As for unintentional threats to GPS, the DHS risk estimate lists space debris, space weather, defective software, and good old-fashioned human mistakes, among other things. Of these threats, space weather is the most potentially catastrophic, according to Norwegian geophysicist Pal Brekke, whose country was hardest hit by the January outage. Eruptions of high energy radiation from the sun (known as solar flares) and ejections of electrically charged gases have disabled satellites in the past.
With satellites and the chips inside them getting smaller as technology progresses, "one particle from the sun that penetrates a satellite can ruin things,” Brekke says. “It wouldn't take that large of an event to take out all GPS."
* * *
So far, mitigating the loss of GPS signals has involved two approaches. One is interoperability with other global navigation satellite systems like Russia's GLONASS (which also failed due to a ground control error in 2014) or the European and Chinese systems, both of which are expected to be up by 2020. The other is better clocks, says Lombardi, the NIST metrologist, who's published numerous articles on the topic. "The typical cell tower clock has an oscillator similar to that of a wristwatch," he says, "and can drift out of tolerance in minutes without a signal." How long a clock can maintain time on its own, called "holdover," also affects electrical grids, many of which rely on GPS-dependent devices called synchrophasors used to precisely regulate current flow, as well as help locate faults in the network. A lack of such timing technology was the reason it took some Canadian technicians three months to locate failures after the infamous blackout of 2003.
Chip-scale atomic clocks the size of a penny are a promising new technology that can hold time for about a day, but are currently too expensive to deploy widely. Moreover, hedging and holdover still aren't backups for when space-based signals are simply unavailable.
The bulk of a more promising, comprehensive backup system already exists, right here on the ground. After the sextant but before GPS, navigators around the world used Long Range Aids to Navigation, or “LORAN,” a terrestrial system of transmitters and receiving equipment first developed during WWII. By the mid-1990s, Loran "tower chains" provided coverage for North America, Europe, and other regions in the Northern Hemisphere. Its use declined in favor of the much finer accuracy of GPS after it became available for civil use in 1995, but the U.S. Coast Guard continued working on an improved system using the existing infrastructure. If adopted, "Enhanced" LORAN, or eLoran, could provide positioning accuracy comparable to GPS. Broadcast at hundreds of thousands of watts, the signal is virtually un-jammable, and unlike GPS, can even be received indoors, underwater, and in urban or natural canyons. It also turns out that eLoran can provide a UTC time signal with sub-microsecond time resolution across a large geographical area.