Does GPS Work Underground?by Fred Decker
Since the beginning of this century, lightweight handheld GPS devices have become inexpensive and widely available. Working from signals broadcast by a series of orbiting satellites, these miniature devices can triangulate your location anywhere on the earth's surface. Once your initial position is established, they can also track your movements, determine your speed and perform many other calculations. However, the technology requires line of sight with the satellites.
There are several places that handheld GPS units won't work or work poorly. If you're inside a large building or a mall, there is little chance your GPS device will be able to get a fix on enough satellites to calculate your position. The same holds true if you're underground, whether in an underground shopping complex, a tunnel, a subway or a naturally occurring cave. Even above-ground hikers can occasionally find themselves without a usable signal if they're beneath a rocky overhang or at the bottom of a wooded ravine. That's an unavoidable limitation of the GPS technology.
GPS devices gather signals from a series of sophisticated satellites originally placed in orbit by the U.S. military. Each is in a fixed or "geostationary" orbit around the earth and contains extremely accurate atomic clocks. Your GPS compares the signals from multiple satellites, calculates how far away they are from its own internal time, and uses those figures to pinpoint your position. Any time your handheld GPS can lock onto signals from three satellites, it can tell you where on the earth's surface you're located. If it can find signals from four or more satellites, it can also tell you your altitude.
Knowing that, you might expect your GPS to give you an error message when you drive through a tunnel or otherwise lose your signal. In practice, that doesn't usually happen. Once the GPS establishes your initial position, it continually recalculates your location in real time. That provides your speed and direction of travel. When you enter a tunnel, your unit loses contact with the satellites. However, it knows how fast you're going, on which street or highway and in which direction. From that, your GPS makes an educated guess -- traditionally called "dead reckoning" -- about your position, until it is able to reestablish contact with positioning satellites.
There are a number of applications for an underground navigation and location system similar to GPS, and several rival technologies have been investigated. For example, Japanese company Aichi Steel developed a device that calculates a position using the earth's magnetic fields and an accelerometer. Combined with GPS technology, it would theoretically provide seamless navigation above and below ground. An underground system developed in Switzerland for cave exploration places four transmitters over the cave, which can connect to GPS satellites in the normal way. They receive signals from a transmitter carried by the explorer and can triangulate that signal to provide an accurate position.
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