GB2456150A

GB2456150A - A transmitter providing a signal indicating a time difference between respective reference clocks of a first and second global navigation satellite system

Info

Publication number: GB2456150A
Application number: GB0800088A
Authority: GB
Inventors: Surinder Singh Thind
Original assignee: Samsung Electronics Co Ltd
Current assignee: Samsung Electronics Co Ltd
Priority date: 2008-01-03
Filing date: 2008-01-03
Publication date: 2009-07-08
Anticipated expiration: 2028-01-03
Also published as: GB0800088D0; GB2456150B

Abstract

A mobile device 5 is disclosed for receiving navigation signals 51a-c, 52a-c, from satellites 11a-c, of a first global navigation satellite system (GNSS) 1, having a first reference clock 100, and from satellites 21a-c, of a second GNSS 2, having a second reference clock 200 respectively. The mobile device is arranged to receive a timing signal 54 from a transmitter 4 at a terrestrial location, the timing signal comprising data indicative of a time difference between the first and second reference clocks 100,200. The mobile device generates an indication of a location of the mobile device based on the received timing signal 54 and a plurality of navigation signals received from a corresponding plurality of satellites including at least one satellite from each GNSS.

Description

LOCATION SYSTEM AND METHOD

Field of the Invention

The present invention relates to location systems and methods using navigation signals transmitted from orbiting satellites to determine location, and to mobile user equipment for use in such systems and methods

Background to the Invention

A variety of global navigation satellite systems (GNSSs) are known, and include the GPS (global positioning system), GLONASS, GALILEO and other systems, some of which are partially implemented and some of which are merely proposals at this time Each of these systems typically comprises a reference clock (which may be located and maintained in a terrestrial ground station) and each of the satellites of the system carries an on-board clock which is synchronised with the appropriate reference clock. The orbiting satellites, which are commonly referred to as space vehicles (SVs) transmits a respective navigation signal (a radio signal) for reception by suitable apparatus (which we will refer to as mobile user equipment) at a terrestrial location (i e. a location on or close to ground level). The formats of the navigation signals transmitted by the various SVs is defined and well known. For example, in the GPS system each space vehicle or satellite transmits a navigation signal in the form of a repeating navigation message having a duration of 121/z minutes. Although the formats of the different navigation signals vary from system to system, a general feature of each format is that the navigation signal comprises data which is indicative both of the position of the transmitting space vehicle and of a time (or timing) of transmission of the signal from * that space vehicle according to the respective reference clock This information is **** needed for the user equipment receiving the signal to calculate its distance from the particular transmitting space vehicle from the time taken for the transmitted signal to * 30 reach the user equipment For user equipment adapted to work with just one GNSS, in order to be able to calculate **** a position (and provide an indication of that position or location to a user, typically by means of a screen) the user equipment must be receiving signals from at least four SVs of that system (at least initially, during the so-called acquisition phase). Four signals are required rather than three because to calculate location coordinates in three dimensions (altitude, latitude and height) requires three satellites, and a fourth satellite is required for timing reference generation A known problem is that in certain locations it may not be possible to receive the minimum of four signals required for location calculation and at such locations the provision of a location indication to a user is interrupted As a partial solution, user equipment is known that is able to receive navigation signals from more than one GNSS Generally this will Increase the number of navigation signals available to the user equipment at a particular location and enables the user equipment to provide indications of location of positions where a device adapted for use with just a single GNSS could not. For example, such dual-system user equipment may be at a position where it can receive only three navigation signals from one GNSS, but in which it can receive two navigation signals from a second GNSS From these five signals (from a combination of SVs on the first and second GNSSs the user equipment is able to calculate its location, even though the reference clocks of the first and second systems are different, It is this difference between the reference clocks which means that five signals are required, rather than four if there are no more than three signals available from any one of the systems Although these dual-system receivers offer improved coverage over the single-system devices, there are still locations at which an insufficient total number of navigation signals are available for the user device to determine position It is therefore an object of certain embodiments of the invention to provide location systems, methods and user equipment which overcome, at least partially, one or more of

the problems associated with the prior art

* ** Summary of the Invention * * S * *5 S... * S

According to a first aspect of the present invention there is provided a location system comprising: *...

