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`US 20050159174Al
`
`(19) United States
`(12) Patent Application Publication
`Bar-On et al.
`
`(10) Pub. No.: US 2005/0159174 Al
`Jul. 21, 2005
`(43) Pub. Date:
`
`(54) MOBILE COMMUNICATION STATIONS
`METHODS AND SYSTEMS
`
`(76)
`
`Inventors: David Bar-On, Rehovot (IL); Mark
`Shahaf, Ashdod (IL); Salomon Serfaty,
`Doar Gaash (IL); Baruh Hason, Tel
`Aviv (IL)
`
`Correspondence Address:
`MOTOROLA, INC.
`1303 EAST ALGONQUIN ROAD
`IL01/3RD
`SCHAUMBURG, IL 60196
`
`(21) Appl. No.:
`
`10/508,297
`
`(22) PCT Filed:
`
`Mar. 3, 2003
`
`(86) PCT No.:
`
`PCT/EP03/02221
`
`(30)
`
`Foreign Application Priority Data
`
`Mar. 28, 2002
`
`(GB) ......................................... 0207344.3
`
`Publication Classification
`
`Int. Cl.7 ....................................................... H04Q 7/20
`(51)
`(52) U.S. Cl. ....................... 455/456.6; 455/457; 342/450
`
`(57)
`
`ABSTRACT
`
`A method for use in a mobile communication system for
`estimating the location of a target mobile station (MS) which
`includes receiving signals from the target MS (N) at a
`plurality of receivers (MSl, MS2, MS3) in different loca(cid:173)
`tions and calculating the location of the target MS (N) from
`information obtained from the received signals, wherein at
`least one of the receivers (MSl) is a searching MS and
`wherein the calculation of location is carried out by the
`searching MS (MSl). The method may involve the target
`MS and the searching MS or MSs communicating by a direct
`mode of operation (DMO) link. The searching MSs may
`conveniently have GPS attachments. A mobile station which
`may be used as the target MS or as a searching MS and a
`system incorporating the same is also disclosed.
`
`N
`
`MS
`
`2
`
`MS
`
`2a GPS
`
`1
`
`MS
`
`Ja GPS
`
`1a GPS
`
`Page 1 of 11
`
`SAMSUNG EX-1053
`
`

`

`Patent Application Publication Jul. 21, 2005 Sheet 1 of 3
`
`US 2005/0159174 Al
`
`2
`
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`
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`
`Page 2 of 11
`
`

`

`Patent Application Publication Jul. 21, 2005 Sheet 2 of 3
`
`US 2005/0159174 Al
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`Page 3 of 11
`
`

`

`Patent Application Publication Jul. 21, 2005 Sheet 3 of 3
`
`US 2005/0159174 Al
`
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`Page 4 of 11
`
`

