There are 66 Iridium satellites up there, and they’re about 25 times closer to the ground than GPS satellites are, and that proximity means their signals are much stronger when they arrive at Earth. Where a GPS signal might be too weak—like in an old brick apartment building amidst other brick apartment buildings—an Iridium signal does just fine.

There’s gotta be a downside, though, or everyone would have used this for location and timing in the first place. And here it is: “The accuracy is not quite as good as GPS,” says O’Connor. Where GPS gives your spot with about 5 to 10 meters of accuracy, Satelles’ method is only about 20 to 30 meters (the timing accuracy is comparable). “When you’re in an environment where you can receive GPS, GPS is better,” says O’Connor. But if you’ve got no GPS, or GPS has got your spot wrong, or you need to know for sure for sure you’re not being spoofed, Iridium might be the way to go.

Private industry isn’t the only place concerned with GPS’s vulnerabilities. The military is nervous, too. “[GPS] has become a single point of failure,” says Dave Tremper, a program manager at the strategic technology office for Darpa, the military's advanced research branch. “Up until the early 2000s, there was a variety of other alternatives,” says Tremper. But after GPS came along, "they all went away."

Now, the Defense Department wants options again. Over in the Army, engineers are developing what is basically a small-scale GPS constellation on the ground. The transmitters, tested at White Sands Missile Range in October 2017, are called “pseudolites”—pseudo + satellites. Their high-power transmissions are harder to jam, and easier to pick up in a dense forest, than the weak GPS signals that have to come from space.

And for the past decade or so, Darpa has also dug into alternatives for “PNT,” an acronym that stands for “position, navigation, and timing.” One of its programs is called Adaptable Navigation Systems, and its technology is already on some ships and Humvees. Instead of building new broadcasters, engineers figured out how to pull position information from signals that already exist. Darpa's sensors pick up things like television broadcasts, cell tower transmissions, and lightning strikes, and they can pull in imagery. The systems then compare that data to catalogs of reference information—like, perhaps, where a TV station or cell tower is.

To grok how such a system could use images, think about what you could do with Google Street View, says Tremper. If you have a dashcam in your car, software could compare its view to Google's catalog, trawling through it in a sophisticated set of reverse image searches. With a smart system, “you could figure out pretty well where you are,” says Tremper.

Good as images may be, no single signal gives a complete location and time. Darpa fuses the TV, cell, and visual (and other) findings into "a collective solution."

In the process of working on Adaptable Navigation Systems, Darpa researchers also tried to capitalize on the very low frequency transmissions that drone across the planet. These radio waves can travel tens of thousands of kilometers, so it only takes a few broadcasters to cover the globe. "There are already very low frequency transmitters out there at known locations," says Tremper, "so we don’t necessarily need to create new ones." A newer Darpa program, called Stoic, is trying to hook into this type of signal to triangulate location, accounting for how part of the atmosphere shifts in real-time and affects the transmissions.

The agency has at least five other PNT projects running. Because while GPS is great—"a science and technology marvel," as Tremper says—it can't be all things to all people all the time, even though it's currently a lot of things to a lot of people a lot of the time. And that won't fly forever, as high-power spoofing and jamming and whatever else become more common. GPS signal, for all its powers, is weak. "It’s easy to disrupt it. It’s easy to interfere with it," says Tremper. "If you’re dependent on something that’s whispering to you, and somebody else comes over and starts yelling ... you’re lost.”

But maybe not if you have a backup.

GPS Gaffes