* 30 a first global navigation satellite system (GNSS) comprising a first reference clock and a first plurality of orbiting space vehicles (SVs), each comprising a respective clock synchronised with the first reference clock and each transmitting a respective *.SS navigation signal comprising data indicative of a position of the respective SV and of a time of transmission of the respective navigation signal from the respective SV according to the first reference clock; a second global navigation satellite system (GNSS) comprising a second reference clock and a second plurality of orbiting space vehicles (SVs), each comprising a respective clock synchronised with the second reference clock and each transmitting a respective navigation signal comprising data indicative of a position of the respective SV and of a time of transmission of the respective navigation signal from the respective SV according to the second reference clock, a transmitter arranged at a terrestrial location and arranged to transmit a timing signal comprising data indicative of a time difference between the first and the second reference clocks, and mobile user equipment adapted to receive the timing signal and the navigation signals from the SVs of the first and second GNSSs, the user equipment being adapted to process the received timing and navigation signals and to generate an indication of a location of the user equipment based on (i e from) said timing signal and a pturality of navigation signals received from a respective plurality of said SVs, the respective plurality of SVs including at least one SV of the first GNSS and at least one SV of the second GNSS Advantageously, by providing the timing signal to the user equipment, the user equipment can generate an indication of its location from fewer navigation signals that would be the case if the timing signal were not provided In particular, provision of the timing signal reduces the number of navigation signals required by the user equipment to determine a position fix by one. Thus, whereas with the prior art dual-system user equipment at least five navigation signals were required to determine position (when fewer than four signals were available from each system), in embodiments of the invention the user equipment can determine its location from just a combination of four navigation signals (three from one system, and one from the other, or alternatively two * from each) Thus, location systems embodying the invention are able to provide I...

improved coverage Additionally, the use of navigation signals from more than one * GNSS by the user equipment can enable location to be determined with greater * 30 accuracy In certain embodiments the transmitter is a base station of a terrestrial radio * communications network (which may also be referred to as a terrestrial radio access ****** * network, or TRAN). For example, in certain embodiments the transmitter is a base station of a GSM network, a node B of a UTRAN network, or an e-node B of an E-UTRA network, It will be appreciated, however, that these are many examples, and the transmitter may be a different ground or building-based transmitter in alternative embodiments Although the user equipment is adapted to determine its location (and provide an indication of location, for example in the form of a display to a user, or a signal to other equipment) from a combination of signals from the two global navigation systems together with the timing signal, in certain embodiments of the invention the user equipment will also be adapted to generate the indication of location from the navigation signals from just one of either of the systems, provided of course that a sufficient number of those signals from one system is available. Typically, this means that the user equipment will be able to generate an indication of its location from just four navigation signals received from one of the systems, in which case the timing signal is not required However, in such circumstances, the user equipment may be further adapted to make use of additional navigation signals received from the other navigation system together with the timing signal, if available, for example to improve the accuracy of its location indication It will be appreciated that in embodiments of the invention, the timing signal transmitted from the terrestrially-located transmitter will, in the vicinity of that transmitter, have a significantly greater strength than the navigation signals from the orbiting space vehicles For example, the timing signal may be a relatively strong radio signal transmitted from the base station or other transmitter of a telephone (or other communications) network, and its strength may be sufficient such that it can be received by the user equipment even when there are buildings or other obstacles between the transmitter and the user equipment. This provision of the timing signal from a * terrestrially-based transmitter further improves the coverage of the system as a whole, in * that the number of locations in which user equipment cannot determine its position are **** reduced. *

