Gizmodo:

If I were to plug the gadget into my car, it would jam up the Global Positioning System signals within a 16-foot radius, rendering my smartphone’s Google Maps app useless and disabling any tracking devices that might be on my vehicle. That may sound harmless enough, but when one considers that thousands of lives (everyone in an airplane right now, for instance) and billions of dollars depend on reliable and accurate GPS signals, it’s easy to understand why my little jammer and others like it are illegal to use, sell, or manufacture in the United States. Every time I turn it on, I could incur a $16,000 fine.

But they’re easy to get online, and I’m not the only one who has ordered one.

Why do people risk the fine to jam GPS?

He’s seen truckers trying to avoid paying highway tolls, employees blocking their bosses from tracking their cars, high school kids using them to fly drones in a restricted area, and even, he believes, undercover police officers using them to avoid tails.

On the downside, your efforts could cause planes to crash. So there’s that.