How Does a GPS Work?

Author: Thomas Rodgers

Your GPS unit communicates with satellites, which send out radio signals. Since the speed of those radio signals is a known factor, the GPS receiver is able to use that speed to calculate its distance--and therefore the distance of the one holding it--from a satellite. This calculation puts you and the GPS in an imaginary sphere.

So imagine yourself holding your GPS and standing in a big circle (See: "A" in the above image) on the ground. This reflects the information from a single satellite.

The GPS will do the same thing with the other satellites with which it can communicate via a clear line-of-sight. So for the second satellite, imagine another circle which intersects with the first. (See: "B" above) This second calculation will put your near the intersection of the two circles (See: "A^B").

Then add a third satellite and another intersecting circle. This third calculation will place you in the intersection of the three circles. This is considered a "2D" (or two-dimensional) mode, figuring your location horizontally only. In this case, you can input your elevation, if you know it, for more accuracy with the GPS unit. Keep in mind that 2D navigation can be unreliable.

If your GPS can communicate with 4 or more satellites, your location will be more accurate and you'll be in "3D" mode, which will include an elevation calculation. This is what you're aiming for.

Your unit display will indicate whether you are in 2D or 3D mode. If it's displaying 2D, try moving around and getting into a more open area, free of obstacles like tall trees, to hopefully get to 3D mode if at all possible.

**You should always check your "EPE" or "Estimated Position Error," which will display on your GPS, usually right on the same screen that indicates how many satellites it's communicating with--the satellite page. This will also show you how accurate, or perhaps inaccurate, the position fix is. An EPE of 20 feet or less is good. The lower the EPE the better.

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Article Source: ArticlesBase.com - How Does a GPS Work?