In a system for geopositioning receivers in areas substantially inaccessible to satellite signals, multiple access points configured to periodically transmit management frames (i) via a single shared communication channel and (ii) using a modulation scheme associated with a rate of at least 50 Mbps. A database stores respective locations for each of the access points. A portable computing device is configured to retrieve the location of the access points from the database and, when moving through a region in which the of access points are disposed, receive at least one management frame from each of the access points within a limited time interval on the single shared communication channel. The portable computing device is further configured to determine a current position of the portable computing device based on the management frames using the retrieved locations of the access points.