****** * 30 In certain embodiments, the system further comprises a transmitter arranged at a terrestrial location and to transmit a second timing signal comprising data indicative of a setting of at least one of the reference clocks. In such systems the user equipment is then arranged to receive and process this second timing signal and to generate an indication of location from the two timing signals and a plurality of navigation signals By providing this information on at least one of the reference clocks from a terrestrial transmitter, the system is able to further reduce the minimum number of navigation signals required at a particular location to determine position In particular, if the user equipment is provided with the timing information for one of the reference clocks, and timing information regarding the difference between the first and second reference clocks, then it should be able to calculate position from just three received navigation signals, Irrespective of whether those three signals are from a single GNSS or from a combination of space vehicles from the two systems In certain embodiments, the user equipment is adapted to receive and process navigation signals from space vehicles of at least one further GNSS, and to use navigation signals received from that further GNSS together with the timing signal or signals and the navigation signals from the first and second GNSSs to determine its indication of location. In such examples, the timing signal may be arranged to carry data indicative of the timing differences between the first, second, and further reference clocks In certain embodiments, the first GNSS is the GPS system and the second GNSS is the GLONASS system. However, it will be appreciated that in alternative embodiments the GNSS systems may be different For example, the first GNSS may be selected from a list comprising GPS, GLONASS, Galileo and other systems, and the second GNSS may be a different system selected from that list, such that possible combinations include GPS/GLONASS, GPS/Galileo, GLONASS/Galileo etc Similarly, the third or further GNSS in certain embodiments may be a different system selected from that list According to a second aspect of the present invention, there is provided mobile user equipment adapted to receive navigation signals from space vehicles of a first global navigation satellite system (GNSS), having a first reference clock, and from space vehicles of a second GNSS, having a second reference clock, the user equipment being further adapted to receive a timing signal comprising data indicative of a time difference between the first and second reference clocks, and to generate an indication of a *..

* 30 location of the user equipment from (i e by processing) the received timing signal and a plurality of navigation signals received from a corresponding plurality (i.e group or combination) of space vehicles including at least one space vehicle of each GNSS **** *.*.** * In certain embodiments the first GNSS is the GPS system and the second GNSS is the GLONASS system Again, however, in alternative embodiments the first and second GNSSs may be any combination of different systems selected from a list comprising GPS, GLONASS, Galileo and other systems In certain embodiments the user equipment is adapted to receive and process navigation signals from SVs of at least one further GNSS, having a respective further reference clock, and to use navigation signals received from the SVs of the at least one further GNSS together with the timing signal and the navigation signals from the first and second GNSSs to determine an indication of location The mobile user equipment may, for example, be a mobile telephone, a PDA, a computer, a portable dedicated GNSS receiver, a vehicle-mounted device, or some other mobile (and optionally portable) equipment Another aspect of the invention provides a vehicle incorporating mobile user equipment in accordance with the above aspect Another aspect provides a method of generating an indication of a location, the method comprising* providing user equipment at said location; receiving, with (i e. at) the user equipment, a plurality of navigation signals from a corresponding plurality of space vehicles (SVs), said pluraiity of SVs including at least one SV from a first global navigation satellite system (GNSS), having a first reference clock, and at least one SV of a second GNSS, having a second reference clock; transmitting a timing signal comprising data indicative of a time difference between the first and second reference clocks, receiving at the user equipment said timing signal, and * using the user equipment to process the received timing signal and received **.

plurality of navigation signals and generate an indication of said location based on the received timing signal and the received plurality of navigation signals.

Thus, the received timing signal and the received plurality of navigation signals are used to determine the indication of location (it is derived I calculated from them).

In certain embodiments said transmitting comprises transmitting from a transmitter located at a terrestrial location For example, the transmitter may be a base station of a terrestrial radio communications network, such as a base station of a GSM network, a node B of a UTRAN network, or an e-node B of an E-UTRA network. These are merely examples however; the list is not exhaustive and the transmitter may be of a different type in alternative embodiments.

In certain embodiments said plurality of navigation signals comprises a minimum of four navigation signals The method may further comprise using the mobile equipment to generate an indication of a location of the equipment in the absence of said timing signal from a minimum of four said navigation signals from a respective four SVs of a single one of said GNSSs The method may further comprise using the mobile equipment to generate an indication of a location of the user equipment in the absence of said timing signal from a minimum of five said navigation signals from a respective five SVs comprising at least one SV from each GNSS.

The method may further comprise transmitting a second timing signal comprising data indicative of a setting of at least one of said reference clocks, receiving the second timing signal at the user equipment, processing said second timing signal, and generating an indication of a location of the user equipment based on (i e from) said timing signals and said plurality of navigation signals In such embodiments, said p'urality of navigation signals may comprise a minimum of three navigation signals In certain embodiments the method further comprises receiving at the user equipment navigation signals from SVs of at least one further GNSS, and using the navigation * ** signals received from the SVs of the at least one further GNSS together with the timing signal or signals and the navigation signals from the first and second GNSSs to determine an indication of location * *.I**S * 30 Another aspect provides a method of operating mobile user equipment adapted to receive navigation signals from space vehicles of a first global navigation satellite system (GNSS), having a first reference clock, and from space vehicles of a second eQS.

