,3. Pmr In certain embodiments the local transmitter transmits data to the mobile device enabling the mobile device to determine (calculate) path loss. Thus, in certain embodiments the transmitter I sends the transmit power P, and the antenna gain G as part of the down link information to mobile receiver. In other words, that information is transmitted to the mobile device (such as a phone with GPS capabilities, or a dedicated GNSS receiver) as * part of the data signal Si. The receiver of the mobile device 2 then measures received . power, and its RX antenna gain is a known quantity (re antenna data). Using this data the path loss is now evaluated by the mobile device.

S

* 30 Path loss can also be stated as a function of path length d and wavelength A of radiation S... * . 555*

L = (4lTd/A)2 So with path loss known, the distance between the transmitter I and receiver 2 is evaluated; d = (A/4Tr).(L112) -I ir f' p. 12 U -ni*1Tj. -. mrJ For one ISM band A is around 0.125m (corresponding to frequency 2.4GHz) (ISM bands include bands 902-928 MHz, 2400-2483.5 MHz and 5725-5850 MHz) Assume path loss is 40dB ( 10000 linear) Path length = (10000)112. O.1251(4.rr) = 0.995m 10.

Path length information can be used in embodiments of the invention to calculate the position of the receiver 2 relative to the transmitter 1. Path length is a measure of direct line of sight distance and does not give true location coordinates of mobile user. In this case a mobile user could be anywhere on a circle with radius r at distance determined by the path length (as shown in figure 1).

The distance of a user (of the mobile device 2) from the transmitting antenna I can be obtained from path length obtained from path loss and from knowledge of the height of antenna in certain embodiments, as shown in fig. 6 (calculating position of mobile user from antenna).

R= (D2....,2)1/2 horizontal limiting radius of coverage r = (d2 -h2)1'2 horizontal distance of user from antenna * 25 S..

This distance or radius of user location is less than the limiting radius of coverage R a.... determined by the range 0 of the transmitter. This does not give exact location but :ii indicates where the user is with reference to transmitting antenna. To obtain more exact :" coordinates of the mobile device 2, at least two antennae are required with specified locations, as shown in the embodiment illustrated in fig. 7 (obtaining user coordinates from two antenna locations). * .5 * S

S

In fig. 7 it is assumed that the mobile user is located at location L2, in between two antennae 11 a, 11 b and the coverage area of each antenna is such that user is able to receive data signals Sla, Sib from both antennae. The antennae are separated by distance s and both are located in the same horizontal plane, ie same height with respect to the user. Distances dl and d2 are the path lengths with respect to anti and ant2 respectively. Angles 01 and 02 are the angles of path lengths of antennal and antenna2 with the horizontal plane containing the antennae.

From the triangle formed by d1, d2 and S, angle 01 can be calculated using cosine rule for triangles; d22 = d12 +2 -2.d1.s.cos 01 Antenna separation $ is obtained from the coordinates of the antennae in horizontal plane (which are provided to the mobile device in this example in the respective data signals Sla and Sib); s = (x2 -x1) ; in x-axis S=(y2-yi);jny-axis cosO1 = (d12 +s2 -d22)I(2.d1.s) Height h1 can be calculated from, h1 = d1. (1-cos2 �)hI2 This gives the vertical displacement of the user's location L2 with respect to the antennae.

To obtain exact user coordinates, the position on the x -y plane is required; this can be obtained by considering triangle formed by ri,r2 and antenna separation s projected onto the x-y plane.

* .*** On the x-y plane the user location in polar form with respect to antennai is (rl, 01) and x and y can be determined from; * U x = r1.cos 0 and y = r1s,n 0 -i.e. r1 and $ need to be determined From triangle formed by d1,h1 and r1 rl=(d12-h12)"2 r22 = r12 + 2 -2. r1. s. cos E cos ei = (r12 + s2 -r22)/( 2. r1. s) x = (d1-h12)112. (r12 + -r22)I( 2. r1. s) y = (d12 -h12)l'2. (1 -cos2 O)t'2 z = This specifies user location L2 with reference to antenna 1; the same case can be made for location coordinates with reference to antenna2.

