. Also, fine-tuning of the location estimate is possible: after the first estimate, more assistance can be given. The E-OTD (Enhanced-Observed Time Difference) method uses the signals received from at least three base stations to measure the difference in times they take from each BTS to the MS. The time difference is used as an input to triangular measurements to estimate the MS location. When the location is calculated in relation to the BTS locations, those coordinates have to be known by the system. Also the base stations have to be synchronised, and this is handled with LMU units, which compare the BTS time to absolute time. The MS has to have special software to be able to support the E-OTD method. The U-TDOA (Uplink-Time of Arrival) method is in a sense the reverse of E-OTD. In U-TDOA the MS signal is measured by at least three base stations, and as in E-OTD the measurements are done by the MS. The arrival time of the data sent by the MS is registered in each BTS. The time is compared to the absolute time to get the difference in arrival times, and from this the MS location can be calculated using triangular measurement. U-TDOA does not require any updates or changes to the MS. On the other hand, all base stations need to be able to know the absolute time, which requires additional hardware, LMU units, for each BTS. The Cell ID (CI) method in a GSM network uses the coordinates of the serving BTS as the location estimate for the mobile phone. As this method is purely network-based, it LOCATION-BASED SERVICES does not require any additional features to the mobile phone, SO it is economic. The CI method can be enhanced with two features, Timing Advance (TA) and Received Signal Level (Rx), which increase the accuracy of the location estimates. The TA feature utilises TA measurements, and Rx utilises the neighbouring cell measurements, more particularly the signal levels measured for the neighbouring cells. Both TA and Rx measurements are an existing feature of GSM networks, SO TA and Rx measurement results are