GNSS, having a second reference clock, and to receive a timing signal comprising data indicative of a time difference between the first and second reference clocks, the method comprising receiving, at the user equipment, a plurality of said navigation signals from a corresponding plurality of said space vehicles (SVs), said plurality of SVs including at least one SV from each of the first and second GNSSs; receiving, at the user equipment, a said timing signal, and using the user equipment to process the received timing signal and received plurality of navigation signals and generate an indication of a location of the user equipment based on the received timing signal and the received plurality of navigation signals Yet another aspect provides a transmitter arranged to transmit a timing signal comprising data indicative of a time difference between a first reference clock of a first global navigation satellite system and a second reference clock of a second global navigation satellite system

Brief Description of the Drawings

Embodiments of the invention will now be described with reference to the accompanying drawings, of which Fig. 1 is a schematic representation of a location system embodying the invention; Fig 2 is a schematic representation of another location system embodying the invention; Fig. 3 is a schematic representation of user equipment embodying the invention receiving navigation signals from a plurality of space vehicles in an urban environment; * .* * S * ** S...

Fig. 4 is a flow chart illustrating a location method embodying the invention; and

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* SS,*.

* 30 Fig. 5 is a flow chart illustrating another location method embodying the invention.

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* Detailed Description of Embodiments of the Invention *.S.SS * Referring now to Fig 1, a location system embodying the invention comprises a first global navigation satellite system GNSS 1 comprising a first reference clock 100 and a first plurality of orbiting space vehicles ha, 11 b, 11 c The first system 1 comprises a terrestrially-located ground station 10 in which the first reference clock 100 is located and maintained The space vehicles ii contain on-board clocks (not shown in the figure) synchronised with the reference clock 100 and each SV ii is arranged to transmit a respective navigation signal Sla, Sib, and Sic comprising data indicative of a position of the respective SV ii and of a time (or timing) of transmission of the respective navigation signal from that SV according to the reference clock 100 Similarly, the system comprises a second GNSS 2, with a respective second reference clock 200 located in a second ground station 20 The second system space vehicles 2i each comprise a clock synchronised with the second referenced clock 200 and the signals S2a, S2b and S2c transmitted from the SVs 21 each comprise data indicative of the position of the respective SV 21 and of a time of transmission of the respective signal according to the second clock 200. In the figure, the earth is denoted generally by reference letter E and mobile user equipment 5 is located close to ground level in this example The system further comprises a transmitter 4 arranged on the ground (in this example, but may be located differently in alternative embodiments) and this transmitter transmits from a suitable antenna 41 a timing signal S4 for reception by the user equipment 5. The timing signal comprises data indicative of a time difference between the first and second reference clocks 100, 200 It will be appreciated that this information or difference between the respective clocks may be obtained and provided to the transmitter 4 in a variety of ways, and the invention is not limited to any particular one of these methods The user equipment 5 is adapted to receive the navigation signals Si and S2 from the space vehicles and the timing signal S4 from the ground-based transmitter 4 and is able to generate an indication of its current location from those signals it receives at that location If the user equipment 5 is in receipt of timing signal S4, then it need only be in receipt of four navigation signals in order to determine its location, whether those four signals all come from space vehicles of one navigation system or from a combination of SVs including at least one SV from the first system 1 and at least one SV 21 from the second system 2 S... * S

In this first embodiment, the system additionally comprises a second transmitter 6 I.....

