Abstract:
A method of providing an estimate of the location of first and second devices MS 1 , MS 2  comprising the steps of obtaining at least one range measurement from the first device MS 1  to a known reference point BS 1 , GPS SVs; obtaining at least one range measurement from the second device MS 2  to a known reference point BS 1 , GPS SVs, the second device MS 2  being located near to the first device MS 1 ; and calculating an estimate of the location of the devices MS 1 , MS 2  using range measurements obtained with respect to both the first and second devices MS 1 , MS 2.

Description:
RELATED APPLICATION 
     This is a continuation application of application Ser. No. 09/882,083, filed Jun. 15, 2001, now U.S. Pat. No. 6,646,603. 
    
    
     FIELD OF INVENTION 
     This invention relates to a method of determining the location of first and second devices. 
     BACKGROUND TO INVENTION 
     Recent progress in the field of GPS has enabled GPS receivers to be produced relatively cheaply leading to their widespread adoption and use. For example, one might envisage a home networking environment in which is provided a mobile telephone with GPS capability for providing its location to a cellular telephone network operator in-the event of an emergency call; a TV with GPS capability for providing TV access control, say as described in U.S. Pat. No. 5,621,793; and a personal computer with GPS and Internet capability for retrieving location specific information from a web site, say a local weather report. 
     In such a home networking environment, three GPS receivers are provided, each returning substantially the same location. However, especially indoors, it is unlikely that all three GPS receivers will each be able to acquire the four GPS space vehicle (SV) signals normally required to obtain a position fix, or at least not all of the time. One reason for this is obscuration of the GPS signals by buildings, walls and other urban paraphernalia. 
     U.S. Pat. No. 5,451,964 (“Babu”) discloses a mobile GPS receiver station operating with a reference GPS receiver station wherein reference station pseudoranges and co-ordinates are transmitted from the reference station to the mobile station. In particular, Babu discloses where the reference pseudoranges are used to generate pseudorange corrections at the mobile station which are then applied to corresponding mobile station pseudoranges, themselves used to determine the position of the mobile station. By corresponding, one means that Babu discloses uses reference station pseudoranges to make corrects to mobile station pseudoranges necessarily obtained from the same satellites. Babu is in effect an implementation of differential GPS (DGPS) but slightly differently from conventional DGPS implementations in that the differential corrections are applied to individual mobile station pseudoranges rather than the resultant position fix. 
     It is an object of the present invention to provide a method of determining the location of a device which is more effective indoors. 
     SUMMARY OF INVENTION 
     According to a first aspect of the present invention, there is provided a method of providing an estimate of the location of first and second devices which are located near to each other, the method comprising the steps of: 
     obtaining at least one range measurement from the first device to a corresponding reference point; 
     obtaining at least one range measurement from the second device to a corresponding reference point, the second device being located near to the first device; and 
     calculating an estimate of the location of the devices using (i) range measurements obtained with respect to both the first and second devices and (ii) the co-ordinates of the corresponding reference points, 
     and wherein at least one range measurement obtained from the second device was obtained with respect to a reference point to which no range measurements were obtained from the first device. 
     Such a method enables an estimate of the location of the devices to be calculated in circumstances where it is impossible to unambiguously determine the location of the first device using only range measurements obtained from the first device, or the second device using only its range measurements obtained from the second device. 
     In particular, but not exclusively, the or each range measurement obtained with respect to the second device and for each such range measurement an indication of the associated reference point, e.g. in the form of a position co-ordinate, are provided to the first device for calculating an estimate of the location of the devices, preferably using a wireless communications link. 
