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Old 11-06-1999
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A Few GPS Basics

Navigating with GPS means having accurate, all-weather, real-time position fixes that are continuously updated. Now introduce GPS with electronic charts and you have a very powerful navigational tool.

With electronic charting, you can view the constantly updated GPS position superimposed on the nautical chart. This spares you the work of plotting latitude and longitude coordinates and laying out your projected courses on a paper chart. At a glance, you can see where you are, where you've been and where you're going.

The Global Positioning System (which does indeed have global coverage) is operated by the Air Force. With 21 satellites now orbiting Earth, GPS is the military's primary radio-navigation system. At any given time, at least four satellites are available to the GPS navigator.

Each satellite transmits a UHF signal. In a manner similar to loran, a GPS receiver calculates position based on the differences in the time that the signals are received. The information from three satellites is as accurate as a calculated, two-dimensional fix. As more satellites are in view, accuracy increases (when altitude-very useful to the Air Force-can also be established).

The Global Positioning System provides two levels of positioning service: Precise Positioning Service (PPS) and Standard Positioning Service (SPS). The military has access to the encrypted Precise Positioning Service, which provides accuracies of better than 20 meters (66 feet). But for use outside the military, the government applies Selective Availability to the PPS signal. This essentially degrades it to the Standard Positioning Service signal.

The Standard Positioning Service signal provides accuracy of 100 meters (about 330 feet) with 95-percent certainty. This means that if you draw a circle with a 100-meter radius around your actual geographic location, 95 percent of the GPS position measurements for your location would fall within that circle. What this means to you is that your boat's position, as shown on the electronic chart, should be viewed as an approximate location, not an exact one. To be safe draw an imaginary circle of ambiguity with a radius of 100 meters around your on-screen position. It's important to recognize that you could actually be anywhere within that circle. Most ship icons on the electronic chart can be increased in size to represent this circle of ambiguity.

The accuracy of GPS can also be affected by factors such as the tendency of radio signals to bounce from trees, buildings, terrain features and other obstructions (also called multipath propagation); and atmospheric distortion. Through advanced filtering and processing techniques, these effects can be minimized to some extent by GPS receivers.

Next time we'll talk about differential GPS.

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