The use of the Global Positioning System (GPS) has revolutionized navigation in the last decade. For the first time in history even novices are able to pinpoint their position anywhere on Earth any time, day or night. Mariners can confidently find their way around hazards even in fog or steer directly toward an unseen destination beyond the horizon. GPS has effectively transformed what was once an art into an exact science.

To retain a military advantage, the U.S. government adopted a technique known as Selective Availability, or SA for short, to reduce the accuracy for civilian users. SA was implemented to prevent unfriendly nations from using the GPS system for acts of aggression against the United States. The amount of error can be varied by advancing or retarding clock information transmitted to users through the satellites. Even with SA, GPS is far superior to anything available in the past.

Now, the best navigation system in the world just got better. In a surprise move, United States president Bill Clinton announced that SA will be turned off as of May 2, 2000. The Clinton Administration now believes the importance of GPS as a world utility far outweighs the risk to national security.

All this comes as good news to boaters. Previously, civilian users could expect an accuracy of about 100 - 200 meters from their GPS receivers, good enough for most applications. Without SA, 10 - 20 meters will be common. Best of all, no changes to existing equipment are needed.

If greater precision is required, mariners can take advantage of a refinement known as Differential GPS. This makes use of an accurately located shore-based station transmitting correction information to specially equipped GPS receivers nearby, reducing the error to 1-3 meters. The U.S. is currently in the process of expanding its network of DGPS stations to cover more coastal and inland areas. This will benefit companies developing new GPS-based services such as rental car guidance systems and freight tracking. There may never again be an opportunity to get lost.

While navigation may appear to have reached the ultimate state of perfection, mariners are reminded that GPS should be regarded as an aid and not to rely on one method for position information. Constant checking and diligence is still required by the 21^st century seafarer.