FR2794313A1 - Geographic positioning system for mobile telephone involves measurement of transmission power levels in current and adjacent cells and use of coordinates of current and adjacent cells - Google Patents

Abstract

To determine position of mobile telephone (MS) placed in cellular zone (CO) with base station (BTSO), power levels of signals transmitted from nearby base stations are determined and two cells with highest power are selected. The base stations (BTSO, BTSM1,BTSM2) coordinates are provided and calculation of mobile telephone position carried out depending on respective signal levels.

Description

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Geographical location of a mobile radiotelephone terminal
The present invention relates to the geographical location of a mobile radiotelephone terminal.

 Many applications related to moving goods or people require a geographical location of a device. As location means are mainly used a geostationary satellite guidance system, called GPS (Global Positioning System) or, for regions served, systems called terrestrial GPS such as that proposed by the MOBILOC service. There are also odometric navigation systems for locating a motor vehicle on a cartographic background, by associating map-matching techniques.

 The present invention aims to provide a geographical location of a mobile radiotelephone terminal itself displaying its own location, without using the previous location systems, and advantageously using some power measurement means included in the terminal.

 To this end, the method for geographically locating a mobile radiotelephone terminal in a cellular radiotelephone network, is characterized in that it comprises the following steps when the terminal is in a current cell: measuring in the terminal the power levels in channels issued by radio stations

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 base in neighboring cells to said current cell, - select two neighboring cells having base stations with the highest power levels, - provide to the terminal the geographic coordinates of three base stations constituted by the base station in said cell current and the base stations in said two neighboring cells selected, and - determine the geographical coordinates of the mobile terminal as a function of the measured power levels and geographical coordinates provided of the three base stations.

 Previously, it is intended to periodically broadcast an identity of a location area in the radiotelephone network including said current cell and cell identities in the location area with geographic coordinates of the base stations in those cells. The step of providing then comprises a loading of the geographic coordinates at least of said three base stations in the terminal in response to the identities of said three cells previously stored in the terminal and recognized in the identities broadcast.

 According to a first embodiment, the step of broadcasting may be a periodic broadcast by at least one transmitter means of a broadcast network substantially covering the location area. The identities of cells in the location area with geographical coordinates of the base stations in these cells are then loaded into a receiver connected to the terminal in response to the identity.

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 location area addressed by the terminal to the receiver and recognized in the periodic broadcast to load the coordinates of said three base stations in the terminal.

 According to a second embodiment, the step of broadcasting may be a periodic broadcast in a predetermined channel periodically transmitted by the base station of the current cell. The identities of cells in the location area with geographical coordinates of the base stations in these cells are then loaded into the terminal in response to the location area identity previously stored in the terminal and recognized in the periodic broadcast.

 The identity of a cell preferably includes a frequency number of a "beacon" channel assigned to the cell and an identity code of the base station in the cell.

 The determining step is based on triangulation and more specifically includes steps of determining distances between the terminal and said three base stations as a function of power levels measured for said three base stations based on a model of predetermined radio-frequency field propagation, and determining the geographical coordinates of the mobile terminal according to the three determined distances and provided geographical coordinates of said three base stations.

 In order not to relocate the terminal when the latter has not moved or moved a little, the method comprises, before the step of measuring, a step of comparing a difference between levels of

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 power measured relative to the current cell respectively at a current time and a predetermined period of the process, with a predetermined power threshold so as to perform the steps of the method set out above only when at least said difference is greater than said threshold .

 The method may comprise a step prior to validating a location of the selection terminal on a screen or by soliciting a key of the terminal in order to perform a periodic location only at the request of the user of the terminal.

 The method can be included in a monitoring cycle of location and power level measurement in the current cell performed in the terminal, at each cycle period or every Q cycle periods.

 Other features and advantages of the present invention will emerge more clearly on reading the following description of several preferred embodiments of the invention with reference to the corresponding appended drawings in which: FIG. 1 is a schematic block diagram of a radiotelephone network and an associated data broadcast network according to the invention; FIG. 2 is an algorithm of the steps of a monitoring and cell change method for a mobile radiotelephone terminal, including a method of geographical location of the terminal according to the invention; and FIG. 3 is an algorithm of the main steps of the geographic location method of the mobile radiotelephone terminal according to a preferred embodiment of the invention.