     Where the range measurement information is provided from the second device to the first device using a short range communications link such as Bluetooth, one can safely assume that the devices are close together and that the resultant position fix is a good estimate of the true position of the first and second devices. Also, having calculating an estimate of the location of the devices at the first device, the estimate may then be provided to the second device. 
     Preferably, the first and second devices have reciprocal functionality in that the or each range measurement obtained with respect to the first device and for each such range measurement an indication of the associated reference point may also be provided to the second device for calculating an estimate of the location of the devices. 
     Alternatively, the or each range measurement obtained with respect to both the first and second devices and for each such range measurement an indication of the associated reference point may be provided to a third device for calculating an estimate of the location of the first and second devices. 
     At least one range measurement obtained with respect to either the first or the second device may be a pseudorange range measurement. 
     For example, the position determining means of the second device may include a GPS receiver wherein at least two range measurements obtained with respect to the second device are pseudorange measurements from a GPS SV. Also, the position determining means of the first device may also include a GPS receiver wherein at least two range measurements obtained with respect to the first device are pseudorange measurements from a GPS SV; and wherein at least 5 pseudorange are obtained with respect to either the first or the second device in total. I.e. sufficient to obtain a position fix. 
     NB. In order to calculate an estimate of the location of the first device using pseudorange measurements obtained with respect to both first and second devices, clocks errors of both the first and second devices must be resolved, i.e. the clock errors between the synchronized GPS SVs and each devices. Fortunately, however, pseudoranges may be more conveniently obtained than absolute range measurements and this may justify the extra computation. 
     Alternatively, where the position determining means of the first and second device both includes a GPS receiver and the first and second device are synchronized, a single pseudorange measurement from a GPS SV obtained with respect to either the first or second device may contribute to the position determination. Without synchronicity, the benefit of a device obtaining a single pseudorange is offset by the extra clock error introduced. 
     At least one range measurement obtained with respect to the first and/or the second device may be a range measurement to a ground based reference point. This information may then be combined with either other ground and or spaced based range or pseudorange measurements in order to obtain a position fix. 
     For example, this may be an absolute range measurement determined by a time-of-arrival measurement with respect to a fixed base station, and may be conveniently provided when the first and/or second device is a mobile cellular telephone able to communicate using a wireless communications link which is independent of any cellular basestation network. Synchronicity between a mobile cellular telephone and the cellular telephone network basestation with which it is registered is relatively easy to achieve, and thus an absolute range measurement may be readily obtained. 
     Also provided in accordance with the present invention is a device able is provide an estimate of the location as claimed in claims  19  to  27 . 
     According to a second aspect of the present invention, there is provided a method of determining the position of a first GPS receiver with the assistance of a second, portable GPS receiver comprising the steps of:acquiring GPS signals in the second GPS receiver and deriving GPS signal information therefrom; 
     providing the GPS signal information from the second GPS receiver to the first GPS receiver; and 
     acquiring GPS signals in the first GPS receiver using the GPS signal information provided by the second GPS receiver, and determining the position of the first GPS receiver therefrom. 
     The term “portable” is intended to mean that it could be conveniently carried from place to place by a human and would normally be carried from place to place by a human. As such, portable, would include a mobile telephone, a personal digital assistant (PDA), a palm-top computer or lap-top computer, all incorporating a GPS receiver. 
     Preferably, the first and second GPS receivers have reciprocal functionality in that the first GPS receiver may also acquire GPS signals and derive GPS signal information therefrom, and provide GPS signal information to the second GPS receiver; and that the second GPS receiver may acquire GPS signals using the GPS signal information provided by the first GPS receiver. 
