According to Extreme Tech, the Defense Advanced Research Projects Agency (DARPA) is currently working on developing a new GPS system that "doesn’t depend on satellites and is resistant to jamming."
In the paper, DARPA hints at how it’s going to develop a positioning system without the use of satellites: “The need to be able to operate effectively in areas where GPS is inaccessible, unreliable or potentially denied by adversaries has created a demand for alternative precision timing and navigation capabilities. To address this need, DARPA is investing in radically new technologies that have the potential to deliver GPS-quality position, navigation and timing information for military systems, including novel inertial measurement devices that use cold-atom interferometry; chip-scale self-calibrating gyroscopes, accelerometers and clocks; and pulsed-laser-enabled atomic clocks and microwave sources.”