GPS is a free service, provided by the US Department of Defense allowing the exact definition of any GPS receiver's position within a digital map. It takes 21 GPS satellites to cover the entire surface of the earth, with an active backup of three satellites. By continuously aligning the current position on the digital map with the route calculated prior to the start, the navigation software can easily provide the user with visual and audio instructions.
For the navigation to be started, the GPS receiver requires all information broadcast by the GPS satellites. Apart from positioning data, these are correction factors as well as almanac data. The complete transfer of a full set of data takes 12.5 minutes. This represents the maximum time it takes a GPS receiver to determine its first position, if either no satellite data are stored or they're outdated. This amount of time will be required when first powering on the receiver, after having it switched off for a longer period of time, or when switching it back on after having significantly changed the device's position while disabled (e.g. during air travel).