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.