Cellular radio location system

DETAILED DESCRIPTION According to the invention, there is provided a method of determining the location of a mobile unit of a cellular radio system having a plurality of base stations, comprising the steps of determining the differences in timing between the base stations' transmissions as measured at the mobile unit, determining from the timing differences the differences in the distance of the mobile unit from each of the base stations, and deriving the location of the mobile unit from the differences in distance so determined, characterised in that the time division frame structures of the control channels of at least some of the base stations within radio range of the mobile unit are synchronised, and the mobile unit determines the differences in timing at the mobile unit of a characteristic feature of the time division frame structure broadcast by the control channel of each base station.
By using the control channel the mobile unit is able to make use of the existing radio link quality monitoring systems used to establish whether a handover should take place, and does not need to establish full communication with any of the base stations.
Preferably the characteristic feature used is a training signal transmitted by each base station, which is correlated with a reference training signal stored by the mobile unit.
Such a "synchronisation burst" (SCH), and its correlation process already form part of the GSM standard for characterising the radio path for the purpose of identifying candidate base stations for handover.
The method of the invention can therefore make use of these existing signals and correlation analysis programming.
However, for position determination it is preferred that the system should use the first identified correlation of the reference signal, rather than the strongest as is use d for handover determination.
This ensures that the most direct signal path, rather than a stronger but more indirect path, is used for distance calculation.
The derived location may also be time averaged to minimise the effects of spurious results from reflected signals, which would make the apparent distance between the base station and the mobile unit longer than it really is