Global Positioning Satellites
Global Positioning System, or GPS, satellites transmit signals that are received by a variety of equipment. Handheld units, automobile units and wrist receivers all use satellite signals to help find the way. The signal only goes one way, as GPS units only receive and do not send or transmit a signal. GPS units, whether they be mounted or handheld, require an unobstructed view of the sky. In other words, there can be no solid objects between the antenna of the GPS unit and the satellite that is sending the signal. As this is the case, GPS units are typically only used outdoors. A failing of GPS units is they often do not perform well within forested areas. Since outdoor activities is a major reason many people purchase GPS units, this is a factor that should be strongly considered.
How GPS Signals Work
Global Positioning Satellites transmit information that includes its location and the current time. GPS satellites are dependent upon very precise time synchronization. For this reason, clocks on board the GPS satellites are synched with atomic clocks at the U.S. Naval Observatory. The result is that all GPS satellites transmit a signal at precisely the same time. These signals, moving at the speed of light, will reach your unit at different times due to the distance each satellite is from your specific location. The handheld unit calculates the amount of time it takes for the signal from up to four satellites to reach the unit. By estimating this amount of time, a triangulation or three dimensional reference point is plotted. Many units will indicate the number of satellites that are being detected by a unit at any given time. It is not uncommon for up to 8 of the 24 orbiting GPS satellites to show as a signal source.
Determining Your Position
The transmission from the GPS satellites not only includes time but distance. This is crucial as a GPS must be able to locate satellites. The GPS unit will estimate the distance to a satellite and determine its location at a point on the satellite. By calculating its location on several satellites, the unit determines an intersection point and thus provides a pinpoint location for the unit and user.
Accuracy of GPS
So exactly how accurate is your GPS unit? Just how close can it come to giving you an exact reading on your location? A rule of thumb is that the type of receiver being used will greatly determine the accuracy. For example, most handheld units will be accurate to within 30 feet or so. However, other types of units such as those mounted in vehicles, are more accurate, due to the use of a stationary receiver unit. This unit is used to provide constant calculations and corrections which allow these units to provide much greater accuracy. In fact, the accuracy can be increased to as close as 5 feet.
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