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 With reference to FIG. 1, a mobile station MS, hereinafter referred to as a mobile radiotelephone terminal, is located at a current instant t in the location area covered by a mobile service switch MSC1 in a digital cellular radiotelephone network RR according to FIG. GSM standard. The switch MSC1 is associated with a VLR1 visitor location recorder and, in many cases, is connected to two AC self - routing switches of the PSTN switched telephone network and serves several subsystems radiologically covering the location area.

Only two pairs of mobile service and recorder switches MSC1 - VLR1 and MSC2 - VLR2 and only two radio subsystems served by the switch MSC1 are shown in Figure 1.

 The location recorder VLR1 contains for each visitor subscriber whose mobile terminal is present in the coverage area at current instant t, in particular data communicated by a home location register (HLR, not shown) connected to the recorders VLR1, VLR2 through the signaling network of the RR radiotelephone network. The data in the VLR recorder includes an International Mobile Subscriber Identity (IMSI) and Subscriber Subscription Profile. The switch MSC1 manages the establishment and termination of communications with the visiting terminals and the mobility of the terminals during a transfer from one cell to another cell in the coverage area, or to a cell in another zone. cover, in collaboration with the VLR1 recorder.

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 Each radio subsystem comprises a base station controller BSC1, BSC2 which controls handovers and regulates the radio resource of several base stations of which four BTS0s BTS1, BTS4 and BTSN-1 or two BTS2s and BTS3s are shown in the figure. 1, covering radio cells included in the location area. At each base station are allocated predetermined carrier frequencies each designated by an Absolute Radio Frequency Channel Number (ARFCN) and different carrier frequencies allocated to neighboring base stations. A carrier supports multiple time slots (slots or bursts) dynamically allocated for the most part to communications with mobile terminals.

 In practice for a communication, the ARFCN number is that of the carrier of a downlink from the base station to the terminal and the frequency of a corresponding uplink is deduced from the number ARFCN and the constant difference between frequencies of rising and falling tracks.

 Each base station transmits on a predetermined carrier frequency a "beacon channel" so that a mobile terminal in the vicinity of this base station periodically measures the power level of the modulated signal transmitted at constant power that it receives. in this beacon channel.

 For example, the mobile terminal periodically polls the beacon channels of N = 6 base stations to measure N = 6 power levels, including that of the base station BTS0 to which it is temporarily attached, so that it selects the station base associated with the highest measured power level and settles on the beacon channel

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 of the selected base station. These periodic power measurements are carried out in the mobile terminal MS as soon as it is powered up, in idle state and during communication in order to monitor the beacon channels of the base station to which the terminal is attached and N-1 = 5 other neighboring base stations at the home base station. The mobile terminal can thus change cell (handover) when the power level measured in the beacon channel of the home base station is less than a predetermined threshold.

 According to the GSM standard, the beacon channel consists of three channels, one channel for FCCH (Frequency Correction CHannel), one channel for time synchronization SCH (Synchronization CHannel) and one channel for broadcasting data specific to the sub-channel. BCCH (Broadcast Control CHannel) radio system.

 In particular, the BCCH channel of the base station of a given cell contains data such as the minimum level of reception and the maximum power at which the mobile terminal must transmit, the number of the location area LAI (Location Area Identification) which contains the location area code LAC (Location Area Code) determined by the operator of the radiotelephone network RR, the identity of the Cell Identity (CI) cell, that is to say the identity of the corresponding base station within the location area, and the list of beacon channel frequencies of N = 5 neighboring cells to the given cell. The SCH channel for the given cell provides the mobile terminals with all the data necessary for synchronization and also an identity code, called "color code" BSIC (Base Station Identity).

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 Code), which distinguishes two base stations transmitting beacon channels on the same frequency and which can be received in a mobile terminal MS located substantially midway between two base stations. Thus, the pair composed by the number of an ARFCN frequency allocated to the given base station and the BSIC color code unambiguously determines, in a one-to-one manner, the given base station in the radiotelephone network RR.

 As for the FCCH channel, the mobile terminal MS periodically measures the effective radiated power (ERP) of the pure sinusoidal signals at the carrier frequencies emitted by the N = 6 neighboring cells in order to transmit the result of these measurements in a channel. SACCH (Slow Associated Control CHannel) amount control to the corresponding radio subsystem which decides whether a handover is to be carried out in particular by comparing the measured powers with each other and at predetermined thresholds.

 FIG. 2 shows an example of a combination of the geographical location LG of the mobile radiotelephone terminal MS according to the invention and of a simplified cell selection algorithm essentially comprising six steps E1 to E6.

