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The GPS (Global Positioning Satellite) system was originally designed and built by the U.S. Department of Defense exclusively for military applications. It was designed to provide guidance for missiles, smart bombs and other military toys. The system itself consists of a number of satellites in a stationary orbit around our planet. While these satellites send out continuous signals towards the earth, a GPS receiver can seek out and lock onto the satellite signals. Once the GPS obtains a signal, it can use the known position of the satellites to calculate its own position on earth with pinpoint accuracy (within about 30 feet).

After acquiring the necessary satellite signals (which usually takes a minute or so), a GPS unit provide the following information:
*an accurate readout of the user's position - displayed as latitude and longitude or as a UTM grid reference. (Since all topo maps have a 1 km square UTM grid overlaid on them, most people use the UTM reference.) 
*precise elevation - which is particularly useful in paddling.
*exactly what the time is - since GPS calculations are based in part on time.

GPS units allow you to store the coordinates of various places you've visited before, or the coordinates for places you've never been to. Then when paddling or traversing in the wild, you can simply tell the unit which of those points you want to head towards, and it will guide you the most direct route to that point. All along the way, it provides you with a continuous readout of where you are, how far you are from that point, and which way you need to turn to get there.

The question remains: Are GPS units the navigational lifesaver that some imply they are, or is the GPS still just an elaborate toy best suited for techies or those who have every gadget under the sun? 


OLD VS NEW GPS
It is important to note the differences between the earlier GPS units and the newer 12-channel receivers. The earlier units relied on "multiplexing receivers," which meant that they used one or two channels to do each of the following: 

1. Track satellites.
2. Find a satellite.
3. Lock onto it.
4. Read it.
5. Then release that satellite to find the next.

Since a GPS needs at least three satellites to achieve a two dimensional position fix and at least four satellites to achieve location and elevation (a three dimensional fix), the older GPS units had to piece together and then average all of this navigational information. As a result, the earlier units typically experienced more problems and provided less reliant data.

On the other hand, the 12-channel receivers sold today use each channel to individually lock onto and hold a satellite. The best four signals are used to provide accurate, reliable information almost instantaneously.


THE U.S. GOVERNMENT'S ROLE IN GPS
When the GPS system was originally put into place by the U.S. Military, readings on GPS units were subject to "selective availability." This meant that the U.S. Department of Defense purposely degraded the accuracy of civilian GPS units, making the signals they received less accurate than they actually could be. While military users could get pinpoint accuracy from GPS units, those without top secret clearance received a signal with purposely introduced inaccuracies within 100m (328 ft.). 

However, in May 2000, the U.S. government did away with selective availability, and the accuracy of GPS units was increased tenfold. What was once a 100m (328 ft.) accuracy rating is now a 10m (30 ft.) accuracy.

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