`

`US 2005/0159174 Al
`
`Jul. 21, 2005
`
`1
`
`MOBILE COMMUNICATION STATIONS
`METHODS AND SYSTEMS
`
`FIELD OF THE INVENTION
`
`[0001] The present invention relates to mobile communi(cid:173)
`cation stations, methods and systems. In particular it relates
`to determination of the location of mobile stations.
`
`BACKGROUND OF THE INVENTION
`
`[0002] Mobile
`for
`systems,
`communications
`radio
`example cellular telephony or private mobile radio commu(cid:173)
`nications systems, typically provide for radio telecommuni(cid:173)
`cation links to be arranged between a plurality of subscriber
`units, often referred to in the art as mobile stations (MSs ).
`The term mobile station (MS) generally includes both hand(cid:173)
`portable and vehicular mounted radio communication units,
`radiotelephones and the like.
`
`[0003] Mobile radio communications systems are distin(cid:173)
`guished from fixed communications systems, such as the
`public switched telephone networks (PSTN), principally in
`that mobile stations can move in geographical location to
`accompany their user and in doing so encounter varying
`radio propagation environments.
`
`[0004] Mobile radio communications systems and mobile
`stations used in them may operate in one of two main modes,
`namely a trunked mode of operation (TMO) and a direct
`mode of operation (DMO). TMO communications use the
`infrastructure supplied by the system operator, especially
`base transceiver stations (BTSs), to deliver communications
`to and from the MS of each user or subscriber serviced by
`the system. Resources available in TMO are shared between
`the many MSs using the system. Systems operating in TMO
`are often referred to as cellular because a multiplicity of
`BTSs provide service to MSs in a network of overlapping
`regions known as cells. In contrast, DMO is a method that
`provides the capability of direct communication between
`two or more MSs without use of any associated operator's
`infrastructure. Some MSs may be dual mode operating using
`either TMO or DMO.
`
`[0005] Methods are known for the location of a MS
`operating in a TMO cellular communication system to be
`determined. Such methods include a so-called TDOA or
`time difference of arrival method and a GPS (global posi(cid:173)
`tioning system) based method. The TDOA method involves
`sending signals from a given MS to at least three BTSs the
`locations of which are known precisely or receiving in a MS
`signals from at least three such BTSs. In the case where the
`signal from the MS is received by the three BTSs, using
`accurate time reference (GPS usually), and measurement of
`the transit delay for the signal to reach each BTS of the
`signal from the specific MS, the system can calculate the
`location of the MS.
`
`[0006]
`In emergency situations, it should be possible to
`search for lost people who are accompanied by an active MS
`by estimating the location of the MS. Examples of situations
`where such searches are likely to be necessary include those
`needed where people become lost at sea or in a forest, or on
`mountain, or where they have become partially or totally
`buried following an explosion, avalanche etc. Unfortunately,
`the typical accuracy of the known location estimation meth(cid:173)
`ods which have been implemented in practice is only about
`
`several hundred metres and such method rely on the exist(cid:173)
`ence of a BTS infrastructure in the area where the search is
`to be made. In consequence, these known methods are of
`limited help where location precision much better than 100
`metres is needed.
`
`SUMMARY OF THE PRESENT INVENTION
`
`[0007] According to the present invention in a first aspect
`there is provided a method for use in a mobile communi(cid:173)
`cation system for estimating the location of a target mobile
`station which includes receiving signals from the target
`mobile station at a plurality of receivers in different locations
`and calculating the location of the target mobile station from
`information obtained from the received signals, wherein at
`least one of the receivers comprises a searching mobile
`station and wherein the calculation of location is carried out
`by the searching mobile station using a method based upon
`time or time difference of arrival of signals travelling
`between the target mobile station and the plurality of receiv(cid:173)
`ers.
`
`[0008] The method according to the invention desirably
`uses at least three receivers, including the said searching
`mobile station (MS), to receive signals from the target
`mobile station (MS). These receivers, additional to the
`searching MS, may be MSs which are operable to commu(cid:173)
`nicate with the said searching MS by direct mode of opera(cid:173)
`tion (DMO) links. Alternatively, one or two of these addi(cid:173)
`tional receivers could be a receiver of fixed location, e.g. in
`a base transceiver station (BTS). However, communications
`involving a BTS, i.e. by TMO, are inherently more complex
`than those which can be established between MSs by DMO,
`e.g. because in TMO different channels are employed for
`uplink and downlink communications and signals sent via a
`BTS generally have to travel longer distances which will
`introduce inaccuracies into the location determination pro(cid:173)
`cedure to be described. Thus, it is desired that at least one,
`preferably two, of these receivers, additional to the said
`searching MS, are also MSs which are operable to commu(cid:173)
`nicate with that MS by direct mode of operation (DMO)
`links. Where these three receivers are MSs, they are referred
`to herein as first, second and third searching MSs.
`
`[0009] Similarly, it is possible for the radio signals from
`the target MS to be sent to the first searching MS by a TMO
`radio link. However, in this case, the complexity problem
`mentioned above again arises. Thus the radio signals from
`the target MS to the first searching MS are desirably sent by
`a DMO link.
`
`[0010] Similarly, the signals sent by the target MS to one
`or both of the receivers other than the first searching MS, e.g.
`the second and third searching MSs, may involve a TMO
`link, even if the said receivers themselves are MSs, but
`desirably these signals are sent by a DMO link.
`
`[0011] The receivers other than the first searching MS
`need to be connected to transmitters, e.g. in transceiver units
`comprising the second and third searching MSs, to commu(cid:173)
`nicate with the first searching MS in order to send to the first
`searching MS signals providing information relating to
`finding the location of the target MS.
`
`[0012] The time or time difference of arrival method of
`determining location is known per se and is described for
`example in GB2368240B. The benefit of selecting such
`
`Page 5 of 11
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`