     Also, it is desirable that the first GPS receiver provide this assistant whilst necessarily being able to determine its-own position, i.e. not be able to acquire the four GPS signals normally necessary to do so. 
     Also provided in accordance with the second aspect of the present invention is a GPS receiver unit  31  to  33 . 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The above and other features and advantages of the present invention will be apparent from the following description, by way of example, of implementations of methods of the present invention with reference to the accompanying drawing in which: 
         FIG. 1  shows, schematically, a pair of mobile cellular telephones MS 1 , MS 2  each comprising a GPS receiver with an additional wireless communication capability independent of a cellular telephone network; and 
         FIG. 2  illustrates, schematically, the interaction of the pair of mobile cellular telephones MS, MS 2  of  FIG. 1  in the context of adjacent piconets. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , mobile cellular telephone MS 1  is shown comprising a communications transmitter and receiver (C Tx/Rx)  16  connected to a communications antenna  15  and controlled by a microprocessor (μc)  12  for two-way communication with a cellular telephone network base station BS 1  with which it is registered. 
     In addition, cellular telephone MS 1  comprises a GPS receiver (GPS Rx)  11  connected to a GPS antenna  10  and also controlled by the microprocessor (μc)  12  for receiving GPS spread spectrum signals transmitted from orbiting GPS satellites. When operative, the GPS receiver  11  receives NAVSTAR SPS GPS signals through the GPS antenna  10  and pre-processes them, typically by passive bandpass filtering in order to minimise out-of-band RF interference, preamplification, down conversion to an intermediate frequency (IF) and analog to digital conversion. The resultant, digitized IF signal remains modulated, still containing all the information from the available satellites, and is fed into a memory of the microprocessor  12 . The GPS signals may then be acquired and tracked for the purpose of deriving pseudorange information. Of course, such methods for GPS signal acquisition and tracking are well known, for example, see chapter 4 (GPS satellite signal characteristics) &amp; chapter 5 (GPS satellite signal acquisition and tracking) of GPS Principles and Applications (Editor, Kaplan) ISBN 0-89006-793-7 Artech House. 
     Cellular telephone MS 1  also comprises a Bluetooth communications transmitter and receiver (BT Tx/Rx)  14  connected to a Bluetooth communications antenna  13  and controlled by the microprocessor (μc)  12  for short range, two-way Bluetooth communication. As illustrated in  FIG. 1 , such communication may be with another mobile cellular telephone MS 2  and independent from the cellular telephone network. Cellular telephone MS 2  is substantially identical to cellular telephone MS 1 . 
     Consider the scenario where cellular telephone MS 1  is required to determine its position, say to provide its location to the emergency services operator (termed public safety answer point in the U.S.) in the event of an emergency call, but its GPS receiver  12  is only able to acquire two of the four GPS signals required to provide an unambiguous position fix. In such a scenario, using its Bluetooth communication transmitter (BT Tx)  14 , cellular telephone MS 1  broadcasts a request to other Bluetooth receivers in the vicinity requesting that they respond by providing supplemental ranging information. Cellular telephone MS 2  located nearby receives the request and at the time of the request is able to see two GPS satellites, both different from those acquired by cellular telephone MS 1 , and from them take pseudorange measurements. Also, cellular telephone MS 2  is able to make an absolute range measurement using a time-of-arrival technique with the base station with which it is registered. 
     In response to the request from cellular telephone MS 1 , cellular telephone MS 2  replies by sending its ranging information together with corresponding reference locations in co-ordinate form. The reference information for GPS pseudoranges, i.e. the position of the GPS SVs, may be derived from GPS data message and the position of the base station from which the absolute range measurement from cellular telephone MS 2  is made may be transmitted from the base station to cellular telephone MS 2  prior to its inclusion in the reply by cellular telephone MS 2  to cellular telephone MS 1 . Upon receiving the reply, cellular telephone MS 1  then has addition ranging information available to it to supplement its own, as summarized in Table 1 below. 
     
