An apparatus and a method for calculating an enhanced observed time difference from a mobile station and transmitting the enhanced observed time difference to a servicing mobile location center for a location-based service are provided. The apparatus comprises means for receiving a location-measuring request and assistance data from the servicing mobile location center; means for choosing a base transceiver station, the synchronization channel of which is to be measured, based on response time detected from the location-measuring request and a predetermined priority; and means for calculating an enhanced observed time difference for the chosen base transceiver station and transmitting a location-measuring response, including the calculation result, to the servicing mobile location center. A first priority is assigned to an overlap between a channel list included in the assistance data and a list of a number of nearby cells and a second priority is assigned to a broadcast control channel allocation list except the channel list included in the assistance data and the list of nearby cells.

 
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