Abstract:
The present invention relates to a method of updating the clock bias between the common clock of the satellites of a radio navigation satellite system and the clock of a radio station of an asynchronous cellular radiotelephone system including a mobile device including a radio navigation satellite system receiver for receiving satellite data supplied by at least four satellites and an assistance server for improving the acquisition of satellite data by the mobile device. The method includes the steps of the mobile device receiving the satellite data, the mobile device calculating pseudodistances between itself and the satellites, encapsulating the pseudodistances with the time at which the pseudodistances are calculated, transmitting the pseudodistances and the time at which the pseudodistances are calculated in the form of a radio signal from the mobile device to the assistance server via the radio station, and the assistance server determining the position of the mobile device and estimating the clock bias between the common clock of the satellites and the clock of the radio station using the pseudodistances and the time at which the pseudodistances are calculated.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based on French Patent Application No. 0314699 filed Dec. 16, 2003, the disclosure of which is hereby incorporated by reference thereto in its entirety, and the priority of which is hereby claimed under 35 U.S.C. §119. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method of updating the clock of a radio station of a cellular radiotelephone system including an assistance server for improving the acquisition of satellite data. 
     2. Description of the Prior Art 
     In the field of mobile telephony, it is proving increasingly necessary to be able to locate mobile telephones. 
     To this end, it is known in the art to associate a cellular radiotelephony device, for example of the Global System for Mobile communications (GSM) mobile telephone type, with a radio navigation satellite system (RNSS) receiver such as a Global Positioning System (GPS), GLONASS or GALILEO type receiver, by means of which the mobile device picks up transmissions from satellites to determine its position. Thus in the event of a road traffic accident, for example, the mobile device can calculate and transmit its position. 
     The position of the device may be determined in the following manner: a plurality of satellites transmit continuously a time-stamped signal that is picked up by the receiver. If it is synchronized to the clock of the satellites, the receiver can then measure the propagation time of this signal and deduce therefrom the distance between it and a particular satellite. A receiver of the above kind can determine its position by triangulation using three satellites. Each propagation time measurement represents the radius of a sphere centered on a particular satellite, the receiver being situated on that sphere. With two distance measurements, the position of a receiver is within a circle formed by the intersection of two spheres. A simultaneous third measurement reduces the intersection to two points, one of which is at a great distance away in space and is easily ignored. 
     However, because it is not totally synchronous with the GPS, the clock of the receiver is affected by a clock bias ΔT. In fact, the atomic clocks of GPS satellites are very accurate but the accuracy of the more rudimentary GPS receiver is fatally lower. The clock bias ΔT is the time difference between the receiver clock and the satellite clock and may be as high as several seconds. It is reflected in an error in measuring the GPS signal propagation time and thereby an error c.ΔT in the satellite-receiver distance, where c is the speed of light. This error affects all distances measured by the receiver. As the distances are not perfect because they are affected by the time bias, they are called pseudodistances. The time bias is unknown a priori and must be determined. 
     There is therefore a fourth unknown, for which it is necessary to measure at least one additional distance, and therefore to use at least four satellites, in order to solve a system of four equations in four unknowns. 
     The signal sent by each satellite is a signal whose spectrum has been spread by a phase-modulated pseudorandom code; the GPS receiver must acquire this signal. The receiver generates locally a replica of the signal and then delays the start of this replica until it is superimposed on that coming from the satellite. The delay is determined by correlating the two signals. The value of this delay is the time taken by the signal to propagate from the satellite to the user. This kind of measurement demands extremely high accuracy (better than 100 nanoseconds). The time taken by the signal to travel this distance is of the order of 50 milliseconds. However, because the clock of the GPS receiver is never fully synchronized to that of the satellites, the receiver has to adjust its clock constantly by a process of successive approximations to arrive at the maximum correlation of the two signals. Acquisition of the signal therefore necessitates a time sweep by the receiver. 
     Furthermore, the signal transmitted by each satellite is transmitted at a known frequency of 1575.42 MHz. The Doppler shift of the satellite, on which is superimposed a receiver local clock uncertainty, results in an uncertainty of ±5 kHz in respect of the signal received by the GPS receiver. Now, to obtain a good correlation, the signal generated locally by the receiver must have the same frequency as the signal transmitted by the satellite. The receiver must therefore perform a frequency sweep, in addition to the time sweep, in order to determine the time taken by the signal to propagate from the satellite to the user. 
     The time and frequency sweeps referred to above imply a very long data processing time and require a receiver having a very high computation power. 