       
         
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Ranging information available to cellular telephone MS1 
               
             
          
           
               
                   
                 Device 
                 Information (with reference location) 
                 Unknowns 
               
               
                   
                   
               
               
                   
                 MS1 
                 Pseudorange from GPS satellite to MS1 
                 Clock error 1 
               
               
                   
                 MS1 
                 Pseudorange from GPS satellite to MS1 
                 Clock error 1 
               
               
                   
                 MS2 
                 Pseudorange from GPS satellite to MS2 
                 Clock error 2 
               
               
                   
                 MS2 
                 Pseudorange from GPS satellite to MS2 
                 Clock error 2 
               
               
                   
                 MS2 
                 Absolute range measurement 
                 — 
               
               
                   
                   
               
             
          
         
       
     
     As there are three unknown co-ordinates of the position estimate, i.e. x, y and z, and two clock errors totalling 5 unknowns; and as there are 5 independent ranges/pseudoranges, it is possible for cellular telephone MS 1  to determine its location. Of course, this position estimate is equally applicable to MS 2  and may be provided to MS 2  using the Bluetooth link. 
     Referring to  FIG. 2 , the pair of mobile cellular telephones MS 1 , MS 2  of  FIG. 1  in the context of adjacent wireless piconets P 1 , P 2 , P 3 . I.e. wireless networks of devices in each of which one device is designated a masters M 1 , M 2  and M 3  (denoted by stars) and is able to communicate with slave devices (denoted by small circles) within its piconet. Each master is capable of communication with other master devices in adjacent piconets. 
     For example, a television TV in piconet P 1  without any means to determine its own position may communicate with its master M 1 , for example a personal computer, and requests that it receive an approximate position fix. This may be in order to effect TV access control whereby only when the present location is consistent with an authorized location are the incoming video signals decrypted and displayed. 
     Upon receiving said request, master M 1  communications with masters M 2 , M 3  of adjacent piconets requesting that they broadcast across their respective piconets requests for ranging information of the aforementioned kind. Cellular telephones MS 1 , MS 2  located within piconets P 2  and P 3  respectively receive such a request and transmit their ranging information to master M 1  via their respective masters M 2  and M 3 . Master device M 1  calculates a position fix using the ranging information supplied by MS 1  and MS 2 , and transmits this information to the TV within its piconet so as to enable the TV to effect access control. I.e. the TV uses an estimate of the position of cellular telephones MS 1  and MS 2  as an estimate of its own position. In this example, the position calculation is done at the master M 1  which may itself be without any means to provide a range measurement. If either MS 1  or MS 2  were individually able to determine their position, this position estimate could be provided via respective masters and master M 1  to the television. 
     In order to reduce the time to first fix, a GPS receiver of either or both cellular telephones MS 1  and MS 2  may be provided with base station assistance in order to acquire GPS signals more quickly. Such assistance may include the provision by the base station to the receiver of a precision carrier frequency reference signal for calibrating the local oscillator used in the GPS receiver; the data message for up to date satellite almanac and ephemeris data from which Doppler shift for satellites in view can be determined; and the current PRN code phase. With such assistance, it is possible to sweep only a narrowed range of frequencies and code phases in which the target PRN code is known to occupy, thereby reducing the number of code instances that need to be checked and thus reducing the time for code acquisition. Base station assistance is further described in relation to a fixed base station in U.S. Pat. Nos. 5,841,396 and 5,874,914 which are incorporated herein by reference. 
     Via the piconets, such assistance may be provided to MS 1  and MS 2  either between each other or by a further device in possession of relevant information, say an up to date almanac and ephemeris data. Ideally, MS 1  and MS 2  are each capable of reciprocating such assistance in that one may be able to provide the other with Doppler information relating to the satellites in view, and on occasion receive and use such Doppler information provided by the other. 
     At present GPS is most notably associated with the Navigation System with Time and Ranging (NAVSTAR) GPS, an all weather, spaced based navigation system developed and operated by the US Department of Defense, however, the general principles underlying GPS are universal and not merely limited to NAVSTAR. Accordingly, GPS refers to any positioning system comprising a plurality of radio transmitters at different locations and a receiver which determines its location based on the time of arrival of the transmissions of the radio transmitters. In so far as a telephone is concerned, this would also include base station triangulation in which timing measurements were taken by the base stations and relayed back to the mobile telephone. 
     From a reading of the present disclosure, other modifications will be apparent to the skilled person skilled and may involve other features which are already known in the design, manufacture and use of GPS receivers and component parts thereof and which may be used instead of or in addition to features already described herein. Although claims have been formulated in this application to particular combinations of features, it should be understood that the scope of the disclosure of the present application also includes any novel feature or any novel combination of features disclosed herein either explicitly or implicitly, whether or not it relates to the same invention as presently claimed in any claim and whether or not it mitigates any or all of the same problems as does the present invention. The applicants hereby give notice that new claims may be formulated to such features and/or combinations of such features during the prosecution of the present application or of any further application derived therefrom.