Will iPhone GPS Work When It's out of Range?

The Global Positioning System, or GPS, is a system that the military developed and is now available for civilian use. The system consists of 31 satellites in orbit as of early 2013. The system arranges the satellites so that, at any given moment and at any spot on the planet, at least four of the satellites should be visible in the sky. Your GPS receiver picks up signals from these satellites. Software in your GPS device uses the position of each satellite, the time each signal was emitted, the amount of time it takes the signal to reach your device, and the speed of light to calculate your position. The software in your device then translates the information into latitude, longitude and altitude coordinates, and uses map data to tell you where you are. If the software determines your coordinates are, for example, 40.7484° N, 73.9847° W, map data would translate that to 350 5th Ave., New York, N.Y., which is the Empire State Building.

Because GPS uses satellite signals, your device must have a clear line of sight to the satellites in order for GPS to work directly. Because of this limitation, the iPhone uses complementary systems to help out when GPS is unable to work well, such as during a thunderstorm or when you're indoors. During these times, the iPhone uses data from the nearest Wi-Fi network or the nearest cell towers to locate your phone. All iPhone models from the 4Gs to iPhone 5 also use GLONASS, which is a Russian satellite location system. GLONASS has 29 satellites in orbit, of which 22 are operational as of early 2013. These additional satellites give more speed and precision to the iPhone's location function.

Starting with the 3GS, all iPhones also have a digital compass; the compass works using magnetic fields, and will therefore work just as well out of cell tower range and without satellite visibility.

Apps that temporarily download maps on an as-needed basis save hard-drive space, but will leave you without a map if you're out of network range. If you're planning a camping trip in the wilderness, be sure to get an app that stores its maps on your hard drive and periodically updates them, as do GPS devices such as those by Garmin and TomTom. TomTom's iPhone app stores maps on your phone's hard drive, as does the StreetPilot app from Garmin and the Navigon app, originally published by an independent company but now owned by Garmin. The GPS Kit app from Garafa is designed for outdoor uses such as hiking, and offers a map-caching option for offline use. With a little smart app shopping ahead of time, your iPhone's location chip can tell you where you are, wherever you may roam.