The location L2 of the mobile user with reference to a GPS antenna can then be determined if the location coordinates (x1,y1,z1) of antennal are taken into account.

For $ in range less then rr/2 and greater then 3rr/2, user X ordinate will be; x = + X = x1 -x for other E' values For e1 in range 0 to TT, V values will be V = Yi + Y = Yi -y for all other angles of $ Z = z1 -h1 **.

It will be appreciated that there will be error sources in the location estimation techniques employed by embodiments of the invention. The main sources in location error will be: * 30 a) errors in measuring the received power by the mobile device receiver to determine path loss, that path loss being used to determine path length/user ::::: coordinates. To address this, however, the mobile device receiver can be calibrated to improve measurement accuracy; b) RF signals from other in-band interferers; C) Multi-path/reflected signals -signals arriving at the mobile user antenna 2lcould be direct line of sight signals, and/or reflected signals from a wanted transmitter Interference may be dealt with in embodiments of the invention, as follows. Errors will be introduced in estimating path loss/path length by unwanted RF signals in the vicinity of the mobile user area (i.e. by unwanted signals received by the mobile device 2). A method of reducing these errors is to determine the received signal power in terms of wanted code powers (in embodiments in which the data signal from the transmitter has been modulated using CDMA techniques) rather then raw RF power. To use this approach requires additional processing by baseband in the mobile device, as codes will have to be correlated before power measurement is made -i.e. in the modulation domain.

In the case of in-band interferers, RF signal power measurement will yield a valid signal power in RF domain but measurement in the modulation domain of signal to noise (S/N) ratio will be poor, indicating a strong interferer. In such a case path loss/path length measurement can be discarded as it will lead to incorrect estimation in location. In this case it is best to use the coordinates of the nearest transmitting antenna as an approximation in location -the coordinates of the antenna and the location of the GPS receiver are transmitted to the mobile user, If interference is persistent then as a default condition the mobile device may be arranged to provide an indication of location corresponding simply to the location of the nearest transmitter 1 or the location of the GPS receiver 3.

Multi-path Signals may be dealt with in embodiments of the invention, as follows. In a confined space the data signals Si (e.g. RF signals) can be reflected off walls W and other objects. The reflected signals can then be picked up by a mobile receiver 2. The reflected signal may be exactly the same as the direct line of sight signal, but delayed in time due to the longer transit path, as shown in fig. 8 (line of sight and reflected signals).

Depending on the phase of multi-path signal relative to direct line of sight signal, the *...

following can result: * Both signals may be in phase, and the receiver 2 will see a stronger signal than the direct line of sight signal (due to constructive interference) * A reflected signal may be anti-phase to the direct line of sight signal, and the receiver will see a weaker than direct line of sight signal (due to destructive interference) The above scenarios are the extreme and will lead to incorrect path loss and path length determination, leading to incorrect estimation of user coordinates by receiver 2.

However, these are the extremes and there will also be cases in between these two where signal fading is less.

Certain embodiments of the invention are adapted to deal with multi-path errors, as follows. Direct line of sight (LOS) signals will always be picked up first by the receiver 2; multi-path signals will be delayed, due to the longer path length, and as a consequence will be tower in amplitude, as shown in fig. 9 (direct LOS and multi-path signals). Multi-path signals can add (if in phase) to give a larger amplitude signal.

