In the past 10 years more and more people have discovered the joy of being outside. Fishermen, hunters, hikers, and bikers have all found their time outdoors more enjoyable, successful and safer due to the introduction of compact GPS units. The choices available to the consumer are quite overwhelming, but the competition within the industry has given us the best possible product for a reasonable price.

Over the next few months we will be discussing the basics of GPS usage, and even some advanced features and applications. But before we can get to those, it is important that all of us understand how GPS works-what it can do and what it can’t do. Let’s start with how GPS actually works.

Your GPS works by receiving position information from a manmade constellation of 24 satellites in geosynchronous orbit. At any given time, and at any place on the earth, 12 of these satellites are above the horizon and available for use. The satellite system is operated and maintained by the Department of Defense. The satellites originally transmitted 2 signals, a military signal and a slightly degraded civilian signal. This degradation was called "Selective Availability" or "SA" and was removed in the spring of 2000. All GPS units ever sold are now capable of receiving the more accurate, non-degraded signal.

To receive a position your GPS unit must be able to "see", or acquire, at least 3 satellites. With three satellites acquired your GPS will give you a 2-dimensional reading, stated in latitude and longitude. When your GPS acquires 4 satellites, your GPS will show you a 3-dimensional reading, stated in latitude, longitude and altitude.

Although the satellite signals being sent to your GPS receiver are quite accurate, some problems with accuracy can still occur. Systems have been produced that help the GPS obtain the most accurate position available.

The first system is call Differential GPS or DGPS. DGPS uses land based radio towers to better lock in on a position. By receiving signals from known land based, fixed position radio towers, the DGPS can compensate for any errors present in the satellite signal. DGPS offers consistent accuracy around 10-15 feet! The problem with DGPS is the equipment. Most units sold now are "DGPS Ready". This does not mean that your unit will receive the DGPS signals as is, just that it will accept a DGPS antenna. The DGPS antenna is usually quite bulky-sometimes 3-4 feet tall and looks like a marine radio antenna. This fact alone makes DGPS impractical for GPS users not in a boat or vehicle. Also, DGPS towers are basically placed along the coasts and the Great Lakes, making DGPS useless in some locations or inland about 50 miles.

The second system is called WAAS or Wide Area Augmentation System and basically works like the DGPS system. Again, land based radio towers are used to obtain a highly accurate correction to any error from the satellites. The WAAS system uses 25 ground position stations and a master station on both coasts. The correction information from the 25 ground stations is sent to one of the master stations. Now here is where WAAS will outperform DGPS. Instead of your GPS receiving radio signals from the ground based radio station, the master stations send the collected correction information to one of two stationary satellites which then "beam" the correction information back to earth. Because it uses a satellite signal instead of radio signals, WAAS has one big advantage. The antenna for receiving the WAAS signal can be built right into a GPS unit-even a small handheld- and adds very little weight and bulk. Since the information is sent by satellite, the WAAS system will work virtually everywhere and has a consistent accuracy of less than 10 feet!

Hopefully this little introduction will help you understand how the GPS system works. In the next article we will discuss what equipment and features are needed to have a good GPS unit and to help you with buying a new GPS unit.