英語時差：鞋子里的GPS（下）

But measuring movement is actually pretty tricky. Any small mistake in the measured acceleration results in position errors that grow with time. So even if you’re standing still and just rocking in place, a navigation computer relying solely on the IMU readings may show you wandering off in a random direction. And that’s where radar comes in. Engineers have developed a tiny radar sensor and navigation computer that attach to a shoe to keep track of the space between the shoe’s heel and the ground. Basically, the radar system helps the IMU device more accurately sense when you’re moving and when you’ve stopped, which helps reduce navigation errors. None of this technology is available in your local shoe store yet. But someday it could find its way there.