GPS -- Global Positioning System -- units show the user's position anywhere on the earth to withing about 50 feet horizontally and 500 feet vertically. These units receive signals from US military satellites which are about 12,500 miles from earth. There are 24 active satellites, of which the GPS unit needs to be able to receive a minimum of three to give one's position (or four, if verticaly position is desired). If the receiver has a good view of the sky, it might receive eight satellites at once. The more satellites it receives, the higher the accuracy of the reading. But even if the reception is poor, any GPS will give far greater accuracy than is need for backpacking.
However, while GPS is cool and has some utility for hiking, I find it unncessary. GPS receivers weigh around a pound. If even if one weighed two ounces, I wouldn't take one. Between the maps and my altimeter watch, I find it easy to always know my position on the PCT. The PCT is a heavy duty trail and as such, is usually fairly obvious. When it isn't, knowing roughly where one is and looking at the contour lines for the present altitude almost always gives the exactly location. My watch also has a digital compass, which is sometimes useful for reorienting oneself on cloudy days.
My watch's timer function is quite useful. I typically start the watch's count-up timer when I pass a location which is identified on the map, such as a trail intersection. Knowing my approximate walking speed (2 MPH) and the time-since-last-landmark I can make a good guess about my location. Add in the altitude, and I have a very good idea of where I am. My watch weighs 50 grams and is more useful than a GPS, and the battery will probably last the whole hike.