Satellite navigation has revolutionized the way we get around. It’s cheap, fast and easy to use. But there’s one fatal flaw in the system: It simply can’t work underground.
GPS requires line of sight to satellites orbiting the planet. The great advantage of this is that it works seamlessly all over the world, without needing to be connected to a cellular or Wi-Fi network. This works well in open areas but in built-up areas with tall buildings, or in underground tunnels, the system can fall apart.