Of course, that's the optimal scenario. In reality, there are many possible errors that can occur in the signal between its transmission and its reception - it's a radio signal, after all, and, just as a cell phone will drop a call when the signal is too weak or distorted, GPS can be subject to similar challenges. For example, the speed of light is constant in a vacuum, but it will vary slightly as it goes through the air, and especially through clouds. There are a number of ways that GPS can correct for these errors. These techniques include observing as many signals as possible as often as possible, and clever processing algorithms. What's more, the civilian and military services provided by GPS vary greatly in accuracy. The civilian system was even "dithered" to produce inaccurate results for many years - a process called "selective availability", or "SA". SA is now turned off, and accuracy to a few meters is possible even with relatively inexpensive products.
There are some excellent tutorials located at Trimble and Garmin. The leading publication on GPS is GPSWorld. The official GPS sites are (among others) http://tycho.usno.navy.mil/gps.html and http://www.af.mil/factsheets/factsheet.asp?fsID=119, and you'll find lots of other information on GPS on the Web.
There's an excellent chance that, if you don't already, you'll own a handheld GPS device in the future; such capability might even be built into your cell phone or car (as is already the case with GM's OnStar service). Keep in mind, though, that GPS requires a clear view of the sky. The signals from the satellites are so weak that they can't penetrate solid objects like buildings. But, no matter - GPS is now so accurate and so inexpensive that its value and utility will continue to spread to a vast array of applications around the globe.
Copyright 2003 by Farpoint Group - All rights reserved.