 In step El, the mobile terminal MS is assumed to be set to the carrier frequency, hereinafter referred to as Fo instead of ARFCN, of the current cell Co relative to a current base station BTSo. This carrier frequency Fo is detected in the FCCHo channel of the current cell. The terminal MS has also captured in the broadcast channel BCCHo of the current cell Co and stored the list of (N-1) carriers.

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 (F1, BSIC1) to (FN ~ 1, BSICN-1) defining the "tag" channels of the neighboring cells provided by the home controller MSC1. The mobile terminal is thus considered at a current time t already in standby state or in the course of establishment or communication flow.

 However, as indicated in an initial previous step EO in FIG. 2, during the putting into service of the mobile terminal MS, the terminal has monitored the BCCH channels relating to all the carriers of the RR radiotelephone network, or to stored carriers. in the terminal MS during the last power up thereof, to search for cells for which the received powers are greater than a threshold, and among them select a cell, said current cell, meeting certain power criteria and of quality, in a similar manner to steps E2, E3 and E5 presented hereinafter.

 In the following steps E2 and E3, the mobile terminal MS checks whether it is still located in the current cell. This check is periodic, for example every T = 5 seconds, t denoting in the following the current time of verification. In step E2, the mobile terminal measures the received power level RXLEV (F0, t) in the downlink channel FCCH of the current cell and compares it to a predetermined threshold RXLEVm. If the measured received level is higher than said threshold, the step E3 indicates the transmission power TXPWR from the mobile terminal required for the current base station BTS0 to receive with good quality the transmitted signals in the upstream direction, whatever the position of the MS terminal in the current cell Co. The power TXPWR is compared to a maximum power

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 predetermined PM with which the terminal can transmit. If the transmission power TXPWR is lower than the maximum transmission power PM, the mobile terminal is still in the current cell Co, and a power control in the mobile terminal by the home base station controller BSC1 can be performed in step E4 according to the previous measurements until the next period T.

 When RXLEV (Fo, t) # RXLEVm in step E2, or when TXPWR> PM in step E3, step E5 measures the N-1 = 5 power levels received in the mobile terminal MS relative to the N- 1 = 5 neighboring cells and selects from these cells the cell in which the mobile terminal receives the highest power level [RXLEV (Fn, t)].

 In the next step E6, the current cell Co becomes the cell covered by the current base station BTS0 corresponding to the highest power level. A handover is managed by the network, either at the home switch MSC if the selected cell belongs to the current location area, or between said switch and the switch on which the selected cell depends. The VLR1 recorder updates the IC0 identity of the cell containing the mobile terminal.

 After the transfer to step E6, the mobile terminal records the particular frequency characteristics of the N-1 = 5 cells neighboring the selected cell, detected in the BCCH broadcast channel of this selected cell.

 As shown in FIG. 2, the invention introduces a geographical location LG into the

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 cycle T of steps E2 to E4, between step E3 and step E2, when confirming the membership of the mobile terminal MS to the current cell Co and the possible power control in step E4. The LG location of the mobile terminal mainly comprises steps LGO to LG7 illustrated in FIG.

 The location of the mobile terminal MS is performed when, in the first step LGO, the user of the terminal validates a LOCATION field in a menu displayed on the screen of the terminal, or by soliciting a predetermined key from the keyboard of the terminal. Otherwise the algorithm returns to step E2. As long as the LOCATION remains validated, the steps LG1 to LG7 are cyclically run at period T.

 However, alternatively, in order not to overload the tasks of the terminal due to the location, the location steps LG1 to LG7 are performed only every Q cycles of duration T, ie after t = t1 - QT as indicated at a step optional LGOO in Figure 2. t is the current time, t1 is the time of the last location performed, and Q is an integer for example equal to 12 or 36 so that the location of the terminal is refreshed every minute or all three minutes.

 In step LG1, the mobile radio terminal MS checks whether it has moved since the last previous period t-T (or alternatively t1 QT). This check is based on the comparison of at least one difference in power levels received at the current time t and in the previous period t-T at a predetermined power threshold.

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 SPm. The difference in levels is relative to the current cell Co, and possibly a difference in levels relative to one of the two cells CM1 and CM2 defined below is also compared with the threshold SPm.

 If the difference or if at least one of the two preceding differences is greater than the power threshold SPm, the terminal MS has moved. In the next step LG2, the terminal measures and stores the power levels RXLEV (F1, t) to RXLEV (FN-1, t) of the carriers in the N-1 channels FCCH1 to FCCHN-1 of the neighboring cells C1 to CN -1./ This step LG2 as well as the following LG3 can intervene before the step LG1 in order to compare with the threshold SPm two power level differences relative to two of the three selected cells as below.

