EP-A-0 303 371, the contents of which are hereby incorporated by reference, describes a radio navigation and tracking system which makes use of independent radio transmitters set up for other purposes. The signals from each transmitter, taken individually, are received by two receiving stations, one at a fixed and known location, and the other mounted on the mobile object whose position is to be determined. A representation of the signals received at one receiving station is sent via a link to a processor at the other receiving station, where the received signals are compared to find their phase differences or time delays. Three such measurements, made on three widely spaced independent transmitters, are sufficient to determine the position of the mobile receiver in two dimensions, i.e. its position on the ground. The phase or time offset between the master oscillators in the two receivers is also determined.
“CURSOR”, as the system described in EP-A-0 303 371 is known, is a radio positioning system which can use the signals radiated by existing non-synchronised radio transmitters to locate the position of a portable receiver. Unlike some other systems which use the temporal coherence properties of networks of purpose-built synchronised transmitters, CURSOR makes use of the spatial coherence of the signals transmitted by single transmitters. In a further development (see EP-A-0 880 712 & WO-A-99/21028), the technology has been applied to find the position of a mobile phone handset in a GSM or other digital telephone system, and these are examples of an ‘Enhanced Observed Time Difference’ (E-OTD) method using the down-link signals radiated by the network of Base Transceiver Stations (BTS) of the telephone system.
In the digital mobile telephone application described in EP-A-0 880 712, the contents of which are hereby incorporated by reference, the signals from each BTS within range of the handset are received both by the handset itself and by a fixed nearby receiver, the Location Measurement Unit (LMU), whose position is accurately known. Representations of the received signals are passed to a Mobile Location Centre (MLC) where they are compared in order to find the time difference between them. FIG. 1 shows the geometry of a standard two-dimensional system. The origin of Cartesian co-ordinates x and y is centred on the LMU positioned at O. The orientation of the axes is immaterial, but may conveniently be set so that the y axis lies along the north-south local map grid. The handset, R, is at vector position r with respect to the LMU position O. A BTS, A, is shown at vector position a.
Consider first the signals from BTS A. The time difference, Δta, measured between the signals received at R and O is given byΔta=(|r−a|−|a|)/υ+ε,where υ is the speed of the radio waves, ε is the clock time offset between the clocks in the receivers at R and O, and the vertical bars each side of vector quantities denote that it is the magnitude of the vectors which are used in the equation. The value of ε represents the synchronisation error between the measurements made by the two receivers. Similarly, may be written for two other BTSs (B and C) at vector positions b and c (not shown):Δtb=(|r−b|−|b|)/υ+ε,Δtc=(|r−c|−|c|)/υ+ε.  (1)
The values of Δta, Δtb, Δtc, are measured by the methods disclosed in EP-A-0 880 712 and the values of a, b, c, and υ are known. Hence the equations (1) can be solved to find the position of the handset, r, together with the value of ε.
In WO-A-99/21028, the contents of which are hereby incorporated by reference, it is described how these same time offsets can be measured using locally-created templates in a GSM telephone system as follows. Suppose that the handset R has recorded a short burst of the GSM signals from BTS A. Contained within that recording is the framing structure, synchronisation bursts and other ‘given’ data (or predetermined values) which are a constant feature of those transmissions. The processor within the handset can create a matching template, based on the known structure of the network signals. Received signals can then be matched by the locally-generated template. When the template finds a match, the correlation peak at the position of best match corresponds to the time offset between the received signals and the local clock inside the handset. For the signals radiated by BTS A this measured time offset, Δta1, is given byΔta1=(|r−a|)/υ+αa+ε1,where αa is the time offset of the BTS transmissions and ε1 is the time offset of the handset's internal clock, both relative to an imaginary universal ‘absolute’ clock. The signals from BTSs B and C may also be measured in the same way, givingΔtb1=(|r−b|)/υ+αb+ε1,andΔtc1=(|r−c|)/υ+αc+ε1.  (2)
The same measurements can also be made by the LMU, givingΔta2=(|a|)/υ+αa+ε2,Δtb2=(|b|)/υ+αb+ε2,andΔtc2=(|c|)/υ+αc+ε2,  (3)where ε2 is the time offset of the LMU's internal clock relative to the same imaginary universal absolute clock. Subtracting equations 3 from equations 2 givesΔta=Δta1−Δta2=(|r−a|−|a|)/υ+ε,Δtb=Δtb1−Δtb2=(|r−b|−|b|)/υ+ε,andΔtc=Δtc1−Δtc2=(|r−c|−|c|)/υ+ε,  (4)where ε=ε1−ε2. It will be noted that equations 4 are just like equations 1, and can be solved in the same way to find the position of the handset, r, and the value of ε.
It will be apparent that the CURSOR method as described above, in common with all other methods which use the signals from non-synchronised transmitters, requires a network of LMUs to be set up within the coverage area of the telephone system. These units act as reference points at which the unsynchronised signals radiated by the BTSs are measured for comparison with the same signals received by a handset. Each position measurement requires a match to be made between the signals received by the handset from a number of nearby BTSs, and signals received by an LMU from the same set of BTSs. In practice, it is often difficult to find a match using just one LMU, especially if the LMU network is sparse, since the handset may receive signals from BTSs not received by the LMU, and vice-versa. It is therefore necessary to combine the measurements from two or more LMUs. However, each new LMU brought into the calculation adds a further unknown clock time offset (ε2, ε3 etc.), each of which therefore requires an additional BTS measurement to provide the extra equation needed to solve for all the unknown quantities.
One solution to this problem is presented in WO-A-99/21028 where it is shown how the LMU network can be synchronised. Referring to FIG. 2, suppose that an adjacent pair of LMUs, U1 and U2, can see a common BTS. The positions of the LMUs and the BTS are all known, so a single measurement of the BTS signals by each LMU is sufficient to determine the clock time offset between the LMUs. For example, suppose that the distance from U1 to the BTS is s1, and the distance from U2 to the BTS is s2. U1 measures time offset Δt1 and U2 measures Δt2, given byΔt1=s1/υ+α+ε21,Δt2=s2/υ+α+ε22,  (5)where α is the time offset of the BTS transmissions, and ε21 and ε22 are the time offsets of the LMU internal clocks in U1 and U2 respectively. Subtracting the second equation from the first yieldsε21−ε22=Δt1−Δt2+s1/υ−s2/υ,  (6)which is the relative time offset of the clock in U1 with respect to that in U2. This process may be repeated for a second pair of LMUs, say U2 and U3, and another BTS whose signals can be received by both members of this second pair of LMUs. In this way a synchronisation map may be calculated, which provides the clock offsets of all the LMU internal clocks relative to one of them adopted as a master ‘LMU network clock time’. Having established the LMU synchronisation map in this fashion, a CURSOR position measurement can then include any number of LMUs without the penalty of adding an extra unknown time offset for every LMU, since the relative LMU time offsets are known.
The receivers discussed in the preceding paragraphs make measurements of time offsets. More generally, receivers can measure time offsets, phase offsets (which can be converted into time offsets with a modulo 360° ambiguity), frequency offsets or rates of change of frequency offsets. Though these measurements are of different quantities, the present invention is applied usefully to each of them as, when combined with similar measurements made by a second receiver, they can independently provide positional information. Positioning systems making use of these measurements are discussed in a related U.S. Pat. No. 6,529,165 filed simultaneously herewith.