Using GPS Units

An Intro - So you have heard about this thing called GPS. When someone talks about it you nod your head knowingly, but you really to get a clue...
GPS Constellation The Global Positioning System (GPS) is a constellation of 24 satellites originally put into space by the US AirForce. (There is also a constellation of satellites put up by the Russians called GLONASS .) The satellites are kept in a very precise location in their orbits (by the 50th Space Wing) and their only job is to send out a time signal (at the tone, the time will be...) that is accurate to within millionths of a second.
A GPS receiver (civilians can get them at sports and electronics stores) listens to these time signals and calculates how long it took the signal to get to the receiver. Using this time and the speed of light, the receiver calculates a distance. By finding the distance to 3 or more satellites and knowing the exact position of the satellites (the receivers have this built in), the receiver can calculate your position.
Naturally, any error in these measurements causes error in your calculated position. There are lots of sources of error, but for non-military users, the biggest source is an intentional error called Selective Availability(SA). SA is basically the government lying to you (there is something new). The satellites have a timetable which tells them all to screw up by a certain amount. (The actual amount is constantly changing to prevent users from calculating it and factoring it out.) By fudging the times, the receivers can't actually figure out where you are, and thus your receiver is always off by 50 to 100 meters.
All good capitalists are always trying to think of new ways to beat the system and so they invented Differential GPS (DGPS). DGPS works like this: put a GPS receiver on a precisely surveyed location (that way you know exactly where the receiver is), compare the position that the GPS receiver calculates to the known location and you have the amount that satellites are lying by. Broadcast this difference to other receivers (over FM radio), and those receivers can then subtract the error and know almost exactly where it is (1 to 5 meters).

GPS in Depth - With all this talk about civilian users we are forgetting the real intent of the GPS system: Military Use. The military versions of the receivers are very accurate (we've all seen the movies of the Smart Bombs and Tomohawk missiles used in Desert Storm). They reason that they are more accurate is that the GPS satellites actually send out various signals on two frequencies. (The frequencies are called L1 and L2). L1 carries the standard Coarse Acquisition (C/A)signal that civilian receivers use as well as an encrypted military code called PY code. L2 also carries the PY signal.
A military receiver will use C/A code to lock onto a specific satellite (by the way, military receivers have the code to ignore the SA error) and will then use the PY code to calculate its position. By using both L1 and L2 frequencies, the receiver can eliminate atmospheric interference thus reducing error even further (1-2 meters). All told the system, is fairly slick.

The Receivers - GPS Unit For a recreational user all these facts don't make much difference. What you really want to know is which receiver to buy and how to use it. Well, there are a lot of options available to you but there are a few things to consider:

ANTENNA: One important thing to think about is the antenna. If your GPS receiver can't hear the satellites then it won't do you any good at all. Trees, buildings, and (most importantly for four wheelers) car roofs can all totally block a GPS signal. Not surprisingly, many people who use GPS in their car use an external antenna. I would consider this to be a valuable option "must have" on your hardware.
WAYPOINTS: The way a GPS receiver records your route is by recording your location at set intervals and then "connecting the dots". These positions are called waypoints. If your hardware doesn't have enough memory for these points then you will lose part of your route.
EASE OF USE: The last thing you want on your trip is to spend driving and fun time fiddling with gadgets. Try out potential receivers to see how easy they are to use so that you can spend your time later doing more productive things.
OTHER OPTIONS: Depending on how much you like using GPS there are other options you might consider. Can you connect your GPS to a computer for downloading and printing routes? How about using your GPS with mapping software (both on and off road) to navigate? Does your GPS have built in maps? Can your GPS use a cigarette lighter power adapter or do you need a lot of batteries?
NUMBER OF CHANNELS: This is a measure of how many satellites your receiver can possibly lock onto at any given time. One thing to know is that only 9-10 satellites are ever physically in view so getting a receiver that does more than this is a waste of money!

Most importantly, you want to be able to have fun. Get a GPS that will meet your needs and try not to get lost. If you do, be glad you have your toy. One thing to note, GPS receivers give position very accurately but are not that good at showing direction. If you do get lost, having that good old fashioned compass handy will be very VERY helpful.