* arranged to transmit from an antenna 61 a second timing signal S6 which comprises data indicative of at least one of the reference clocks 100, 200. The user equipment 5 is also adapted to receive this second timing signal S6 and to use it in its location *SSS calculations When the user equipment is able to receive both the first and second *..*.* timing signals S4, S6, it then only needs to be able to receive a minimum of three navigation signals from the orbiting space vehicles, whether those three signals all come from the space vehicles of one system, or from a mixture of space vehicles of the first and second systems Referring now to Fig. 2, this shows another location system embodying the invention, incorporating mobile user equipment 5 which also embodies the invention. In this figure the user equipment 5 is at a location which can be regarded as generally terrestrial, at which it is able to receive four satellite navigation signals These four signals consist of a signal Sla from a space vehicle 1 la of a first GNSS, a second signal S2 from a space vehicle 21 of a second GNSS, a third signal S3 from a space vehicle 31 of a third GNSS, and a fourth signal Sib from a space vehicle lib also of the first GNSS These four signals each comprise data indicative of transmission timing relative to their own reference clocks. The user equipment 5 at the illustrated location is also able to receive a timing signal S4 from a base station 4 and its transmitting antenna 41 This timing signal S4 comprises data indicative of a time difference between the reference clocks of the first and second GNSS and of a time difference between the reference clocks of the second and third GNSS. With this timing information from the signal S4, the user equipment 5 is able to calculate its location from the four received navigation signals Sia, S2, S3 and Sib even though they originate from three different global navigation satellite systems Referring now to Fig. 3, this shows user equipment 5 embodying the invention and located in an urban environment between buildings 81 and B2 In this embodiment the user equipment 5 is adapted to receive satellite navigation signals from both the GPS and GLONASS systems, and a plurality of those satellites is shown in the figure The GPS space vehicles are denoted by ha, hib, lic and lid, and the GLONASS space vehicles are denoted by 21a, 21b, and 2ic As can be seen, building Bi interrupts the lines of sight from the user equipment 5 to satellites 1 la and 21a (and so blocks the navigation signals from those satellites to the user) In the position indicated in the figure the user equipment 5 is, however, still able to receive satellite signals directly from five of the illustrated satellites, those five including satellites ilb, lic and lid of the S..

* GPS system and satellites 21b and 21c of the GLONASS system. The user equipment is adapted to calculate its location from the five navigation signals it can receive from these five visible satellites (and it can do this without requiring an additional timing signal * 35 indicative of a time difference between the reference clocks of the GPS and GLONASS *.** * systems) However, the user equipment 5 is further adapted to be able to receive such a timing signal from a suitably arranged transmitter (not shown in this figure). The transmitter may be a mobile phone or other such transmitter, transmitting a radio signal which is relatively strong compared to the power of the signals received by the user 5 from the satellites The timing signal may thus be able to penetrate or pass through the buildings Bi and B2 Thus, if the user equipment 5 were to move slightly to the right in the figure, such that building Bi still blocks signals from satellites ha and 21a and building B2 then blocks the signals from satellite lid, the user equipment is only able to receive navigation signals (and their associated navigation messages) directly from four satellites (lib, 21b, ilc, and 21c) but is still able to calculate its new location using the received timing signal as well Thus, it will be appreciated that the location system embodying the invention and utilising the timing signal transmitted from a terrestrial transmitter can provide significantly improved system coverage in urban canyon" environments Referring now to Fig 4, this illustrates a method in which user equipment embodying the invention is able to determine its location using signals from five satellites from a combination of the GPS and GLONASS systems The method begins with start step SlO followed by step Si 1 in which assistance data from a network (such as a mobile phone or other such communications network) is obtained If available, this assistance data will relate to the GPS and GLONASS systems. Then in step S12 the user equipment searches for GPS and GLONASS satellites. Then, when receiving signals from the GPS and GLONASS satellites comprising their respective navigation messages, the user equipment in S13 establishes the Almanac and Ephemeris data for the GPS and GLONASS transmitting satellites (in other words it extracts this data from received navigation messages) In this example, the user equipment is calculating location without requiring a signal indicative of reference clock timing differences Thus, in step S14 the user equipment establishes a timing reference from the signals received * from the GLONASS and GPS satellites (and, as described above, in order to do this it must be receiving a minimum of five signals) Then, in step S15 the user equipment S...

30 calculates a difference in the timing references of the GPS and GLONASS systems.

Next, in step S16 the user equipment makes pseudo range measurements on at least 5S*S** * four of the satellites (for example on three GPS satellites and one GLONASS satellite, or some other combination, e.g two GPS satellites and two GLONASS satellites). Finally, *:** in step S17 the user equipment calculates its location coordinates based on the received S...