`

`US 2005/0159174 Al
`
`Jul. 21, 2005
`
`2
`
`method for use as part of the method according to the
`invention is that the target mobile station does not need to
`have accurate time synchronisation with the receivers
`including the searching mobile station(s) although these
`receivers desirably have accurate time synchronisation
`themselves, e.g. by synchronisation with an external syn(cid:173)
`chronisation clock such as the clock of the GPS (Global
`Positioning System) by including GPS receivers in associa(cid:173)
`tion with the receivers.
`
`[0013]
`In the method according to the invention, the first,
`second and third searching MSs may be operable in a search
`mode whereby these MSs co-operate to apply a procedure to
`determine the location of the target MS in one of the ways
`described herein. Such a mode may be initiated in each MS
`by a user applying a suitable control signal to the MS, e.g.
`via a control button or a soft or hard key of a keypad or a
`voice operated command. A GPS attachment may be used in
`conjunction with one or more of these MSs to assist the
`location and synchronisation procedure. One of these MSs,
`selected either by decision making logic functions of the
`MSs themselves or manually by the user of one of the MSs,
`may be designated as the lead MS. The lead MS may issue
`a signal indicating that the search procedure is to begin and
`may later in the procedure carry out calculations to estimate
`the location of the searched for target MS. In the method as
`specified earlier the first searching MS is designated as the
`lead MS. The signal issued by the first searching MS as lead
`MS to initiate the search procedure, e.g. as a polling or
`interrogation signal, may be a broadcast signal. All MSs
`within range which are active (switched on) and operate
`according to the same communication protocol as used in
`the signalling sent by the first searching MS may recognise
`the signal and respond by sending a return signal accord(cid:173)
`ingly. The second and third searching MSs may be pro(cid:173)
`grammed to be prevented from responding when they are set
`to be in the search mode. The lead MS, namely the first
`searching MS, may be operable to select only one respond(cid:173)
`ing MS (at a time) e.g. the responding MS sending the
`strongest signal in the correct protocol, as the MS to be
`searched for. This may be achieved by the first searching MS
`sending a signal to the unselected MSs to prevent further
`response signals in a given period from those MSs. In this
`method, as described earlier, the searched for MS is the said
`target MS.
`
`[0014]
`In the method according to the invention, the target
`MS is desirably operable to send to the first searching MS
`and at the same time to the other receivers one or more
`signals recognised by the first searching MS and the other
`receivers as signals which can be employed in a location
`estimation procedure. This signal or signalling by the target
`MS may be in response to the signal issued by the first
`searching MS as lead MS in the search procedure of the first,
`second and third MSs. The signal may however be generated
`automatically and periodically by the target MS.
`
`[0015] Alternatively, when possible, the signal(s) sent by
`the target MS may be initiated by a user of the target MS
`applying a control function instruction thereto, e.g. via a
`control button or a soft or hard key of a keypad or a voice
`activated control.
`
`[0016] The signal sent by the target MS is employed in the
`method according to the invention to provide measurement
`of the relative distances of the first searching MS and the
`
`other receivers from the target MS. Such distances are
`measured by a known time of arrival or time difference of
`arrival method, e.g. in the manner described later. The
`instant in time when the signal or a recognised component
`part thereof, e.g. the start of a particular time slot, is sent by
`the target MS, is desirably labelled by a code in the signal.
`Such a code may conveniently be recognised by the receiv(cid:173)
`ers including the first searching MS whereby the time of
`receipt of the same signal by the receivers including the first
`searching MS can be accurately recorded. The target MS
`may issue two or more signals each having a different
`labelled code to assist the location procedure.
`[0017] According to the present invention in a second
`aspect there is provided a mobile station which is operable
`as the target MS or as the first, second or third searching MS
`in the method according to the first aspect.
`[0018]
`In a particular form of the mobile station according
`to the second aspect, wherein the mobile station is to be used
`as the first searching MS, the MS has an electro-optical
`display and is operable to show on the electro-optical
`display graphical information of the estimated position
`co-ordinates of the target MS as calculated in the said
`method by the first searching MS, optionally together with
`graphical information of the estimated position co-ordinates
`of one or more of the searching MSs.
`[0019] Thus the present invention provides a new method
`and a mobile station for use therein which allows determi(cid:173)
`nation of the location of a target mobile station accompa(cid:173)
`nying a lost person to be made more accurately than in the
`prior art and beneficially allows searching in areas in which
`no nearby BTS infrastructure is present. The invention is
`particularly suitable for use in a communication system and
`method operating according to TETRA(TErrestrial Trunked
`Radio) standards are known. The TETRA standards are
`operating protocols which have been defined by the Euro(cid:173)
`pean Telecommunications Standards Institute (ETSI).
`[0020] Embodiments of the present invention will now be
`described by way of example with reference to the accom(cid:173)
`panying drawings in which:
`
`BRIEF DESCRIPTION OF IBE
`ACCOMPANYING DRAWINGS
`[0021] FIG. 1 is a schematic illustration of an arrange(cid:173)
`ment of three mobile radio stations being used to search for
`a further a target mobile radio station in a method embody(cid:173)
`ing the invention.
`[0022] FIGS. 2a and 2b are illustrations respectively of
`good and bad arrangements of searching mobile stations
`being used in the method illustrated in FIG. 1.
`[0023] FIGS. 3a and 3b are graphical hyperbolic plots of
`possible position co-ordinates of mobile stations obtained
`from a first measurement iteration of measured distances
`between pairs of the mobile stations taking part in the search
`method illustrated in FIG. 1.
`[0024] FIGS. 4a and 4b are plots similar to those shown
`in FIGS. 3a and 3b for a later measurement iteration of the
`measured distances.
`
`DESCRIPTION OF EMBODIMENTS OF THE
`INVENTION
`[0025] As illustrated in FIG. 1, a search is taking place for
`a lost person having a MS (mobile station) N (e.g. a mobile
`
`Page 6 of 11
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`

