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GPS or the Global Positioning System was originally put into service by the United States military for use as a guidance system for the the branches of the U.S. military. A few years after it was up and running it was made available to civilians and the use of the system was released to the public. The complexity of GPS is hard to explain without going into advanced math and physics. GPS works on the principal of triangulation. By having 3 different vectors or points to measure a single point from, you can determine the estimated location the point. The math involved is applied to GPS to calculate the location of an object by using precise timing. Each GPS sat sends out a constant 30 second telemetry message that consists of it's own clock time, position in the atmosphere and other data.

When a GPS unit is powered on, it initializes it's self and begins to check it's catalog for the most visible sat. The built in catalog is a listing of dates and times, each date and time tells the GPS what satellites is SHOULD be able to see at any particular time. It then takes these satellites and places them in a cue. Usually on the unit's LCD you will see a listing of the satellites along the bottom of the screen. As the unit sees the satellites it will begin to pop up a signal strength meter to show how well it sees the sat. As it begins to see the sat it starts to receive and decode the 30 second telemetry message. One Garmin units, it first shows a gray bar and when the unit has received the message for that particular sat, it then darkens the signal bar to show that it has completely downloaded the message and decoded it. The GPS must receive at least three of these in order to get a accurate position or "fix". If altitude is required, then a fourth sat is required to make that calculation. Once the unit has a fix it will then begin to show the current longitude and latitude of the reciever. Most good units will update the position the screen just about every second. Also, once it has a fix the GPS should also start spitting out complete NMEA sentences from the serial port. These are used mainly for amateur radio and direction finding applications. These sentences can also be fed into a computer's serial port for real time mapping and tracking. I do this with my laptop in my truck. There are many NMEA strings that can be sent. The most common are $GPGGA and $GPRMC. These two sentences provide the most information at once such as position, elevation, heading and speed.

There are many companies that manufacture GPS units. One of my favorite is Garmin, they have been around the longest and even produced the first GPS unit to go in Desert Storm and be used by U.S. Troops. Magellan also produces nice in-expensive units, although some do not have a serial out port and do not have most of the functions of a Garmin. Delorme also produces units. They are mainly for the mapping software they produce. These units are small and usually come in a yellow water-tight case. They are good for trackers that need to be tough and don't require an LCD display. I bought my Tripmate from Delorme's web site for $50. It's a reconditioned model they sell without software.