Okay, you need to get across town to that new bike shop. So, you take out your smartphone and fire up Google maps. Almost instantly it pinpoints you and offers you directions to the shop. How does your phone always know where you are? It uses a technology called GPS. GPS stands for global positioning system. Right now 12,000 miles overhead, there is a network of 30 satellites orbiting the Earth. Each one circles the planet twice a day in 1 of 6 precise orbits. Using onboard atomic clocks, each satellite broadcasts a radio signal that provides the precise time. The radio signal travels at the speed of light over a 180,000 miles per second through space down to Earth to the GPS receiver in your smartphone. But even at that speed there is still a delay between when the satellite sends its signal and when your phone receives it. Using this delay, your phone can actually calculate the exact distance between it and the satellite. But a signal from one satellite is not enough. In order for Google maps to track you, it uses data from at least 3 satellites in a process called trilateration. Here’s how it works. Your phone gets a signal from this satellite, 12,000 miles away. By itself, that’s not all that helpful as you could be 12,000 miles away in any direction. That means you could be located anywhere on this circle. But then your phone gets a signal from this satellite — it’s 10,000 miles away, so you’re also located somewhere on this circle. Combine them together — and you are located at one of two points where the circles intersect. Now you’re getting close to your actual location. If you then find out your phone is 8,000 miles away from this satellite and located on this circle, you know that your location is where all 3 circles intersect. In practice, you need a signal from a 4th satellite to act as a safeguard correcting for any clock errors that may have occurred. Lucky for you, wherever you are on the planet, at least four satellites will be in a direct line of sight and able to send a signal directly to your phone. Sometimes there is a problem with GPS. Have you ever looked at Google maps only to find that the blue dot is a block away from your actual location? Clock errors, atmospheric disturbances, and even reflections from large buildings can interfere with the GPS signal, leading to accuracy problems. Most of the time though, GPS gets you from point A to point B — and everywhere in between. Just don’t use it while you’re riding.