A location tracking unit for use with a location based service system is presented. The location tracking unit comprises: a navigation receiver adapted to implement a location tracking function; data processing means adapted to determine an occupied location according to a first location matching process, the first location matching process using navigation data from the navigation receiver and a first set of geographic data; and data processing means adapted to verify the integrity of the first location matching process based on a second location matching process using navigation data and a second set of geographic data.