 In step LG3, the terminal MS compares the N-1 power levels measured in pairs in order to select the two highest power levels such as RXLEV (FM1, t) # RXLEV (FM2, t)> [RXLEV (Fi, t) with i E [1, N-1] and i # Ml and i # M2.

 The next step LG4 reads the frequency characteristics (F0, BSICo), (FM1, BSICM1) and (FM2, BSICM2), already stored in the terminal MS at step E1, the current cell Co and the cells CM1 and CM2. corresponding to the two highest power levels selected, which identifies these three cells to deduce their locations and to determine the location of the mobile terminal in the following steps.

 The following step LG5 deduces the location coordinates in longitude and latitude (Xo, Yo), (XM1, YM1) and (XM2, YM2) of the three base stations BTSo, BTSM1 and BTSM2 in the cells Co, CM1 and CM2. by

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 correspondence with frequency characteristics (Fo, BSICo), (FM1, BSICM1) and (FM2, BSICM2).

 According to a first embodiment, the RR radiotelephone network is associated with an RD data broadcasting network, for example of the RDS (Radio Data System), SWIFT, or DAB (Digital Audio Broadcasting) type, or even DVB (Digital Video Broadcasting) type. The mobile radiotelephone terminal MS is then connected to a radio frequency receiver REC for such a network. The receiver is for example a digital broadcast receiver installed in a vehicle, as shown in Figure 1, comprising a keyboard and a screen, and able to receive audio messages, images and texts displayed on the screen, relating to road, meteorological or tourist information, in addition to the selective reception of radio programs.

 The broadcasting network RD is, according to the invention, organized substantially like the radiotelephone network RR, that is to say comprises transmitters dispersed over a given territory to cover location zones that are substantially identical to those covered by the switches of the service. mobile MSC. In FIG. 1, two transmitters EM1 and EM2 of the broadcast network RD are represented as associated respectively with the switches MSC1 and MSC2, although alternatively, a switch can be associated with several transmitters, or a transmitter can be associated with several MSC switches, in particular depending on the relief and local locations of transmitters and base stations.

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 According to the invention, each transmitter serving a location area contains a database of base stations and periodically broadcasts in a broadcast channel provided for this purpose, for example every 1 to 3 minutes, in the fast information channel. FIC (Fast Information Channel) of a DAB network, the identity of the cells, that is to say the pairs (F = ARFCN, BSIC) of the base stations belonging to the location area, with their geographical coordinates ( X, Y); the transmitter EM1 broadcasts a list of quadruplets (F = ARFCN, BSIC, X, Y) relating to the base stations of the localization area, such as the base stations BTS0, BTS1, BTS4 and BTSN-1, according to the figure 1. This list is preceded by the identity of the LAI location area that contains the MCC country code, the RRM operator's PLMN and the LAC location code.

 In this first embodiment, prior to step E0, the radiotelephone terminal MS is connected to the digital radio receiver REC by their own peripheral interfaces, and the cell identity broadcast channel is selected periodically and automatically, for example, following the soliciting a specific key or validating a selected field in a menu displayed in the REC receiver. Step LG5 loads and updates the list of base station quadruplets broadcast in a memory of REC receiver, and particularly the coordinates (Xo, Yo), (XM1, YM1) and (XM2, YM2) of the three stations of base BTS0, BTSM1 and BTSM2 in a memory of the terminal MS, in response to the identity of the current location area LAI detected in the broadcast message compared to that provided by the terminal MS which has already

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 memorized during the previous consultation of the BCCHo channel of the current cell Co.

 According to this first embodiment, the cycle of steps LG1 to LG6 is either nested in steps E2 to E4, or is completely independent of them, whether the terminal MS is in a standby state or during communication, or simply in operation and connected to the REC receiver. The radiotelephone terminal MS can be incorporated into the receiver REC.