By using data correlation techniques (on demodulated RX data) and determining the time delay(s) of multi-path signals, certain embodiments are able to measure the power of a direct tine of sight signal and use this to determine path length to estimate user location, rather than raw RF power measurement. To gauge the time delays involved, consider the case where a direct line of sight signal path length is lOm and a reflected signal path is 1 5m; Transit time for direct signal = (lOm/c) = 33.3 nanoseconds Transit time for reflected signal = (15/c) =50 nanoseconds Delay = 16.7 nanoseconds This gives an estimate of the resolution time required by the receiver's base band processing means to overcome multi-path errors. Once the direct line of sight signal is identified in certain embodiments, all other received signals can be ignored for determining path length. S...

If multi-path signal problems are persistently severe then the mobile device may be arranged to provide a location indication corresponding simply to the coordinates of the * S...

* system's GPS receiver 3 as a default condition. *..I.

* * 30 s... Referring now to Fig. 10, this shows another location system embodying the invention.

Here a transmitter 1 has been arranged at a fixed terrestrial location (attached to a wall * WI), with its transmitting antenna 11 positioned so that data signal SI is broadcast into a so-called dead spot or blind spot at which navigation signals N from GPS satellites cannot be received by the mobile device 2. This blind spot is generally located adjacent to the ground surface G, between facing walls Wi and W2 of an urban environment. In other words, the blind spot into which the transmitter 1 transmits can be regarded as being in a canyon. The transmitter 1 in this example is arranged to transmit a fixed, unchanging data signal SI. The transmitter 1 has undergone a set-up procedure in which it has been connected to a GPS receiver 3 positioned at a location in which the receiver 3 was able to receive a sufficient number of navigation signals N to determine its location. The GPS receiver then provided a location signal to'the transmitter 1 by means of temporary connection or link 33. The transmitter 1 was then adapted to store the location data contained in the signal from the GPS receiver and use that data to determine the data in its own broadcast data signal Si. The transmitter 1 also comprises means for inputting a position offset, in the form of a key pad 12. In effect, this key pad 12 is operable to input an offset or adjustment indicative of the difference in position between the GPS receiver 3 and the transmitter 1. The transmitter 1 then uses this adjustment or offset information (or data) in addition to the location data from the GPS receiver to set the data contained in the data signal Si so that that data is more accurately indicative of the location of the transmitting antenna 11, rather than simply being indicative of the position of the receiver 3, when connected. Thus, the transmitter 1 can be regarded as having been programmed or set-up to transmit fixed location data to the mobile device 2 for reception in the GNSS blind spot. In certain embodiments the mobile device may simply pass on that adjusted or offset location data to the user, but in more sophisticated embodiments may derive a revised location indication based on an estimated distance between the mobile device 2 and the transmitting antenna 11 (determined, for example, by path loss calculations as described above).

Referring now to Fig. 11, this shows another location system embodying the invention.

In this embodiment, a programmable transmitter I has been arranged with its * transmitting antenna 11 located inside a building to transmit its data signal Si into an -interior space V. The transmitter 1 comprises means for programming the data to be contained in its broadcast data signal Si. In this example the programming mean is in :" the form of a key pad 12, although it will be appreciated that in other embodiments alternative programming or input means may be employed. The mobile device 2 is S...

* shown in two locations. In location L2a the mobile device 2 is outside the building B and at a position where it can receive a plurality of satellite navigation signals N. In this S...

:* location L2a the device is not able to receive the data signal Si from the internal transmitter. In response to the signal conditions the mobile device 2 operates in a first mode in which it derives an indication of its current location from the received satellite navigation signals N (and not from the data signal SI). It will be appreciated that if position L2a is close to the building B, under certain conditions the mobile device 2 may be able to receive the data signal Si at the same time as it is able to receive a sufficient plurality of satellite navigation signals N. It will also be appreciated that the mobile device 2 may be arranged to use the received signals in a variety of ways to determine how it will currently derive its indication of current position. For example, if a sufficient number of navigation signals are received then even though a data signal or data signals may be received, just the navigation signals may be used to determine location.

