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Re: GPS accuracy
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Posted by Larry West [Email] (#1140) [Profile/Gallery] (more from Larry West) on Mon, 14 Jun 2010 06:17:42 Share Post by Email
In Reply to: GPS accuracy, yaofeng, Sat, 12 Jun 2010 08:24:00
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GPS navigators are wonderful pieces of technology, but they are hardly infallible. There are so many variables that enter into how good a "fix" you get, that it is impossible to make generalizations based on one trip or drive.

It starts in the sky. At 12,550 miles up, a constellation of 24 satellites circle the globe, transmitting their unique information down to receivers all over the globe. The satellites do not know, or care where any of the receivers are.

In your receiver, the CPU takes the information from the satellites that it can see, and from that, determines your latitude and longitude. The more accurate this calculation, the better the "fix". The more "good" satellites your receiver can see, the better the fix. By "good", I mean their position, not whether they are broken...

You need, if I recall, 3 satellites for a basic fix, 4 for a good fix, and 5 or 6 for a 3D fix (that includes altitude).

The best location for a GPS antenna is high and clear of any structure, such as on the roof of a building, boat, plane, car or truck. This is so that the antenna has an unobstructed view of the sky.

Most GPS units have antennas built in, so you're usually not getting that kind of coverage. You're essentially missing anything behind the GPS unit, due to the signals being blocked by the car's structure. This will affect your ability to get an accurate 3D fix, and sometimes a 2D fix.

On top of this, you have the accuracy of the road database that is inside your GPS navigator. I've yet to see one that is completely flawless. Not even Google Maps gets all addresses and locations correct 100% of the time. Generally, I'm happy if I can get onto the right road and block. I can still read numbers...

GPS Navigators are an aide in getting you places. They're not perfect, and they should not be treated as such.
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