# Autonomous GPS

How in the world do autonomous vehicles get from A to B?

##### There is a problem, however, and it’s a big one.

Much like the problem with determining longitude I talked about last time, if the local clock on the receiver is not perfectly synchronised with the clock on the satellite, the time difference will not be correct. Because we are using radio waves that travel at the speed of light – 300 million metres per second – even a small error in the local clock will result in a large error in distance. If the local clock is off by just one millionth of a second, for example, the distance calculation will be out by 300 metres. One solution is to use an extremely accurate clock that is set to the same time as the satellites – in much the same fashion as the marine chronometers of old were set to Greenwich time. The problem with this idea is that only atomic clocks are accurate enough. Atomic clocks are very expensive and tend to be big and heavy.You won’t get a caesium clock to fit into your mobile phone, for example, and even if you could the phone would set you back over \$50,000. We need a way to get around the problem that doesn’t involve having an expensive atomic clock in every receiver.

##### Fortunately, we can use a bit of geometric chicanery to solve the problem.

To begin with, we need to know the positions (x,y,z) of the satellites, as well as the time the transmissions were sent – we calculate these from the ephemeris data and the time of transmission in the satellite messages.