     One solution is to use a server to assist the GPS receiver of the mobile device by increasing its sensitivity by curtailing the time-frequency area to be swept. A server of this kind is described in “Indoor GPS Technology” (F. van Diggelen et al., CTIA Wireless-Agenda, Dallas, May 2001). This technology is known as the assisted GPS (A-GPS) technology.  FIG. 1  represents a telecommunication system  1  using an assistance server  5  of this kind. A mobile device  2  including a GPS receiver, such as a mobile telephone in a GSM type telephone network  4 , is seeking to calculate its position from signals P 1  to P 4  transmitted by at least one of four satellites S 1  to S 4 . To this end, the device  2  sends a request R in the form of a radio signal over the telephone network  4 . The request R passes through a base transceiver station (BTS) type radio base station  3  associated with the cell in which the mobile device  2  is located. The request R is processed by the server  5 , which receives satellite information in real time via fixed radio stations  6  equipped with GPS receivers receiving information K. In response to the request R, the server  5  sends to the mobile device  2  information I that passes through the BTS  3 . The information contains, for example, the ephemerides of the satellites S 1  to S 4 . Using that information, the mobile device  2  can determine the Doppler shift of the satellites and considerably curtail its frequency sweep. Note that there are two types of A-GPS technology, namely mobile station based (MS-based) and mobile station assisted (MS-assisted). In the case of the MS-based technology, the position of the mobile device  2  is calculated by the mobile device itself. In the case of the MS-assisted technology, the position of the mobile device  2  is calculated by the server  5 . 
     In the synchronous code division multiple access (CDMA (or CMA 2000)) telephone networks used in the USA, the time sweep is much less critical since each of the transceivers is synchronized. Thus using an assistance server with a synchronous network greatly curtails the time and frequency sweep. On the other hand, in the case of an asynchronous network such as a GSM or UMTS network using time division multiple access (TDMA), the time sweep aspect remains very important even if an assistance server is used, because the transceiver clocks may vary considerably. 
     One prior art solution to this problem is described in the document “Analysis of GPS time-transfer accuracy in GSM and UMTS networks and possibilities to improve sensitivity” (J. Syrjärinne, ION GPS 2002, 24-27 Sep. 2002) and consists in equipping each base transceiver station (BTS)  3  with a GPS receiver, as shown in  FIG. 1 . The base station  3  can therefore, after calculating its position and the clock bias between itself and the satellites, determine the common clock of the satellites and transmit that GPS clock to the mobile device  2  by marking the information signal  1 . The clock transmitted to the mobile device  2  is fairly accurate in that it is affected only by the transmission time of a few microseconds between the base station  3  and the mobile device  2 . This transmission time is short because the mobile device  2  is in the cell associated with the base station  3  and the diameter of a cell such as a GSM cell varies from 300 m to 30 km, depending on terrain and population density. 
     However, implementing a solution of the above kind causes certain difficulties. Thus this solution is extremely costly, because it entails many modifications of the network and installing a GPS receiver in each BTS type radio base station. 
     The present invention aims to provide a method of updating the clock bias between the common clocks of the satellites of a radio navigation satellite system and the clock of a BTS type radio station of an asynchronous cellular radiotelephone network such as a GSM network, said method enabling mobile devices in said cellular network equipped with an radio navigation satellite system receiver to access this clock bias in a simple and economical way. 
     SUMMARY OF THE INVENTION 
     To this end the invention proposes a method of updating the clock bias between the common clock of the satellites of a radio navigation satellite system and the clock of a radio station of an asynchronous cellular radiotelephone system including a mobile device including a radio navigation satellite system receiver for receiving satellite data supplied by at least four satellites and an assistance server for improving the acquisition of satellite data by the mobile device, the method including the following steps:
         the mobile device receiving the satellite data,   the mobile device calculating pseudodistances between itself and the satellites,   encapsulating the pseudodistances with the time at which the pseudodistances are calculated,   transmitting the pseudodistances and the time at which the pseudodistances are calculated in the form of a radio signal from the mobile device to the assistance server via the radio station, and   the assistance server determining the position of the mobile device and estimating the clock bias between the common clock of the satellites and the clock of the radio station using the pseudodistances and the time at which the pseudodistances are calculated.       

