Trilateration is the process of using geometry to estimate the position of the mobile device using estimated distances traveled by the different “positioning” or “ranging” signals that are received by a mobile device from the different transmitters, satellites, or other types of beacons of a particular positioning network.  If position information like the transmission time and reception time of a positioning signal from a beacon are known, then the difference between those times multiplied by speed of light would provide an estimate of the distance traveled by that positioning signal from that beacon to the mobile device.  Different estimated distances corresponding to different positioning signals from different beacons in that positioning network can be used along with position information like the locations of those beacons to estimate the position of the mobile device.  Positioning systems and methods that estimate a position of a mobile device (in terms of latitude, longitude and/or altitude) based on positioning signals from beacons (e.g., transmitters, and/or satellites) and/or atmospheric measurements are described in co-assigned U.S. Pat.  No.  8,130,141, issued Mar. 6, 2012, and U.S. Patent Application Publication No.  US 2012/0182180, published Jul. 19, 2012. 
Many known approaches for estimating the position of a mobile device use a single positioning network that provides a position estimate with a particular accuracy and a particular level of certainty.  However, as described in the disclosure that follows, improved accuracy and certainty can be achieved using information from two or more positioning networks.