1. A method for building a database of beacon locations within a geographical area of interest, said method comprising: deploying one or more mobile beacon database-building devices throughout said geographical area of interest, each having a GPS receiver and a beacon signal receiver; each mobile beacon database-building device periodically measuring and recording a time-stamped composite GPS signal together with the beacon IDs detectable at the point of measurement; deriving, from each time stamped composite GPS signal, the start-of-transmission and the time-of-arrival (TOA) from an identified GPS satellite; constructing, for each of said identified GPS satellites, a spherical surface of location; centered on said GPS satellite's position at the start-of-transmission, with a radius corresponding to the difference between the time-of-arrival (TOA) and the start-of transmission; storing said spherical surfaces of location, each with the beacon IDs detectable at the point of measurement, in a beacon survey database; estimating beacon locations by determining the points for which the sum of the squares of the distances to each of three or more spherical surfaces of location sharing one of the beacon IDs detectable at the point of measurement is minimized; and storing each estimated beacon location, together with the corresponding shared one of the beacon IDs detectable at the point of measurement, in a beacon location database.