`US 2005/0159174 Al
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`Jul. 21, 2005
`
`3
`
`telephone). Using the mechanisms and procedures described
`below, MSs (mobile stations) 1, 2 and 3 (e.g. mobile
`telephones) being carried by helpers searching for the user
`of MS N are used to detect the location of MS N to find the
`missing person. For the purpose of the location detection,
`each of MSs 1, 2 and 3 is equipped with a GPS attachment
`is connected to the MS providing the MS with an accurate
`measurement of its own position co-ordinates and with an
`accurate time reference. These GPS attachments are receiv(cid:173)
`ers labelled GPSs la, 2a and 3a respectively in FIG. 1. Such
`attachments are known per se in the mobile telephony field.
`The GPS attachments do not have to be permanently con(cid:173)
`nected to the mobile stations. They may be employed as an
`add-on accessory that is used only in emergency cases such
`as when the mobile stations are being employed in a search
`operation.
`[0026] There can be more than three mobile stations used
`in the search procedure, to allow more accurate measure(cid:173)
`ments. However, three mobile stations are enough to put the
`invention into practice. Throughout this embodiment of the
`invention, it will be assumed that DMO signalling is used
`according to the TETRA (Terrestrial Trunked Radio) stan(cid:173)
`dards. This is a set of operational standards for modern
`digital RF communications systems specified by the ETSI
`(European Telecommunications Standards Institute). It
`should be noted, however, that the invention is applicable for
`use in other systems for direct radio-to-radio interfaces
`between the mobile stations.
`[0027] A procedure for searching for MS N involves the
`following steps:
`[0028] A: Initiating Signalling from MS N
`[0029]
`It is necessary for MS N to issue radio signalling by
`which MSs 1, 2 and 3 can determine its location. Although
`it is possible that the person holding MS N could operate MS
`N to cause it to initiate a signal, e.g. by operating a button,
`key or voice controlled device to carry out a suitable control
`function, it is more likely that this will not be possible in
`most cases, e.g. because the person being searched for is
`unconscious. It is also assumed that MS N is not able to
`provide continuous transmission of the appropriate signal(cid:173)
`ling because of the energy drain on the battery this would
`require in currently available terminals. Therefore, a polling
`mechanism provided by the searching mobile stations MS 1,
`2 and 3 will normally be required. A polling signal needs to
`be generated and sent. This is carried out by MS 1 whose
`user is leading the search operation. The purpose is to cause
`MS N which is assumed to be switched on but in a standby
`state to recognise the content of the polling signal in its
`processing and memory functions. In response, MS N will
`automatically generate and transmit the required location
`assisting signalling. The polling signal sent by terminal MS
`1 can be directed to a specific mobile station (in this case it
`is known that MS N is missing) or sent by broadcast to any
`mobile stations in the area which can recognise the polling
`signal. In the latter case, several responses may be received.
`For the purposes of this embodiment of the invention, it is
`assumed that the search is for a specific mobile station, i.e.
`MSN.
`[0030] B. Receipt of Signalling from MS N by the Other
`Mobile Stations MS 1, MS 2 and MS 3
`[0031] Receipt of the signalling from MS N by MSs 1, 2
`and 3 allows the MSs 1,2 and 3 to measure the precise
`
`'receive time' ( also known as 'time of arrival') when a given
`component of the signalling is received by each of the
`mobile stations MSs 1, 2 and 3 as described earlier.
`[0032] C. Transmitting by Mobile Stations MS 2 and MS
`3 to MS 1
`[0033] The recorded receive times by MSs 2 and 3
`together with the position co-ordinates of those MSs
`obtained using the GPS attachments 2a and 3a respectively
`of those MSs is transmitted to lead mobile station MS 1. MS
`1 then has the receive time measurements and location
`co-ordinates for all three searching MSs 1, 2 and 3.
`[0034] D. Calculating by MS 1
`
`[0035] The relative distances from MS N of the MSs 1,2
`and 3 are proportional to the respective receive times by
`those mobile stations. Also, MS 1 has the position co(cid:173)
`ordinates of each of MSs 1, 2 and 3. From these various
`measurements, MS 1 is able to calculate the location of
`terminal 1.
`[0036] Amore detailed description of the procedure which
`may be used to determine the location of MS N is as follows:
`
`[0037] Al: Initiating Signalling from MS N
`[0038] One of the searching mobile stations, MS 1, is
`designated as search lead terminal and transmits a "Search
`Polling Request" (SPRq) command. The command is either
`directed specifically to MS N or is broadcast to all mobile
`stations in the area. The other MSs that are participating in
`the search, i.e. MSs 2 and 3, were set at the start of the search
`procedure to a search co-operation mode by entry of an
`appropriate function signal by their users. In that mode, the
`MSs 2 and 3 are set so that they do not respond to the SPRq
`command. MS 1 is aware, by entry of information by its
`user, that MSs 2 and 3 are set to the search co-operation
`mode.
`
`[0039] When MS N receives and understands the SPRq
`command, it responds with a 'Search Polling Reply' (SPRy)
`message. The content of the SPRy message may contain a
`'counter' to number the messages, although other param(cid:173)
`eters, such as frame and slot number, may beneficially be
`used to uniquely identify when a particular message is sent,
`i.e. to give the message a unique identity (ID). The SPRy
`message, with different message ID labels according to
`when each is sent, may be repeated several times to make
`sure the searching mobile stations, in this case MSs 1,2,3,
`receive it. The number of times it is repeated may be selected
`as follows. Some options are:
`
`[0040] The SPRq command includes the number of
`times the SPRy message should be repeated and the
`time between repetitions.
`
`[0041] The SPRq command includes the period of
`time when the message should be repeated and the
`time between repetitions.
`
`[0042] The above parameters may be pre-defined in
`MSN.
`
`[0043]
`In the case where several MSs respond to the SPRq
`command, MS 1 can select one of the MSs to continue
`searching for and instruct (by a special command signal
`issued by MS 1 when detecting such responses) the respond(cid:173)
`ing MSs other than the selected one to stop transmitting. It
`
`Page 7 of 11
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`