signals (and in particular on the pseudo range measurements obtained from those signals) Referring now to Fig 5, this shows a method in which user equipment embodying the invention calculates its location using just four satellite signals (those four signals originating from a combination of GPS and GLONASS satellites) together with assistance (in the form of information contained in a timing signal) in the timing difference between the GPS and GLONASS systems Once again, the method's start is donated, in this case by reference number S20 Then, in step S21 the user equipment obtains the assistance data from the signal it receives from a transmitter (for example a transmitter of a communications network, such as a cellular telephone network), that assistance data now including information on a timing difference between the clocks of the GPS and GLONASS systems Then, in step S22 the user equipment searches for GPS and GLONASS satellites. Having detected those satellites from their navigation signals, the user equipment in S23 goes on to establish the Almanac and Ephemeris data for the GPS and GLONASS satellite systems from the received navigation signals and the messages they contain Next, in step S24 the user equipment establishes a timing reference for the GLONASS and GPS satellites from the assisted data 0 e from the data contained in the timing signal received from the terrestrial transmitter) Next, in step S25 the user equipment obtains the timing difference between the GPS and GLONASS systems from the assistance data. Next, in step S26 the user equipment makes pseudo range measurements on four of the satellites (for example from three GPS satellites and one GLONASS satellite, or some other combination) Finally, in step S27 the user equipment calculates its location coordinates and typically then provides this information to a user, for example by means of a suitable screen.

It will be appreciated from the above that certain embodiments of the invention are concerned with Location Based Services (LBS) for mobiles and stand-alone GPS/other GNSS receivers *:*::* Certain embodiments improve the availability of satellites in restricted areas (i e. areas in *::* 30 which there may be restricted line of sight satellites or degradation due to multi-path satellite signals) In embodiments of the invention, increasing the number/availability of S..... . . satellites increases the probability of location fix being possible, and also improves the accuracy of location fix : 35 It will also be appreciated that a problem with prior art systems has been poor location accuracy in areas offering restricted view of satellites (e g built up areas, urban canyons"), and the worst case scenario has been that no location determination has been possible due to the number of visible GPS or GLONASS satellites being lower than the minimum required.

Embodiments of the invention, by making use of two or more satellite systems, improve the probability of location fix in restricted areas and improve location accuracy, compared with that currently available with the GPS system alone Using embodiments of the invention, urban canyon location fixes improve both in frequency and accuracy As futher background, using just the GPS system a minimum of four satellites are required to calculate location coordinates in three dimensions, altitude, latitude and height require three satellites, and the fourth satellite is required for timing reference generation. For general use, the GPS system performs well but limitations become obvious when used in built up areas, e g with restricted views of satellites and multi-path signal scenarios This leads to errors in location accuracy and in the worst case condition, no location fix is possible due to lack of visible satellites Combining GNSS systems (e g GPS and GLONASS) for location determination provides advantages In cases where the number of satellites in view is likely to be restricted, the ability to use additional satellites proves useful The possibility of satellite visibility Increases, and this in turn increases the probability of location fix. This is certainly the case with GLONASS system being made available for civilian use with full constellation of twenty four satellites. This in effect doubles the number of satellites visible for a user in any given location See again figure 1 -this shows the Jim iting condition in which three GPS satellites are visible and two GLONASS satellites are visible to the user, and neither the GPS system nor the GLONASS system will give location coordinates on their own With both GPS and GLONASS systems, in the absence of another signal giving inter-system timing information, a total of five satellites are required to generate a location fix *. (e g three GPS and two GLONASS) This is one more satellite then GPS alone. The S..

fundamental reason for this extra satellite is due to the time frame reference difference between GPS and GLONASS -i e the two systems are not synchronized Both GPS and GLONASS have very accurate clocks but they are not synchronized hence the extra satellite is required to establish the difference in the two time references As the : reference clocks in the two satellite systems are very accurate and very stable they are :": unlikely to drift In this case the fifth satellite is not required once location fix is established, i.e. once the timing reference difference between the two systems is established This leads to overall four satellites in tracking mode, e g. two from GPS and two from GLONASS or other combination In certain embodiments, location coordinates can be determined using four satellites only In the absence of the timing signal, the only reason for using a fifth satellite is to determine the time difference between GPS and GLONASS system of satellites, Once this has been established only four satellites are required for location coordinates In a mobile environment, and in accordance with an embodiment of the invention, the service provider could provide a timing signal indicative of a timing difference between the GPS and GLONASS systems over the air interface i e. as in Assisted GPS. With such a feature then only four satellites will be required in various combinations from the GPS and GLONASS systems With this feature, a mobile user can receive the latest up to date timing information of the GPS/GLONASS systems from service provider base stations and this will increase the probability of location fixes in restricted areas Using the GLONASS system offers benefits Location accuracy is deliberately limited in the GPS system, at best to a few meters in open sky conditions for civilian use This is done by data encryption and deliberately varying the reference clock that leads to "dithering" of GPS data, this leads to imperfect pseudo range measurements and inaccurate location calculation. In the GLONASS system this is not the case for civilian use, if user's receiver is capable of synchronizing to GLONASS timing reference then accuracy in the range of centimeters is possible. This typically will require the technique of carrier phase tracking rather then code phase tracking that is commonly used in commercial location receivers -i.e a more advanced receiver is required to operate in this manner.

