but the Defense Department reserves the right to distort the satellite signals to reduce this accuracy to about 100 meters. For accurate navigation this distortion can be overcome by using differential systems. A differential system uses an accurate signal from a known location to correct the distorted satellite systems. Differential GPS systems have an accuracy of about 10 meters.
GPS systems for use as recreational navigation aids can be categorized as handheld and dash-mounted units. Both types of GPS receivers off the same accuracy and very similar standard features including current position, course, speed, distance to destination, etc. The biggest advantage of the handheld units are their size and portability. They generally are about a half-foot long and 3 inches wide and 1 inch thick. Typically they weigh less than a pound. Their power source is 3 to 6 "AA" batteries. This type of unit is handy to use on your boat or to take on a hike or outing. The big disadvantages of handheld units for use as a navigational aid is the size of their screen, the short life of their power supply (usually less than 8 hours), and difficulties that can be encountered in signal quality due to the antenna being blocked. Dash-mounted units wire into your boat's batteries, their antennas are permanently mounted in a position of maximum exposure, many makes are totally waterproof, and usually they have over twice the screen area of a handheld unit. This last feature is particularly important for those of us who don't want to strain our eyes to when trying to read our course in a moving boat. Another nice feature of deck-mounted units is that some of them come with charting systems that shows your current position on a map. This gives you a good intuitive grasp of where you are. Of course the main disadvantage of a deck-mounted unit is that it is not readily portable..
Prices for GPS systems start at about $300.00. Much like buying a car you can aid on features that make the sky the limit. A good dash-mounted unit with background charting capability will run close to $1000.00. If you opt to add differential GPS capabilities you will probably increase the price by $500.00.