Alternatively, the mobile device may be set up to use a combination of received navigation signals and data signals Si to determine position. As another option, the mobile device may be arranged so that any received data signal Si takes preference, in other words that received data signal or signals being used to determine location regardless of whether satellite signals N are being received (and an advantage of this arrangement is that the larger amount of processing required to determine location from received navigation signals is avoided).

Returning to Fig. 11, the mobile device 2 is also shown in a second, internal location L2b at which it is no longer able to receive the navigation signals N, but receives data signal Si. The mobile device is responsive to these signal conditions in this embodiment to operate in a second mode in which it determines its location indication solely from the received data signal SI.

Referring now to Fig. 12, this shows another location system embodying the invention.

The system comprises first and second transmitters la, lb each arranged to receive a location signal 300 from a common GPS receiver 3 arranged at a fixed location so that its antenna 31 is able to receive a plurality of navigation signals N from orbiting satellites.

Each of these two transmitters is programmed with a respective offset OS1 and 0S2 such that the data signal Sla transmitted from the first transmitter la comprises data indicative of the location of the first transmitter la, and the signal Sib from the second transmitter lb is indicative of the different location of the second transmitter lb. The system also comprises a combined receiver-transmitter module 301 arranged at another * fixed location. This module 301 comprises a receiver portion 3c adapted to determine its :::: location from navigation signals N received via a receiving antenna 31c. The module 301 also comprises a transmitter portion Ic adapted to transmit a third data signal Sic comprising data indicative of the location of the module 301 determined by the receiver portion 3c from the received signals N. The various data signals Sla-c are transmitted into a space V. A mobile device 2 inside this space V is arranged to derive a location indication from the received data signals S la-c and provide that indication to a user. In this embodiment the transmitters and the transmitting portion are adapted to transmit signals SI such that they have substantially the same transmitted power. The mobile device 2 is then arranged to determine its location within the space V according to the relative strengths of the data signals it receives. For example, at position L2c, the mobile device 2 receives all three data signals, but signal Si a is substantially stronger than signal Sib, which in turn is substantially stronger than received signal Sic. The mobile device is adapted to read the data contained in these three signals and to weight that data according to the received strengths and so provide a location indication to the user which substantially corresponds to the current location L2c. Similarly, at position L2d the mobile device receives signals Sib and c at substantially the same power levels, with signal Sla being substantially weaker. The mobile device at this position is therefore able to determine that its current location is substantially midway between the locations of the second transmitter lb and the transmitter-receiver module 301, and it provides an indication of position to the user in accordance with that determination.

Moving on to Fig. 13, this shows another location system embodying the invention. In this embodiment, a plurality of transmitters Ia, Ib, Ic are arranged to broadcast their respective data signals Sia-c into a space V in which it is not possible to receive any satellite navigation signals N. This space V may, for example, be inside a tunnel underground, inside a building, or indeed in any other location. In this embodiment each transmitter has been set or programmed such that the data contained in its respective broadcast data signal Si is indicative of a respective location L2a, L2b, L2c inside the space V at which a mobile device 2 is able to receive the signal. Thus, the data signals are indicative of a position or location at which the signal may be received, rather than * strictly being indicative of the location of the transmitting device. These locations L2a-c are generally on the floor F of the space V. According to the position of the mobile device 2 inside the space V, the broadcast signals Si from the different transmitters are received with different strengths. The mobile device 2 is arranged to determine its ***** * : 30 location (current location) from the relative strengths of the received data signals, and to ***** * provide an indication of current location to a user by means of a visual display on a screen 22 and also by means of an audible signal 23 emitted from the device 2. It will * *: be appreciated that the location indication may be provided in a variety of alternative ways in different embodiments of the invention. The mobile device in Fig. 13 is also operable to receive GNSS navigation signals when outside the space V1 and in this example comprises a GPS receiver. The arrangement shown in Fig. 13 is particularly advantageous for enabling the mobile device 2 to provide up to date location information to a user in environments such as tunnels and other underground environments. The audible indication 23 is particularly useful in conditions in which visibility in the enclosed space V is reduced.

