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Boat Tech: What is GPS

How GPS Works
by Chuck Husick - Current as of May 2009

The GPS consists of earth based command and control stations and a constellation of polar orbit satellites. The system is operated by the U.S. Department of Defense. Each satellite transmits information describing its precise orbital position. All satellites use the same frequency, allowing the use of a fixed tuned receiver. Using position information received from at least three satellites, a GPS receiver / navigator can calculate its two dimensional position on the earth's surface, its latitude and longitude. Signals from four or satellites provide improved accuracy, including the ability to calculate height above sea level (3D position fixing). Although altitude capability is clearly not of great significance for marine use, the stability of altitude information can be a useful check on the overall accuracy of the system. Large altitude changes at times when you are not hauling your boat up or down a mountain may be reason to wonder a bit about the accuracy of the displayed position.

At one time, the number of satellites a receiver could track was a useable evaluation criteria in selecting a GPS. Most of today's GPS receivers are capable of tracking 12 or more satellites. In most circumstances a GPS will use data only from those satellites at a reasonable elevation above the user's horizon. It is rare for more than 8 satellites to be sufficiently above the users horizon for use by the GPS receiver, therefore virtually any GPS capable of dealing with 8 at one time will likely provide all the performance you require.

In order to work properly the GPS antenna must have a relatively clear view of the sky. In general, any position on or near the deck of a boat will be satisfactory. The presence of the mast, boom and rigging of a sailboat at least a few feet distant won't create noticeable problems. The antenna should not be located close to and at the same height as a radar antenna. Locations more than a few feet from a VHF or SSB antenna are usually satisfactory. It is not advisable to mount the GPS antenna high on a sailboat's mast. The antenna motion created by the roll and pitch of the vessel will complicate life for the GPS while offering no compensating advantage.

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