Global Positioning System, or GPS for short, is a navigational system which lets users determine their position anywhere in the world by analyzing the radio signals from a network of 24 orbiting satellites. Push a button, and a receiver small enough to hold in your hand will tell you where you are and how fast you are moving, displaying the information, if you so desire, on a map. Enter a waypoint a location the user programs in and many machines will tell you what course to steer to get to it, altering the course as wind and current changes your position, so that navigation is reduced to following the dotted line on a computer-like screen. These receivers, accurate to within about 300 feet in the form commercially available, have become standard equipment on vessels large and small in the past few years. Ferry captains use waypoints to record their routes, so that they can take the same one each time.
Many fishermen use waypoints to record the location of traps or nets.
Although the same jobs can be done, weather permitting, with nothing but a chart and a compass, GPS has come to be for many the easy, routine way to navigate. Those who have come to depend on it, however, may get a rude shock Saturday. Because GPS relies on a network of satellites whose position changes constantly, the receivers need to know the exact date and time to figure positions. Rather than keeping track of dates using a calendar of months, days and years, the internal timekeepers in GPS receivers count weeks, starting on the date the system was created, Jan. 6, 1979. As time goes on, tiny inaccuracies in the formula which calculates the satellites positions are compounded, getting larger and larger.