Referring again to fig. 1, another embodiment of the invention has the same general form, but differs from the previously described embodiment in certain details, as follows.

In this alternative embodiment the connection (or data feed) 32 is wireless. Also, the location signal provided from the receiver 3 to the transmitter is a GPS data signal comprising the raw data from the plurality of received navigation signals N. In other words the transmitter is not receiving a calculated position Ir the receiver; instead it is receiving a signal which, in effect, is a combination of the data messages from the plurality of SVs, and which is indicative of the location of the receiving antenna 31 because it is possible, with suitable processing, to calculate the position of antenna 31 from the signal. The transmitter I is then arranged to retransmit the received GPS data in a corresponding data signal Si from its antenna. Thus, the mobile device 2 receives GPS data via the transmitter 1. The mobile device is then arranged to calculate the position of the GPS receiver antenna 31 from the data signal SI (using correlation techniques to determine the respective time delays from the transmitting SVs, and the GPS data itself, which of course is indicative of the positions of the respective SVs). In certain embodiments the mobile device may be arranged simple to indicate that calculated location as its current location, but in other embodiments is adapted to indicate a modified location (albeit derived from the location of the GPS receiver 3) as described above. Conveniently, the link 32 may be a Bluetooth TM Jink, with the GPS receiver being arranged to transmit its GPS signal (derived by at least stripping the * carrier from the received signal corresponding to the combined SV signals N) in that form. **** *.*** * * * * * * ** * *

Claims

1. A location system comprising: a transmitter arranged at a terrestrial location and adapted to transmit a data signal indicative of a terrestrial location; and a mobile device adapted to receive said data signal and to receive navigation signals from space vehicles of at least one global navigation satellite system, the mobile device being further adapted to provide an indication of a current location of the mobile device, th mobile device being arranged to derive said indication of current location from either said data signal or a plurality of said navigation signals, according to the signals received by the mobile device at the current location.

2. A location system in accordance with claim 1, wherein the transmitter is arranged to transmit the data signal in an ISM (Industrial, Scientific, Medical) band.

3. A location system in accordance with claim I or claim 2, wherein said data signal is indicative of the location of the transmitter.

4. A location system in accordance with any preceding claim, wherein the transmitter comprises a transmitting antenna and said data signal is indicative of the location of the transmitting antenna.

5. A location system in accordance with any one of claims I or 2, wherein said data signal is indicative of a location different from the location of the transmitter.

6. A location system in accordance with claim 5, wherein said data signal is indicative of a location at which the mobile device is able to receive the data signal at a strength above a predetermined threshold. S**S.

* : 30

7. A location system in accordance with claim 5, further comprising a receiver * arranged at a terrestrial location, the receiver being adapted to receive navigation * , signals from space vehicles of at least one global navigation satellite system and to * generate from said received signals a location signal indicative of the location of the receiver, the receiver being further arranged to provide said location signal to the transmitter, and wherein said data signal is indicative of the location of the receiver.

8. A location system in accordance with any preceding claim, wherein the transmitter comprises means for setting said data signal.

9. A location system in accordance with claim 8, wherein the transmitter comprises input means for inputting a location signal indicative of a terrestrial location, the transmitter being arranged to determine said data signal according to a location signal input via said input means.

10. A location system in accordance with claim 9, wherein the input means comprises connection means for connecting the transmitter to a global navigation satellite system receiver to receive the location signal from the receiver.

11. A location system in accordance with claim 9 or claim 10, wherein the transmitter further comprises offset means for setting said data signal such that it is indicative of a location offset from the location indicated by the location signal.

12. A location system in accordance with any one of claims 1 to 7, further comprising a receiver arranged at a terrestrial location, the receiver being adapted to receive navigation signals from space vehicles of at least one global navigation satellite system and to generate from said received signals a location signal indicative of the location of the receiver, the receiver being further arranged to provide said location signal to the transmitter, and the transmitter being arranged to determine said data signal according to the location signal.