 In the next step LG6, the mobile terminal MS determines its geographical coordinates (XMS, YMS) by triangulation.

déterminer la distance 11 MS, BTSoII, 1 1 MS, BTSM1 , MS, BTSM211 entre le terminal MS et chacune des trois stations de base en fonction du niveau de mesure de puissance RXLEV(Fo, BSICo, t) , RXLEV(FM1, BSICM1, t) , RXLEV(FM2, BSICM2, t) de cette station (étapes E2, LG3) sur la base d'un modèle de propagation de champ radioélectrique prédéterminé, par exemple le modèle de OKUMARA HATA, puis à déduire les coordonnées géographiques (XMS, YMS) du terminal mobile en

fonction des trois distances Il MS, BTS0| I , IIms, BTSMll1 et #MS, BTSM2# et des coordonnées géographiques diffusées et mémorisées (Xo, Yo), (XM1, YM1) et (XM2, YM2) des trois stations de base BTS0, BTSM1 et BTSM2'
En variante, les altitudes Zo, ZM1 et ZM2 des antennes des trois stations de base BTSo, BTSM1 et BTSM2 sont également diffusées et chargées avec les longitudes et lattitudes de ces trois stations de base à l'étape LG5. L'altitude du terminal mobile MS lui est transmise par un altimètre incorporé ou relié au récepteur REC pour déterminer plus précisément les trois distances précitées et les deux coordonnées Stage LG6 consists essentially of

determine the distance 11 MS, BTSoII, 1 1 MS, BTSM1, MS, BTSM211 between the terminal MS and each of the three base stations as a function of the power measurement level RXLEV (Fo, BSICo, t), RXLEV (FM1, BSICM1 , t), RXLEV (FM2, BSICM2, t) of this station (steps E2, LG3) on the basis of a predetermined radio-frequency field propagation model, for example the OKUMARA HATA model, and then to deduce the geographical coordinates ( XMS, YMS) of the mobile terminal in

function of the three distances Il MS, BTS0 | I, IIms, BTSMll1 and #MS, BTSM2 # and geographical coordinates broadcast and stored (Xo, Yo), (XM1, YM1) and (XM2, YM2) of the three base stations BTS0, BTSM1 and BTSM2 '
Alternatively, the altitudes Zo, ZM1 and ZM2 of the antennas of the three base stations BTSo, BTSM1 and BTSM2 are also broadcast and loaded with the longitudes and latitudes of these three base stations at step LG5. The altitude of the mobile terminal MS is transmitted to it by an altimeter incorporated or connected to the receiver REC to determine more precisely the three distances mentioned above and the two coordinates

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 (XMS, YMS) of the mobile terminal at step LG6.

 The last step LG7 displays the geographical coordinates of the mobile (XMS, YMS) on the screen of the terminal MS when the user of it has requested them by selecting a particular field in a menu. Optionally, these XMS and YMS coordinates are transmitted to a separate navigation aid device or integrated into the REC receiver, or implemented as a specific application in a portable microcomputer to display the location of the mobile terminal on a portion of the road map displayed on the receiver screen.

Then the algorithm goes from the display step LG7 to step E2.

 If at step LG1, the mobile radiotelephone terminal MS finds that it has not moved or moved a little since the last previous period tT, or alternatively (t1 - QT), that is to say if [ RXLEV (Fo, t) - RXLEV (F0, tT)] # SPm, or possibly [RXLEV (FM1, t) - RXLEV (FM1, tT)] # SPm, the location of the terminal MS is that determined during a LG6 previous step and the algorithm goes from step LG1 to the LG7 display step, or directly to step E2.

 According to a second embodiment of the invention relative to the step LG5, the radiotelephone network RR itself broadcasts the geographical coordinates (Xo, Yo), (XM1, YM1) and (XM2, YM2) at least base stations. BTSo, BTSM1 and BTSM2, or alternatively the three geographical coordinates (Xo, Yo, Zo), (XM1, YM1, ZM1) and (XM2, YM2, ZM2) at least of the three aforementioned base stations. More specifically, the identity of the LAI location area with the

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 list of quadruplets (F = ARFCN, BSIC, X, Y) relating to the base stations of the location area is transmitted in a specific downlink channel by each base station included in the location area. The specific downlink channel is preferably the CBCH (Cell Broadcast Channel) channel multiplexed in the Common Control Channel (CCCH) common control channel broadcast by the base station and which can be used to address short messages, particularly relating to meteorology or for example, at all the terminals in the cell covered by the base station.

 Software according to the algorithms of the invention shown in FIGS. 2 and 3 is implemented in the ROM containing the operating system of the SIM (Subscriber Identity Module) microprocessor card of the mobile radiotelephone terminal MS.

 The geographical location indicated by the mobile terminal can also be used to record the geographical position of a site, for example where a mobile measuring device is placed to measure the radio-frequency field emitted by a radio or television broadcasting station, or by a base station.