     Thanks to the invention, in an asynchronous network of the GSM type, an assistance server such as an A-GPS server, by using the pseudodistances and the time at which said pseudodistances are calculated, knows the clock bias between a radio station such as a BTS and the satellites of a radio navigation satellite system such as the Global Positioning System. Thus the clock bias of a particular radio station is updated each time that an assistance server calculates the position of a mobile device in a cell type area associated with said station: this applies to mobile devices using an MS-assisted technology. This updating method is therefore a self-supported, simple and economical method that does not necessitate any structural modification of the network, such as adding radio navigation satellite system receivers to all the radio stations. 
     In a first embodiment, the time at which the pseudodistances are calculated corresponds to time information supplied by the radio station. 
     Thus the time at which the pseudodistances are calculated is determined on the basis of the signal exchanged between the mobile device and the radio stations; in the case of a GSM signal, for example, the pseudodistances are time-stamped by means of the structure of the GSM message, which is divided into hyperframes in turn divided into superframes in turn divided into multiframes in turn divided into frames in turn divided into bit periods; a number corresponding to the pseudodistances calculation time may be associated with each of these elements (hyperframe, superframe, multiframe, frame, bit). The server then determines the position of the mobile device and the time difference between the time-stamp of the pseudodistance calculation and the clock of the GPS satellites. The server therefore has direct access to the clock bias between the clock of the radio station and the GPS clock. 
     In a second embodiment, the time at which the pseudodistances are calculated is the time according to the clock of the mobile device at which the pseudodistances are calculated, the method including the following steps:
         the mobile device inserting into the radio signal the time it sends the radio signal,   the radio station inserting into the radio signal the time it receives the radio signal,   the assistance server determining a first clock bias between the clock of the mobile device and the common clock of the satellites,   the assistance server estimating a second clock bias between the clock of the mobile device and the clock of the radio station by establishing the difference between the sending time and the receiving time, and   deducing the clock bias between the common clock of the satellites and the clock of the radio station by establishing the difference between the first and second clock bias values.       

     In this embodiment, the assistance server, such as an A-GPS server, knows the clock bias between the radio station BTS and the GPS satellites from the difference between the two clock bias values. Note here that the second clock bias, which is that between the clock of the mobile device and the clock of the radio station, is determined by the server by establishing the difference between the time-stamp for sending by the mobile and the time-stamp for reception by the station. This second clock bias is accurate to within the travel time between the mobile and the station. That travel time is of the order of a few microseconds and is negligible compared to the second clock bias values, which are of the order of several seconds. 
     The clock bias between the common clock of the satellites and the clock of the radio station is advantageously stored in an update database. 
     The present invention further provides an assistance server for improving the acquisition of satellite data by a mobile device, the server including a database for the clock bias between the clock of the radio stations of an asynchronous cellular radiotelephone system and the common clock of the satellites of a radio navigation satellite system. 
     The clock bias values are advantageously updated by a method according to the invention. 
     The present invention further provides a method of improvement by an assistance server of the acquisition of satellite data by a mobile device including a radio navigation satellite system receiver in an asynchronous cellular radiotelephone system, the method including the following steps:
         the mobile device sending an assistance request to the assistance server via a radio station associated with it, and   the server identifying the clock bias between the common clock of the satellites and the clock of the radio station as updated by an updating method according to the invention.       

     Thus the improvement method relying on an assistance server advantageously uses the method of the invention of updating the clock bias. 
     A first embodiment of the method includes a step of the assistance server transmitting assistance data including:
         time information associated with the radio station extracted from the clock bias between the common clock of the satellites and the clock of the radio station as updated by the updating method, and   the time of the common clock of the satellites extracted from the clock bias between the common clock of the satellites and the clock of the radio station as updated by the updating method.       

     It is advantageous if the time information associated with the radio station is transferred as a reference time of the BTS radio signal in the form of hyperframe, superframe and multiframe references and a bit number, and the time of the common clock of the satellites is associated with the BTS reference date. 
     A second embodiment of the method includes the following steps:
         the assistance server transmitting assistance data including a clock bias between the common clock of the satellites and the clock of the radio station as updated by the updating method, and   the radio station inserting into a radio signal sent to the mobile device the time the radio station sends the radio signal.       

     Thus the assistance data supplied to a mobile device includes the updated clock bias between the common clock of the satellites and the clock of the radio station associated with the mobile device. Because the radio signal further includes the time-stamp for sending of the assistance data radio signal by the radio station, the mobile device can obtain a precise estimate of its clock bias with respect to the radio station and, simply by establishing a difference, a precise estimate of its clock bias with respect to the satellites. In fact, on receiving the radio signal, the mobile knows, to within the travel time, which is negligible, the time-stamp of the clock of the base station and the clock difference between the base station and the satellite system. 