13. A location system in accordance with claim 12, wherein said data signal is indicative of the location of the receiver.

14. A location system in accordance with claim 12 or claim 13, wherein the receiver and transmitter are comprised in a single, common module.

* : 30 ***** *

15. A location system in accordance with claim 12 or claim 13, wherein the locations * of the transmitter and receiver are different. **�. ** *. * *

16. A location system in accordance with any one of claims 12 to 15, wherein the transmitter further comprises offset means for setting said data signal such that it is indicative of a location offset from the location of the receiver.

17. A location system in accordance with any one of claims 12 to 16, wherein said location signal further comprises timing information determined by said received navigation signals and indicative of timing in at least one of said global navigation satellite systems, the data signal comprises timing information determined by the timing information in the location signal, and the mobile device is arranged to use the timing information in a received data signal to maintain synchronisation with space vehicles of at least one said global navigation satellite system.

18. A location system in accordance with any one of claims 12 to 17, wherein said receiver is a GPS receiver.

19. A location system in accordance with any preceding claim, wherein the transmitter is arranged to transmit said data signal into an interior space of a building.

20. A location system in accordance with any preceding claim, wherein the transmitter comprises a transmitting antenna, and the transmitter and mobile device are adapted such that the mobile device is able to receive the data signal and derive said indication of position from the received data signal only when the mobile device is within a predetermined range of the transmitting antenna.

21. A location system in accordance with any preceding claim, wherein the mobile device is operable to derive said indication of current location from the data contained in a received data signal.

22. A location system in accordance with any preceding claim, wherein the mobile S. device is operable to derive said indication of current location from the data contained in, "s.' and from a received strength of a received data signal.

S

* : 30

23. A location system in accordance with claim 22, wherein the transmitter comprises * a transmitting antenna and the mobile device is adapted to determine a distance , between itself and the transmitting antenna according to a strength of a received data ***.

signal, and to derive said indication of current location from the data in the received data signal and from said distance.

24. A location system in accordance with claim 23, wherein the transmitter is adapted to transmit additional data in said data signal for reception by the mobile device, the mobile device being arranged to determine a path loss for a received data signal from the strength of the received ddta signal and said additional data, and to determine said distance from the path loss.

25. A location system in accordance with any preceding claim, wherein the mobile device is arranged to monitor signal strengths of the data and navigation signals it receives and to derive said indication of current location from either said data signal or a plurality of said navigation signals according to the strengths of the signals it receives at the current location.

26. A location system in accordance with any preceding claim, wherein the mobile device is arranged to monitor signal to noise (SIN) ratios of the data and navigation signals it receives and to derive said indication of current location from either said data signal or a plurality of said navigation signals according to the S/N ratios of the signals it receives at the current location.

27. A location system in accordance with any preceding claim, comprising a plurality of said transmitters, each arranged at a respective terrestrial location and adapted to transmit a respective data signal indicative of a respective terrestrial location, and wherein the mobile device is adapted to receive said data signals and to derive said indication of current location from either at least one data signal or a plurality of said navigation signals, according to the signals received by the mobile device at the current location.

28. A location system in accordance with claim 27, wherein the mobile device is operable to derive said indication of current location from the data contained in, and from the relative strengths of a plurality of received data signals.

* : 30 *

29. A location system in accordance with claim 27 or claim 28, wherein the mobile device is operable to derive said indication of current location from the data contained in S..

the strongest data signal received at a current location.

30. A location system in accordance with any one of claims 27 to 29, further comprising a plurality of receivers, each receiver being arranged at a respective terrestrial location and adapted to receive navigation signals from space vehicles of at least one global navigation satellite system and to generate from said received signals a respective location signal indicative of the respective location of the receiver, each receiver being further arranged to provide said respective location signal to a respective one of said plurality of transmitters1 and each transmitter being arranged to determine its respective data signal according to the respective location signal.