`US 2005/0159174 Al
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`Jul. 21, 2005
`
`4
`
`will be assumed that in this case the selected MS is MS N.
`The search procedure will then proceed only for MS N. This
`procedure can then be repeated later, after MS N has been
`located, for locating the other mobile stations.
`
`received and a 'bias' component due to the geometries
`involved in the measurements and positions of satellites. By
`using time differences T12 and T13, the contribution of the
`bias component which is similar for all MSs is cancelled out.
`
`[0044] Bl: Receipt of Signalling from MS N
`[0045] When MS N transmits the SPRy message, MSs 12
`and 3 receive it. They all record the ID of the message (e.g.
`the "counter" of the frame/slot number) and the time the
`message was received. MSs then send this information to the
`MS which initiated the SPRq command, i.e. MS 1, using a
`"Search Polling Reply Data" (SPRd) message. The SPRd
`message also contains the position co-ordinates obtained
`from GPS attachments 2a and 3a of the MSs 2 and 3. This
`procedure is repeated for each received SPRy message.
`[0046] Cl: Transmission of Receipt Time by MSs 2 and 3
`to MS 1
`[0047] The MSs 2 and 3 may be operable to send a signal,
`whose content is recognised by MS 1, giving measurement
`information relating to the search method, including the x
`and y position co-ordinates of MSs 2 and 3 and the respec(cid:173)
`tive receive times of given signals from MS N.
`
`[0048] Dl: Calculation of MS N's Location by MS 1
`[0049] When MS 1 has received the SPRd messages from
`MSs 2 and 3, it initiates a procedure to perform the calcu(cid:173)
`lation of MS N's location, e.g. as described further below,
`using the data received from MSs 2 and 3 and the data it
`received from MS N. After the first iteration of this proce(cid:173)
`dure, the calculation may not be accurate enough or may not
`be optimised. In this case, the user of MS 1 may instruct the
`users of MSs 2 and 3 to change positions ( and of course MS
`1 can change its position also). This instruction can be made
`by sending radio messages to the MSs 2 and 3 for the
`attention of their users, e.g. by short text message and
`display at the receiving MS, or by speech message ( or any
`other way). The position change instruction may also con(cid:173)
`tain the desired new location of the MS as calculated by MS
`1 (e.g. 'move 30 metres to the north').
`
`[0050] The position of MS N may be determined by
`triangulation. One well-known implementation of triangu(cid:173)
`lation is based on the Time Difference of Arrival (TDOA) of
`radio signals at the MSs 1, 2 and 3. MS 1 can, with the
`information received by itself and gathered from MSs 2 and
`3, calculate differences in receive time of the searching MSs
`considered in pairs.
`If Tl, T2 and T3 denote the time of arrival of a
`[0051]
`signal transmitted by MS N by MSs 1, 2 and 3 respectively,
`MS 1 can calculate the differences T12= Tl-T2 and T13=
`Tl-T3. Calculating these differences will reduce errors in