     The present invention further provides a method of calculating the position of a mobile device including a radio navigation satellite system receiver in an asynchronous cellular radiotelephone system, the method including the following steps:
         an assistance server improving the acquisition of satellite data by the mobile device by a method according to the invention,   the mobile device acquiring the satellite data,   the mobile device calculating pseudodistances between the mobile device and the satellites from the satellite data, and   the assistance server or the mobile device determining the position of the mobile device.       

     It is seen here that the method of calculating the position of a mobile device works as much for an MS-assisted type mobile device (in which the position is determined by the server) as for an MS-based type mobile device (in which the position is determined by the mobile device). 
     The present invention further provides a mobile device of an asynchronous cellular radiotelephone system including a radio navigation satellite system receiver and means for determining the position of the mobile device by the method according to the invention. 
     The present invention finally provides a method of updating the clock bias between the common clock of the satellites of a radio navigation satellite system and the clock of a radio station of an asynchronous cellular radiotelephone system including a mobile device including a radio navigation satellite system receiver for receiving satellite data supplied by at least four satellites and exchanging a radio signal with the radio station and an assistance server for improving the acquisition of satellite data by the mobile device, the method including the following steps:
         the mobile device receiving the satellite data,   the mobile device calculating pseudodistances between itself and the satellites, and   the mobile device determining its position and estimating the clock bias between the common clock of the satellites and the clock of the radio station using the pseudodistances and the time at which the pseudodistances are calculated, the time at which the pseudodistances are calculated corresponding to time information supplied by the radio station.       

     A method of the above kind enables updating from a mobile device using an MS-based technology of the clock bias between the common clock of the satellites of a radio navigation satellite system and the clock of a radio station. 
     Other features and advantages of the present invention will become apparent in the course of the following description of one embodiment of the invention, which is given way of illustrative and nonlimiting example. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  represents a prior art telecommunication system. 
         FIG. 2  represents a telecommunication system using a first embodiment of a method of the invention of updating the clock bias. 
         FIG. 3  represents a telecommunication system as represented in  FIG. 2  using a method of improvement by an assistance server of the acquisition of satellite data in a first embodiment of the invention. 
         FIG. 4  represents a telecommunication system using a second embodiment of a method of the invention of updating the clock bias. 
         FIG. 5  represents a telecommunication system as represented in  FIG. 4  using a method of improvement by an assistance server of the acquisition of satellite data in a second embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  has already been described in relation to the prior art. 
       FIG. 2  represents a telecommunication system  100  including an assistance server  500 . As in  FIG. 1 , a mobile device  200  including a GPS receiver, such as a mobile telephone of a GSM type telephone network  400 , is seeking to calculate its position from pseudorandom signals P 01  to P 04  transmitted by four satellites S 01  to S 04 . To this end, the device  200  sends a request, not shown, in the form of a radio signal over the telephone network  400 . That request passes through a base transceiver station (BTS) type radio base station  300  associated with the cell in which the mobile device  200  is located and is processed by the server  500 , which receives satellite information in real time via fixed radio stations  600  equipped with GPS receivers, not shown, for receiving that information. 
     The mobile device  200  calculates the pseudodistances between itself and the four satellites S 01  to S 04  from the signals P 01  to P 04  and from assistance information supplied by the server  500  enabling it in particular to determine the Doppler shift of the satellites. 
     The mobile device  200  uses an MS-assisted technology, i.e. its position is calculated by the server  500  and not by the mobile device  200  itself. 
     The pseudo-distances are therefore transmitted in the form of a radio signal RA to the assistance server  500 . Before reaching the server  500 , this signal RA passes through the base station  300  and then the GSM telephone network  400 . 
     The signal RA further includes a time-stamp DE corresponding to the time of measurement of the pseudodistances by the mobile device  20 . The GSM signal is used for this time-stamping. The pseudodistances are time-stamped by means of the structure of the GSM message, which is divided into superframes in turn divided into frames in turn divided into bit periods; a number corresponding to the time of calculation of the pseudodistances may be associated with each of these elements (hyperframe, superframe, multiframe, frame, bit). 
     The server  500  receives the signal RA after routing via the base station  300  and the network  400 . 
     The server  500  then determines the position of the mobile device and the time difference between the pseudodistance calculation time-stamp and the clocks of the GPS satellites. The server  500  thus has direct access to the clock bias ΔH=H sat −H BTS  between the clock H sat  of the satellites S 01  to S 04  and the clock H BTS  of the base station  300 . 
