There used to be a terrestrial radionavigation system called Loran-C that could be used for positioning and timing, but it was decommissioned in 2010. Loran-C

There used to be a terrestrial radionavigation system called Loran-C that could be used for positioning and timing, but it was decommissioned in 2010. Loran-C was a high-power, low-frequency signal as opposed to the very-low-power, high-frequency signal of GPS. This made Loran-C less susceptible to jamming. It was a nice backup. http://en.wikipedia.org/wiki/Loran-C http://en.wikipedia.org/wiki/Loran-C