GPS units navigate by satellites that circle the earth twice a day at an altitude of almost 11,000 miles, yet the system can take you back to within a boat length of a location you saved in memory. Thinking about that makes my head hurt, so like most other fishermen, I prefer to just take the system for granted.

Things weren't always this good. Back when GPS was introduced, it had a stated accuracy of 100 meters or better, not impressive enough to get many folks to give up their old Loran-C units and take a chance on the new system. Accuracy was intentionally degraded by a feature called Selective Availability (SA), reportedly to prevent an enemy from using our own system against us.

That feature was set to zero by presidential order during the Clinton administration, and accuracy improved tenfold. Getting within a hundred feet of a desired location was much more desirable, and the system began to attract more attention.

GPS was billed as the new space-based system that would eventually replace Loran and other ground-based forms of radio navigation, but ships at sea and planes in the air needed better accuracy. Differential GPS was the answer and, ironically, it gave the spaced-based system new ties to the ground.

The maritime industry adopted DGPS using towers built on surveyed locations near major harbors and inland waterways. GPS receivers in the towers record their position as provided by the GPS satellite system. They then compare the received position with their known surveyed position, and consider any difference as position error. If, for instance, GPS says the tower is 100 feet south of its known surveyed location, then DGPS transmits 100-foot position corrections to all DGPS receivers up to 350 miles away.

The Wide Area Augmentation System (WAAS) was built to support the precision approaches and landings of aircraft. This system uses ground-based receivers in surveyed locations strategically located near airports.

Both differential systems can increase GPS accuracy another tenfold, getting us to within 10 feet of a desired location, but WAAS has won the overall popularity contest. In fact it's difficult to find a new GPS receiver that doesn't come out of the box with WAAS built in.

Marine DGPS usually requires the installation of a separate radio receiver and antenna to pick up corrections from local towers and pass them on to the main GPS unit. WAAS corrections are broadcast from satellites positioned over the equator, and are picked up directly by antennas built into WAAS GPS units or their antenna modules.

Boats cruising offshore or on open freshwater lakes typically have no trouble getting WAAS reception. The farther north you go, the lower the satellites broadcasting WAAS corrections are in the southern sky and the shorter an obstruction needs to be to block the signal.

If your favorite waters have cliffs or mountains on the southern shore that block WAAS reception, it might be worthwhile to find out if you are close enough to a marine DGPS tower to receive corrections. Go to www.navcen.uscg.gov/dgps/coverage/Default.htm and click on your state to see a map of current DGPS coverage.

Getting top accuracy from your GPS unit requires following a couple of simple guidelines. First, make sure it is set to use the appropriate type of differential corrections (WAAS or DGPS). Most units allow you to set an alarm that lets you know when correction signals are not being received, and using it can keep you from overestimating your unit's accuracy. It is critical that you mount your antenna where it gets as close to a clear, 360-degree view of the sky as is possible on your boat. If this isn't possible, give it your best shot. If your boat has radar, don't position the GPS antenna on the same plane as the radar's scanning antenna.

Finally, no matter how good your GPS accuracy gets, be sure to verify your position with your depth finder, your eyes and (most importantly) your common sense.