Now that you’re considering getting a gps chartplotter, you’re probably more aware of everyone talking about their GPS, or getting a new GPS, but what is GPS and how exactly does it work?

GPS stands for Global Positioning System, and consists of an array of 24 satellites in orbit around the Earth. The satellite network was originally placed in orbit by the US Department of Defense and intended for military purposes. The huge benefit to the military was that you could determine your position anywhere on the Earth, regardless of weather conditions, day or night. However, in the 1980’s, the system was made available for civilian use, even though it would not be until 1995 that the full compliment of 24 satellites would be in orbit.

The 24 gps satellites orbit the Earth at an altitude of ~11,000 miles, travelling at a speed of ~7000 mph. This allows the satellites to complete two full orbits in just under 24 hours. The satellites are positioned so that no matter where you are, you should be able to receive the signal from at least 6 satellites . It is the ability to receive multiple satellite signals that gives the gps system its accuracy. But how?

Each satellite continuously transmits a signal containing its precise location and time. All satellite signals are monitored by 3 control stations around the world, and are relayed to the master control station in Colorado Springs, which maintains each satellite’s highly precise orbit.

Your car’s gps, your boat’s marine gps chartplotter receives these satellite signals and determines your position through a process called satellite ranging. Since each satellite transmits both time and location, your receiver calculates the time delay from each satellite, which it then converts to a distance. Since each satellite acts as a reference point in space, by knowing your relative position from at least 3 different satellites, your gps will calculate where on Earth those relative distances intersect, This is displayed as your location. When you simultaneously receive a signal from at least 4 different satellites, your gps can determine your 3-dimensional position (longitude, latitude, and altitude) and your velocity. Naturally, the more satellites you receive a signal from, the more accurate your position and speed can be determined. Standard gps technology can calculate you location to within 50 ft (15 meters).

Some newer gps systems are WAAS compatible (such as a GPS WAAS chartplotter). WAAS is the Wide Area Augmentation System, which consists of 25 ground based reference stations (and two master stations – one on each coast of the United States) that provide corrected gps signal information. The basic gps signal is corrected for orbit and clock drift, plus transmission delays caused by changing conditions in the atmosphere. Use of this corrected signal increases the accuracy of the gps system to less than 10ft. It should be noted that all WAAS stations are located in the Northern Hemisphere, so there is no signal correction available for those navigating in the Southern Hemisphere.

There is no fee for using either the gps or WAAS system. All you need is a compatible receiver and you’ll soon not only know exactly where you are, but you’ll also be able to navigate from one location to another. The ability to navigate from your known location to a specific destination on the open ocean is the primary application of the gps system when used in a GPS Chartplotter.