     This clock bias ΔH is stored in a database  700  of the server  500 . 
     In this way, in an asynchronous GSM type network, the A-GPS assistance server  500  knows the clock bias between the radio base station (BTS)  300  and the GPS satellites. The clock bias of the radio station  300  is updated each time that an assistance server calculates the position of a mobile device that is located in the cell associated with the station  300  and uses an MS-assisted technology. The updating of the database  700  each time that a mobile device using an MS-assisted technology seeks to determine its position is therefore self-supported. 
     Note, however, that it is equally possible to maintain the database  700  by means of a mobile device using an MS-based technology located in the cell associated with the station  300 . In that case, the mobile device calculates its position itself. One solution is to calculate that position by taking for the pseudodistance calculation time reference the time-stamp of the GSM signals exchanged with the base station. Thus the mobile device simultaneously calculates its position and the clock bias ΔH=H sat −H BTS  between the clock H sat  of the satellites S 01  to S 04  and the clock H BTS  of the base station  300 . The mobile device can then forward its position and the clock bias to the server in order to update the database. 
     The use of the database  700 , including the updating of the clock bias between the clock H sat  of the satellites S 01  to S 04  and the clock H BTS  of the base station  300 , will become clear in the light of  FIG. 3 , which represents a system  100  identical to that of  FIG. 2  except that a second mobile device  201  seeking to obtain its position relative to the satellites S 01  to S 04  this time receives an assistance signal AS from the assistance server  50 . 
     To illustrate the use of the database  700 , consider a mobile device  201  in the same cell as the mobile device  200  represented in  FIG. 2 . The assistance data AS passes through the network  400  and the base station  300  before reaching the mobile device  201 . 
     When the server  500  receives a request for assistance, not shown, from the mobile device  200 , that request includes information from which the base station  300  can be identified. The server  500  therefore searches the database  700  for the updated clock bias ΔH between the clock H sat  of the satellites and the clock H BTS  of the base station  300 . 
     The server  500  extracts from this clock bias ΔH the time-stamp for calculation of ΔH by the base station and the corresponding time of the satellite clock H sat . 
     This information is inserted into the assistance data signal AS, which is then transmitted to the mobile device  200  via the network  400  and the base station  300 . This information may be inserted in the following manner:
         transmitting a BTS time pulse locked to an event (hyperframe, superframe, multiframe, frame, bit), and   associating that time pulse with a GPS time.       

     The mobile device  200  then knows the base station time-stamp and the corresponding time of the satellite clock H sat . It suffices for the mobile device  200  to wait for the time-stamp for calculation of ΔH by the base station to lock onto the corresponding time of the satellite clock H sat . 
     The satellite clock H sat  enables it to synchronize its own clock and to curtail the time sweep it performs in order to acquire the pseudorandom signals P 11  to P 14  from the satellites S 01  to S 04 . 
     The mobile device  200  uses the pseudorandom signals to calculate the pseudodistances. 
     Note that the mobile device  200  can use either an MS-based technology (in which case it calculates its position directly from the pseudodistances) or an MS-assisted technology (in which case it supplies the pseudodistances to the server  500 , which calculates its position). 
       FIG. 4  represents a telecommunication system  10  including an assistance server  50 . As in  FIG. 1 , a mobile device  20  including a GPS receiver, such as a mobile telephone of a GSM type telephone network  40 , is seeking to calculate its position from pseudorandom signals P 01  to P 04  transmitted by four satellites S 01  to S 04 . To this end, the device  20  sends a request, not shown, in the form of a radio signal over the telephone network  40 . That request passes through a base transceiver station (BTS) type radio base station  30  associated with the cell in which the mobile device  20  is located and is processed by the server  50 , which receives satellite information in real time via fixed radio stations  60  equipped with GPS receivers, not shown, for receiving that information. 
     The mobile device  20  calculates the pseudodistances between itself and the four satellites S 01  to S 04  from the signals P 01  to P 04  and the assistance information supplied by the server  50 , enabling it in particular to determine the Doppler shifts of the satellites. 
     The mobile device  20  uses an MS-assisted technology, i.e. its position is calculated by the server  50  and not by the mobile device  20  itself. 
     The pseudodistances are transmitted in the form of a radio signal RA′ to the assistance server  50 . Before reaching the server  50 , this signal RA′ passes through the base station  30  and then the GSM telephone network  40 . 
