Global positioning system (GPS) devices triangulate your position by calculating the signal from your device to two or more satellites. When you are moving, your direction on the ground can be computed in relation to true north; this is called your track angle.
A GPS track angle is defined as a measurement of the direction a vehicle is moving in, which also determines average velocity. It's not the same thing as the direction the vehicle is pointed in. The track angle is sometimes referred to as your "heading," although this is technically incorrect. Heading should be calculated based on the direction your device is pointing (as with a magnetic compass) and therefore cannot be determined from the direction you are moving (technically called the "velocity vector").
GPS triangulation can be used to pinpoint your location on a geographical grid-- commonly a map overlay, which can provide real-time driving or walking directions. Your track angle is determined based on subsequent measurements of your location over time, which determines your direction and average velocity.
Global positioning systems work by connecting to satellites in geosynchronous orbit over the Earth's equator. Geosynchronous means that the satellite remains in the same location in relation to the ground, in sync with the Earth's rotation. In theory, this provides global range to GPS devices; in practice, line of sight to at least one satellite is necessary.
GPS devices have become increasingly smaller since their introduction and can now be included on a single computer chip. This alllows for their inclusion on handheld devices such as cell phones or PDAs.
GPS devices must correct for many potential sources of error when calculating your position, including deviation of your signal through the Earth's atmosphere and varying distance from your location to two or more satellites. Even the speed of light isn't fast enough to avoid discrepancies of microseconds between the time your GPS device registers with satellites, and these microseconds must be accounted for before your location can be calculated.
Amazingly, GPS triangulation serves as an additional proof of Einstein's theory of relativity. Einstein predicted that time would pass differently for objects moving at faster speeds, and GPS satellites prove this: At over 8,600 miles per hour, relativistic effects cause satellite onboard clocks to run at a difference of 38 millionths of a second per day. GPS engineers counter this by deliberately setting the frequency of onboard clocks to run slower than real time when the satellite is at rest, which causes their clocks to run in sync with earthbound measurements.
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