The Global Positioning System - By: R Harlow

The advent of the Global Positioning System, or GPS as it is more commonly known, has changed the fundamentals of marine navigation unlike any other marine equipment before. Although long range navigation systems such as Decca, Loran and SATNAV were in use prior to the advent of GPS, none of them provided the sort of reliability and accuracy that the GPS provides. Therefore, more traditional methods of fixing a vessel's position such as star and sun sights, and dead reckoning were still considered to be the primary means of position fixing.
The GPS has completely changed this scenario and is now considered to be the primary means of position fixing for most marine vessels. A GPS can reliably fix a vessel's position to within 3 meters, 90 per cent of the time. Further accuracy to less than a meter can be obtained with the use of Differential GPS (DGPS). These levels of position fixing accuracy are sufficient to allow navigators of most vessels to confidently use the GPS to ensure their vessel's safety in open waters. The GPS can also help fishing vessels accurately locate areas where fish spawn.
Although the GPS is almost a miraculous piece of equipment, it must be borne in mind that like any other piece of equipment, it can be prone to certain errors. Relying on the GPS position alone without verifying your position through other means can sometimes lead to disastrous consequences. In order to fully appreciate the functionality and limitations of the GPS it is necessary to have an idea of how the GPS functions in the first place.
Most GPS receivers rely on the NAVSTAR system to obtain the vessel's position. This system, which is operated by the US Department of Defense, consists of 24 satellites that orbit at a height of about 12,550 miles above the surface of the earth. These satellites are so placed that at any given point in time, a GPS receiver anywhere on the earth will have access to at least four of these satellites. This is crucial to positional accuracy, because in order to determine a vessel's position accurately, a GPS receiver must be able to access at least 3 satellites simultaneously.
Each orbiting satellite continuously sends information in the form of messages that can be decoded by GPS receivers. These messages contain information on the time that the message was sent, the location and general health of the satellite, and a rough map of the location of all the GPS satellites. The GPS receiver uses the time difference between when the message was sent to when it was received to calculate its distance from the satellite. In this manner, when simultaneous messages are received from more than one satellite, the GPS receiver is able to calculate the position of the vessel through the process of triangulation. In order to calculate the position of a vessel in two dimensions, i.e., in terms of latitude and longitude, a GPS receiver must receive a signal from at least 3 healthy satellites. The signal from the 3rd satellite is used to compensate for any time discrepancies when determining the vessel's position. All GPS receivers are fitted with an audible or visual alarm that will warn the user in case a sufficient number of healthy satellites are unavailable to fix the vessel's position.
GPS receivers are available in all shapes and sizes, from handheld models to robust fixed mounted ones. Since this is one piece of equipment that you will be relying on almost all the time to fix your vessel's position, it makes sense to invest in a decent model from a reputable manufacturer. Some of the best-known manufacturers of GPS receivers include Furuno, Garmin, and Raytheon Marine.

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