But BAE systems has introduced a scheme, NAVSOP (Navigation via Signals of Opportunity), which employs common radio, TV, Wi-Fi and cell phone signals to triangulate location to within "several feet" — a degree of precision equal to or better than satellite systems.
It uses all sorts of other signals as well, from GPS satellite to air traffic control. The system can even learn and evolve by taking signals that were originally unidentified and using them to build increasingly reliable and more exact fixes on location.
     Shifting to the cheap and nimble NAVSOP would not require infrastructure investments in transmitter towers and the like, because it takes advantage of whatever is already in place. 
     Larger models are in development, but NAVSOP chips are approximately the size of a coin and work with a tiny radio receiver.
     ...Overreliance on GPS signals is rampant in day-to -day life from data networks, financial systems, health networks, rail, road, aviation and marine transport, to shipping and agriculture. And military platforms commonly use GPS to find their position, navigate and execute missions.     With different systems sharing GPS dependency, a loss of signal could cause the simultaneous failure of many things people rely on daily.
Last year, the European Commission estimated that six to seven percent of its countries' GDP, representing a whopping $1 trillion, is already dependent on satellite radio navigation in Europe alone.