Throughout the description and claims of this specification, the words "comprise" and "contain" and variations of the words, for example "comprising" and "comprises", means **. "including but not limited to", and is not intended to (and does not) exclude other moieties, additives, components, integers or steps S...

:: 30 Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires, In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

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Features, integers, or characteristics descnbed in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith * S. ** . * ** S... * S S...

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Claims

1. A location system comprising a first global navigation satellite system (GNSS) comprising a first reference clock and a first plurality of orbiting space vehicles (SVs), each comprising a respective clock synchronised with the first reference clock and each transmitting a respective navigation signal comprising data indicative of a position of the respective SV and of a time of transmission of the respective navigation signal from the respective SV according to the first reference clock, a second global navigation satellite system (GNSS) comprising a second reference clock and a second plurality of orbiting space vehicles (SVs), each comprising a respective clock synchronised with the second reference clock and each transmitting a respective navigation signal comprising data indicative of a position of the respective SV and of a time of transmission of the respective navigation signal from the respective SV according to the second reference clock, a transmitter arranged at a terrestrial location and arranged to transmit a timing signal comprising data indicative of a time difference between the first and the second reference clocks; and mobile user equipment adapted to receive the timing signal and the navigation signals from the SVs of the first and second GNSSs, the user equipment being adapted to process the received timing and navigation signals and to generate an indication of a location of the user equipment based on said timing signal and a plurality of navigation signals received from a respective plurality of said SVs, the respective plurality of SVs including at least one SV of the first GNSS and at least one SV of the second GNSS

2 A system in accordance with claim 1, wherein the transmitter is a base station of a terrestrial radio communications network

3 A system in accordance with claim 2, wherein the transmitter is one of a base *. 30 station of a GSM network, a node B of a UTRAN network, and an e-node B of an E-UTRA network. * I

4. A system in accordance with any preceding claim, wherein said plurality of navigation signals comprises a minimum of four navigation signals

I

**.* 35

5. A system in accordance with any preceding claim, wherein the mobile equipment is further adapted to generate an indication of a location of the user equipment in the absence of said timing signal from a minimum of four said navigation signals from a respective four SVs of a single one of said GNSSs 6 A system in accordance with any preceding claim, wherein the mobile equipment is further adapted to generate an indication of a location of the user equipment in the absence of said timing signal from a minimum of five said navigation signals from a respective five SVs comprising at least one SV from each GNSS 7 A system in accordance with any preceding claim, further comprising a transmitter arranged at a terrestrial location and to transmit a second timing signal comprising data indicative of a setting of at least one of said reference clocks, the user equipment being arranged to receive and process said second timing signal and to generate an indication of a location of the user equipment based on said timing signals and said plurality of navigation signals 8. A system in accordance with claim 7, wherein said plurality of navigation signals comprises a minimum of three navigation signals 9 A system in accordance with any preceding claim, wherein the user equipment is adapted to receive and process navigation signals from SVs of at least one further GNSS, and to use navigation signals received from the SVs of the at least one further GNSS together with the timing signal or signals and the navigation signals from the first and second GNSSs to determine an indication of location 10 A system in accordance with claim 9, wherein the at least one further GNSS comprises a third GNSS having a third reference clock, and the first timing signal comprises data indicative of a time difference between the third reference clock and at least one of the first and second reference clocks * ** * S S * S. **** 11. Mobile user equipment adapted to receive navigation signals from space vehicles of a first global navigation satellite system (GNSS), having a first reference clock, and * from space vehicles of a second GNSS, having a second reference clock, the user equipment being further adapted to receive a timing signal comprising data indicative of a time difference between the first and second reference clocks, and to generate an S...