Claims (9)

A method for geographically locating a mobile radiotelephone terminal (MS) in a cellular radiotelephone (RR) network, characterized in that it comprises the following steps when the terminal is in a current cell (Co): measuring (E2, LG2) in the terminal levels

 of power (RXLEV (Fo, t) to RXLEV (FN-1 't)) in channels transmitted by base stations in neighboring cells to said current cell (Co), - selecting (LG3) two neighboring cells having base stations (BTSM1, BTSM2) with the highest power levels, - providing (LG5) to the terminal (MS) the geographical coordinates of three base stations constituted by the base station (BTSo) in said current cell (Co ) and the base stations (BTSM1, BTSM2) in said two selected neighboring cells, and - determining (LG6) the geographical coordinates (XMS, YMS) of the mobile terminal as a function of the measured power levels (RXLEV (Fo, BSICo, t ), RXLEV (FM1 'BSICM1, t), RXLEV (FM2, BSICM2, t)) and provided geographic coordinates ((Xo, Yo), (XM1, YM1), (XM2, YM2)) of the three base stations ( BTSo, BTSM1 'BTSM2) - 2- A method according to claim 1, comprising previously a step of periodically diffusing (LG5) an identity (LAI) of a location area in the radiotelephone network (RR) including said current cell (Co) and identities (F , BSIC) of cells in the location area with geographic coordinates (X, Y) of the base stations (BTS) in these cells, and <Desc / Clms Page number 19>  the providing step (LGS) comprises a loading of the geographic coordinates at least of said three base stations (BTSo, BTSM1, BTSM2) in the terminal in response to the identities of said three cells previously stored in the terminal and recognized in the broadcast identities.  3 - Process according to claim 2, wherein the step of broadcasting is a periodic broadcast by at least one transmitter means (EM1) of a broadcast network (RD) substantially covering the location area, the identities (F, BSIC) of cells in the location area with geographic coordinates (X, Y) of the base stations (BTS) in these cells being loaded into a receiver (REC) connected to the terminal (MS) in response to the area identity location (LAI) addressed by the terminal (MS) to the receiver (REC) and recognized in the periodic broadcast.  4 - Process according to claim 2, wherein the step of broadcasting comprises a periodic broadcast in a predetermined channel issued periodically by the base station (BTSo) of the current cell (Co), and the identities (F, BSIC) of cells in the location area with geographic coordinates (X, Y) of the base stations (BTS) in these cells being loaded into the terminal (MS) in response to the location area identity (LAI) previously stored in the terminal and recognized in the periodic broadcast.  5 - Process according to any one of claims 2 to 4, wherein the identity of a cell comprises a number of the frequency (F = <Desc / Clms Page number 20>  ARFCN) of a "beacon" channel assigned to the cell and an identity code (BSIC) of the base station (BTS) in the cell.  6 - Process according to any one of claims 1 to 5, wherein the step of determining (LG6) comprises steps of: - determining distances (#MS, BTS0 #, #MS,

 BTSM1, 11 MS, BTSM2) between the terminal (MS) and said three base stations (BTSo, BTSM1, BTSM2) as a function of the measured power levels (RXLEV (Fo, t), RXLEV (FM1 't), RXLEV ( FM2, t)) for said three base stations on the basis of a predetermined radio-frequency field propagation pattern, and - determining the geographical coordinates (XMS, YMS) of the mobile terminal according to the three determined distances and geographical coordinates provided ((X0, Yo), (XM1, YM1) (XM2, YM2)) of said three base stations (BTSo, BTSM1, BTSM2).  7- Method according to any one of claims 1 to 6, comprising before the step of measuring (LG2), a step of comparing (LG1) a difference between power levels (RXLEV (F0, t) - RXLEV ( Fo, tT)) measured relative to the current cell (Co) respectively at a current time (t) and at a predetermined period (T; QT) of the method, with a predetermined power threshold (SPm) in order to perform the the steps set forth in claim 1 when at least said difference is greater than said threshold. <Desc / Clms Page number 21>  8- Method according to any one of claims 1 to 7, comprising previously a step for validating a location (LGO) of the terminal by selection on a screen or by soliciting a key of the terminal (MS).  9- Method according to any one of claims 1 to 8, included in a monitoring cycle of location and power level measurement (E2, E3, E4) in the current cell (Co) realized in the terminal, to each period (T) of the cycle or all the Q periods of the cycle.