31. A location system in accordance with claim 30, further comprising a receiver arranged at a terrestrial location and adapted to receive navigation signals from space vehicles of at least one global navigation satellite system and to generate from said received signals a location signal indicative of the location of the receiver, the receiver being further arranged to provide said location signal to each of said plurality of transmitters, and each transmitter being arranged to determine its respective data signal according to the location signal.

32. A location system in accordance with claim 31, wherein each transmitter is arranged such that its data signal is indicative of a location offset from the location of the receiver by a respective amount.

33. A location system in accordance with any preceding claim, wherein the mobile device comprises a GPS receiver.

34. A transmitter for a location system in accordance with any preceding claim.

35. A transmitter-receiver module comprising a transmitter for a location system in accordance with any one of claims 1 to 33, and a receiver adapted to receive navigation signals from space vehicles of at least one global navigation satellite system and to generate from said received signals a location signal indicative of the location of the I.....

* : 30 receiver, the receiver being further arranged to provide said location signal to the * transmitter.

36. A mobile device for a location system in accordance with any one of claims I to 33.

37. A mobile device in accordance with claim 36, the mobile device being operable in a first mode in which it generates said indication of current location from a plurality of said navigation signals, and a second mode in which it generates said indication of current location from at least one said data signal.

38. A mobile device in accordance with claim 37, further adapted to switch between said modes according to the signals it receives at a current location.

39. A method of providing an indication of location, the method comprising: arranging a transmitter at a terrestrial location and transmitting a data signal from the transmitter, the data signal being indicative of a terrestrial location; providing a mobile device adapted to receive said data signal and to receive navigation signals from space vehicles of at least one global navigation satellite system; operating the mobile device to derive an indication of said current location of the mobile device from either said data signal or a plurality of said navigation signals, according to the signals received by the mobile device at its current location; and providing said indication of current location with the mobile device.

40. A method in accordance With claim 39, further comprising arranging a GNSS receiver at a terrestrial location, using the receiver to generate a location signal from a plurality of navigation signals received by the receiver at its terrestrial location, providing the location signal to the transmitter, and deriving the data signal from the location signal.

41. A method in accordance with claim 39 or claim 40, wherein transmitting the data signal comprises transmitting the data signal in an ISM band.

42. A method in accordance with any one of claims 39 to 41, wherein deriving the indication of current location comprises determining a path loss for a data signal received by the mobile device from the transmitter.

S S....

* 30

43. A method in accordance with any one of claims 39 to 42, comprising arranging the transmitter to transmit said data signal into a bIi1 spot of at least one of said GNSS systems. 555. S **

*

44. A method in accordance with any one of claims 39 to 43, comprising arranging the transmitter to transmit said data signal into an interior space of a building.

45. A method in accordance with any one of claims 39 to 44, comprising: providing a plurahty of said transmitters; arranging each transmitter at a respective terrestrial location; transmitting from each transmitter a respective data signal indicative of a respective terrestrial location; and operating the mobile device to derive said indication of current location from either at least one data signal or a plurality of said navigation signals, according to the signals received by the mobile device, at the current location.

46. A method in accordance with claim 45, comprising operating the mobile device to derive said indication of current location from the data contained in, and from the relative strengths of a plurality of received data signals.

47. A location system substantially as hereinbefore described with reference to the accompanying drawings.

48. A transmitter substantially as hereinbefore described with reference to the accompanying drawings.

49. A transmitter-receiver module substantially as hereinbefore described with reference to the accompanying drawings.

50. A mobile device substantially as hereinbefore described with reference to the accompanying drawings.

51. A method of providing a location signal substantially as hereinbefore described with reference to the accompanying drawings. *... S... * S

S

I...'. * . * SSS * * . S. * S

S