An apparatus and method for calculating time offsets for imperceptible base stations, which include base stations that have limited or no communication with a mobile station are presented. Time offsets for the imperceptible base stations are computed using transmit time offset information for a plurality of base station pairs received from a non-carrier base station almanac. A time offset between the mobile device at a first location and a serving base station at a second location is also computed. The time offsets for the imperceptible base stations may then be computed using the received transmit time offsets of the plurality of base station pairs and the time offset between the mobile device and the serving base station. The non-carrier base station almanac is built using a mobile device that informs a non-carrier location server of observed time difference of arrival (OTDOA) assistance data it receives from a carrier's location server.