`Tl, T2 and T3 common to all searching MSs. By multiply(cid:173)
`ing the differences T12 and T13 by the speed of light, MS
`1 can calculate the corresponding difference in distance
`(from MS N) between MSs 1 and 2 and between MSs 1 and
`3 respectively. These distances will be denoted as R12 and
`R13 respectively, and are known as pseudo-ranges.
`[0052] Since all of the MSs 1,2 and 3 are likely to be
`situated in a small geographical area, it can be assumed that
`the receivers of their GPS attachments la, 2a, 3a are locked
`to the same satellite constellations. Errors in GPS measure(cid:173)
`ment have two components: a random component that is
`due, for example, to the signal-to-noise ratio of the signal
`
`[0053] Assume that (xi,YJ, i=l, 2, 3 denote the planar
`coordinates of MSs 1, 2 and 3, known to MS 1 by the
`measurements described above. The differences R12 and
`R13 express the loci of points where the difference of
`distance to MS N is constant. This is a hyperbola. If (xn,Yn)
`denote the co-ordinates of MS N, then it is true that
`
`Y(xn-x,)2+(yn-y1)2-Y(xn-x2)2+(yn-Y2)2-R12
`
`Y (xn -x1)2 +(yn -y 1)2-Y(xn -x3)2 +(yn -y3)2-R13
`[0054] This constitutes a non-linear set of equations,
`which will be referred to as Equations 1 and 2, that can be
`solved by computation by MS 1. Equations 1 and 2 can be
`solved using linearisation techniques and an iteration
`method where an initial guess for (xn°,yn°) is made and this
`guess is refined in successive iterations. The method for
`obtaining this mathematical solution is analogous to that
`known in the prior art for TMO systems.
`[0055] Since all of the MSs 1,2 and 3 are likely to be
`situated in a small geographical area, it can be assumed that
`the receivers of their GPS attachments la, 2a, 3a are locked
`to the same satellite constellations. Errors in GPS measure(cid:173)
`ment can occur and have two components: a random com(cid:173)
`ponent that is due, for example, to the signal-to-noise ratio
`of the signal received and a 'bias' component due to the
`geometries involved in the measurements and positions of
`satellites. Thus, the position co-ordinates given by the GPS
`in general are not precise and there could be an error of say
`(llx,lly). This error will manifest itself in the precision of
`determining the position of terminal N.
`
`[0056]
`If (llx,lly) is the error of the GPS positioning of
`MSs 1, 2 and 3, it can be assumed that it is the same for all
`mobile stations. Taking into account the error (llx,lly) Equa(cid:173)
`tion 1 that is being solved by MS 1 can then be written in the
`form:
`
`Y(xn-x,+Ax)2+(yn-Y1+L1.y)2-
`Y(xn-X2+L1.y)2+(yn-Y2+L1.y)2-Rl2
`[0057] and Equation 2 can be written in a similar form.
`Assuming that llx2,lly2-0 and that ((xn-x1)2»2llx(xn-x1 )
`and taking into account that
`
`[0058]
`\"(xn-x 1)2+(yn-Y1)2-
`that
`it can be shown
`Y(xn-x2)2+(yn-y2)2=R12+llR12 Equation 3, where
`
`Equation 4
`
`----;===(X=n =-=X2=) ==JIU+
`
`✓ (xn - X2) 2 + (Yn - Y2) 2
`
`Page 8 of 11
`
`