     The signal RA′ also includes the time-stamp for the pseudodistance measurement determined from the clock of the mobile device  20 . 
     In addition to the pseudodistance measurement time-stamp, the signal RA′ includes a time-stamp DE′ marked by the mobile device  20  and corresponding to the time the mobile device  20  sent the signal RA′. 
     The signal RA′ is received by the base station  30  which stamps it with a time-stamp DR corresponding to the time the base station  30  sent said signal RA in the form of a radio signal. 
     The assistance server  50  receives the signal RA′ time-stamped by the time-stamps DE′ and DR after routing via the network  40 . 
     The server  50  then calculates from the pseudodistances the position of the mobile device  20  and a first clock bias ΔH 1  between the clock H mob  of the mobile device  20  and the common clock H sat  of the satellites: ΔH 1 =H mob −H sat . 
     The server  50  also calculates from the time-stamps DE and DR a second clock bias ΔH 2  between the clock H mob  of the mobile device  20  and the clock H BTS  of the base station  30 : ΔH 2 =H mob −H BTS . 
     Ignoring the transfer time of the signal RA from the mobile device  20  to the base station  30 , which is of the order of a few microseconds, the second clock bias ΔH 2  is given by the equation: ΔH 2 =DE−DR. 
     By establishing the difference between ΔH 2  and ΔH 1 , the server  50  obtains a third clock bias ΔH 3  between the clock H sat  of the satellites S 01  to S 04  and the clock H BTS  of the base station  30 : ΔH 3 =H sat −H BTS . 
     This third clock bias ΔH 3  is stored in a database  70  of the server  50 . 
     In this way, in a GSM type asynchronous network, the A-GPS assistance server  50  knows the clock bias between the radio base station (BTS)  30  and the GPS satellites from the difference between the two clock bias values. The clock bias of the radio station  30  is updated each time an assistance server calculates the position of a mobile device using an MS-assisted technology located in the cell associated with the station  30 . The updating of the database  70  each time that a mobile device using an MS-assisted technology seeks to determine its position is therefore self-supported. 
     The use of the database  70 , including the updating of the clock bias between the clock H sat  of the satellites S 01  to S 04  and the clock H BTS  of the base station  30 , will become clear in the light of  FIG. 5 , which represents a system  10  identical to that of  FIG. 4  except that a second mobile device  202  seeking to obtain its position relative to the satellites S 01  to S 04  this time receives an assistance signal AS′ from the assistance server  50 . 
     To illustrate the use of the base station  70 , consider a mobile device  202  in the same cell as the mobile device  20  represented in  FIG. 4 . The assistance data AS′ therefore passes through the network  40  and the base station  30  before reaching the mobile device  202 . 
     When the server  50  receives an assistance request, not shown, from the mobile device  202 , it searches the database  70  for the updated clock bias ΔH 3  between the satellite clock H sat  and the clock H BTS  of the base station  30 . 
     In a first mode of operation, that clock bias ΔH 3  is used to supply the GPS time to the user (MS) in the assistance data signal AS′. One option is to provide a BTS (hyperframe, superframe, multiframe, frame, bit) reference time (i.e. a pulse) associated with a GPS time. That signal AS′ is transmitted over the network  40  to the mobile device via the base station  30 . Thus, on the occurrence of the BTS reference clock pulse, the mobile device  202  obtains ΔH 3  and consequently the GPS time. 
     In a second mode of operation, the clock bias ΔH 3  is inserted into the assistance data AS′ which is transmitted over the network  40  to the base station  30 , which stamps the signal AS′ with a time-stamp DR 2  corresponding to the time the station  30  receives the signal AS′. 
     Thus the mobile device  202  obtains ΔH 3  and DR 2  and can obtain the common satellite clock H sat  by establishing the difference between the latter two values. 
     The clock H sat  enables it to synchronize its own clock and to curtail considerably the time sweep required for it to be able to acquire the pseudorandom signals P 11  to P 14  from the satellites S 01  to S 04 . 
     Those pseudorandom signals enable the mobile device  202  to calculate the required pseudodistances. 
     Note that the mobile device  202  can use either an MS-based technology (in which case it calculates its position directly from the pseudodistances) or an MS-assisted technology (in which case it supplies the pseudodistances to the server  50 , which calculates its position). 
     Of course, the invention is not limited to the embodiment that has just been described. 
     In particular, the invention has been described in the case of the Global Positioning System but it could consist in a receiver of another radio navigation satellite system, such as a GLONASS or GALILEO type receiver.