indication of a location of the user equipment from the received timing signal and a plural ty of navigation signals received from a corresponding plurality of space vehicles including at least one space vehicle of each GNSS 12 Mobile user equipment in accordance with claim 11, wherein said first GNSS is the GPS system and the second GNSS is the GLONASS system 13 Mobile user equipment in accordance with claim 11 or claim 12, wherein the user equipment is adapted to receive and process navigation signals from SVs of at least one further GNSS, having a respective further reference clock, and to use navigation signals received from the SVs of the at least one further GNSS together with the timing signal and the navigation signals from the first and second GNSSs to determine an indication of location 14. Mobile user equipment in accordance with any one of claims 11 to 13, wherein the mobile user equipment is one of. a mobile telephone, and a dedicated GNSS receiver 15. Mobile user equipment in accordance with any one of claims 11 to 14, wherein the mobile user equipment is portable 16 A vehicle incorporating mobile user equipment in accordance with any one of claims 11 to 15.

17 A method of generating an indication of a location, the method comprising providing user equipment at said location, receiving, with the user equipment, a plurality of navigation signals from a corresponding plurality of space vehicles (SVs), said plurality of SVs including at least * one SV from a first global navigation satellite system (GNSS), having a first reference clock, and at least one SV of a second GNSS, having a second reference clock; transmitting a timing signal comprising data indicative of a time difference between the first and second reference clocks, *.**** * receiving at the user equipment said timing signal, and using the user equipment to process the received timing signal and received plurality of navigation signals and generate an indication of said location based on the **** * 35 received timing signal and the received plurality of navigation signals **.*** * * 18 A method in accordance with claim 17, wherein said transmitting comprises transmitting from a transmitter located at a terrestrial location 19. A method in accordance with claim 18, wherein the transmitter is a base station of a terrestrial radio communications network.

A method in accordance with claim 19, wherein the transmitter is one of. a base station of a GSM network, a node B of a UTRAN network; and an e-node B of an E-UTRA network 21 A method in accordance with any one of claims 17 to 20, wherein said plurality of navigation signals comprises a minimum of four navigation signals 22. A method in accordance with any one of claims 17 to 21, further comprising using the mobile equipment to generate an indication of a location of the equipment in the absence of said timing signal from a minimum of four said navigation signals from a respective four SVs of a single one of said GNSSs 23. A method in accordance with any one of claims 17 to 22, further comprising using the mobile equipment to generate an indication of a location of the user equipment in the absence of said timing signal from a minimum of five said navigation signals from a respective five SVs comprising at least one SV from each GNSS.

24 A method in accordance with any one of claims 17 to 23, further comprising transmitting a second timing signal comprising data indicative of a setting of at least one of said reference clocks, receiving the second timing signal at the user equipment, processing said second timing signal, and generating an indication of a location of the user equipment based on said timing signals and said plurality of navigation signals.

25 A method in accordance with claim 24, wherein said plurality of navigation signals comprises a minimum of three navigation signals I.. *I* * S 26. A method in accordance with any one of claims 17 to 25, further comprising receiving at the user equipment navigation signals from SVs of at least one further S...

GNSS, and using the navigation signals received from the SVs of the at least one further GNSS together with the timing signal or signals and the navigation signals from the first and second GNSSs to determine an indication of location 27 A method of operating mobile user equipment adapted to receive navigation signals from space vehicles of a first global navigation satellite system (GNSS), having a first reference clock, and from space vehicles of a second GNSS, having a second reference clock, and to receive a timing signal comprising data indicative of a time difference between the first and second reference clocks, the method comprising.

receiving, at the user equipment, a plurality of said navigation signals from a corresponding plurality of said space vehicles (SVs), said plurality of SVs including at least one SV from each of the first and second GNSSs, receiving, at the user equipment, a said timing signal, and using the user equipment to process the received timing signal and received plurality of navigation signals and generate an indication of a location of the user equipment based on the received timing signal and the received plurality of navigation signals 28. A transmitter arranged to transmit a timing signal comprising data indicative of a time difference between a first reference clock of a first global navigation satellite system and a second reference clock of a second global navigation satellite system 29 A location system, mobile user equipment, method of generating an indication of a location, method of operating mobile user equipment, or a transmitter substantially as hereinbefore described with reference to the accompanying drawings * I' a. S * S* **. * S

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