`

`US 2005/0159174 Al
`
`Jul. 21, 2005
`
`5
`
`-continued
`
`(
`
`(Yn - Y1)
`
`[0059] That is, from the standpoint of the computation,
`li.R12 is equivalent to an additional error in the measured
`pseudo-range. Assuming that MSs 1, 2 and 3 are located at
`the apexes of an equilateral triangle whose sides are equal to
`300 metres and assuming that the accuracy of the GPS
`position measurement is about 1 metre, the value li.R12 is of
`the order of 1.5 metres.
`[0060] One of the advantages in performing this compu(cid:173)
`tation using the linearisation technique expressed above is
`that one of the steps involves the computation of the
`so-called co-factor matrix defined in the following website
`publication
`(http://www.gmat.unsw.edu.au/snap/gps/gps
`survey/chapl/1 49.htm). This co-factor matrix permits the
`computation of what is known as Geometric Dilution of
`Precision (GDOP). In simple terms, GDOP represents the
`ratio between the error that will be achieved in the estima(cid:173)
`tion of the required position to the optimal error that can be
`achieved. The GDOP in implementing this embodiment of
`the invention is very dependent on the relative positions of
`MSs 1,2 and 3 and MS N. If the MSs 1,2 and 3 are well
`distributed with respect to MS N, i.e. ideally to form an
`equilateral triangle with MS Nat the centre, then the GDOP
`will be low whereas if they are not evenly distributed, the
`GDOP will be high. This is illustrated graphically in FIGS.
`2(a) and (b). FIG. 2(a) illustrates an arrangement of mobile
`stations MS N, 1, 2 and 3 (now shown as small circles)
`which gives a low value of GDOP. In contrast, FIG. 2(b)
`illustrates an example of an arrangement of mobile stations
`MS N, 1, 2 and 3 which gives a high value of GDOP.
`
`[0061]
`In the steps needed to solve Equations 1 and 2
`using a linearisation approach, known from GPS and TDOA
`calculations for example, an MS (MS 1) can compute a
`value for GDOP for the particular calculation. Thus, the
`GDOP value obtained for a given calculation is calculated
`and used to improve the positions of MSs 1, 2 and 3 with
`respect to each other and to MS N. In a typical procedure
`using the measured GDOP value, a very coarse search can
`be carried out initially. MS 1 carries out a location compu(cid:173)
`tation and determines that the GDOP value is large. An
`instruction can then be sent by the user of MS 1 to the users
`of either or both of MSs 2 and 3 to move, to perform new
`measurements and to resend their co-ordinates and receive
`time measurements to MS 1 to see if a better GDOP value
`can be achieved. This procedure may be repeated iteratively
`until a minimum acceptable value of GDOP is attained.
`
`[0062] Graphical (Analytical) Method for Improving
`GDOP
`
`[0063] Once R12 and R13 have been measured, assuming
`that all the measurements are exact, hyperbolae can be
`constructed which are the solutions in the xy plane to
`Equations 1 and 2. From R12, there is obtained a set of
`points (x1 ,y1) (x2 ,y2 ), etc which follow a hyperbola repre-
`
`senting the set of geometric pos1t10ns for all potential
`solutions for (xn,Yn). Similarly the hyperbola obtained using
`R13, points defined by (x1 ,y1), (x3 ,y3 ) etc represent the set
`of geometric positions for all potential solutions for (x