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Global Positioning System (GPS) is a navigational aid originally developed for the military. The system simply receives signals. It is the applied technology that gives the GPS its versatility. If you have ever used map and compass, you will understand a little about how the GPS works. In order to find your position on a map, you need to have three points of reference. The intersecting line from the reference points is where you are. Map and compass work uses triangulation (bearings), GPS uses trilateration (distances) to calculate location. Satellites orbiting the earth emit unique signals that can be received by a GPS. The GPS Software interprets the signal, identifying the satellite that it came from, where it was located, and the time that it took for the signal to reach the system. Once the receiver has both time and distance it begins to determine position. Three satellites provide the intersection point and the fourth is used to check that the positioning is accurate. Accuracy depends upon the synchronization of atomic clocks in the satellites with the clock in the GPS System. Although the clock in the GPS is not atomic, utilizing the fourth satellite gives it that functionality as the internal clock adjusts itself to correct any discrepancy discovered. If you are considering purchasing a GPS, make sure that it can be updated easily. This is especially true if you buy a multi--function GPS or one that is used where conditions change regularly. An in--vehicle GPS soon loses its usefulness if it is not updated as road systems change. Updates vary according to the device being used. They can come in CD/DVD packages or as computer downloads. The user can purchase maps specific to the area in which the GPS will be used or a range of maps and routes. These are available from GPS software companies who will charge proportionally to the sophistication of the software. GPS Units vary in price according to their usefulness. It is possible to buy units for less than one hundred dollars to units costing more than one thousand pounds. What your needs are will be a factor in the cost of your unit. If you are a backpacker then portability is a major consideration. If you are a trucker, you need to be able to find a delivery point as quickly and conveniently as possible. Whatever device you go for, cost is generally related to quality. Buy the best you can afford.
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