No, this technique is different. What cell-phones do is rely on pre-mapped databases of wifi and cell tower locations to do trilateration to get a position. T

No, this technique is different. What cell-phones do is rely on pre-mapped databases of wifi and cell tower locations to do trilateration to get a position. The technique the article describes is basically using any stationary RF source as a reference point to tell if your position estimate is drifting. (It's not unlike using a mountain as a landmark for your direction and location while hiking; you don't have to know precisely where the mountain is, you just have to be able to see it and know that it's not moving.) As you point out, it's dependent on what RF sources are nearby (this is the "opportunistic" part of their SOP signal acronym) but can be used to help compensate for the drift that inertial navigation systems have without having to have a pre-compiled database of RF transmitter locations.