Map Town Ltd. is an exclusive retailer of Garmin International Units. Map Town Ltd. features outdoor recreation, marine, nautical, and pocket PC GPS receivers with compatible software and accessories.
Global Positioning System (GPS) is a satellite-based navigation system made up of a network of 24 satellites placed into orbiting the earth. GPS was originally intended for military applications, but during the 1980's, the government made the system available for civilian use. GPS works in any weather conditions, anywhere in the world, 24 hours a day, 365 days of the year.
The group of 24 satellites beam radio signals from their positions above the earth back to earth's surface. The GPS unit detects the radio signals sent from the satellites and calculates the unit's position. A minimum of 4 satellites cover any spot on the earth at all times.
How Accurate is a GPS Unit?
A GPS receiver calculates it's three dimensional position on earth: latitude, longitude and altitude. Positions are calculated based on the difference in time between when the satellite sent the signal and when the receiver detected the signal. The position the receiver calculates for latitude and longitude (horizontal position) is accurate to approximately 15 m (49.2 ft) without WAAS or about 3 m (9.8 ft) with WAAS. The position the receiver calculates for altitude (vertical position) is accurate to approximately 22.5 m (73.8 ft) with out WAAS or about 4.5 m (14.8 ft) with WAAS.
What is WAAS?
Wide Area Augmentation System is a GPS based navigation and landing system that provides precision guidance to aircraft. WAAS is designed to improve accurracy and to ensure integrity of information coming from the GPS satellite. There are land stations in the US that calculate the amount of error for any GPS satellite. The corrections are up-linked to WAAS satellites that beam the correction to continental USA and Alaska. WAAS enables GPS receivers use the correction to make their positions more accurate.
