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How come?

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You have to watch how closely you place some GPS units together. If they are side by side they often interfere with each other. You should be outside, keep them a few feet apart and repeat the test or place one in a location, write down the coords, and then remove it before placing the other on the same spot.


All things being the same, without any funky unit to unit interference, they should read the same or within a meter or two of each other. The whole theory behind implementing WAAS corrections is that every unit should read close to the same coordinates (uncorrected) at a given location, all other factors being equal. Horseshoes and handgrenades. Close is good, repeated anomoly is bad.

Edited by yogazoo
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My 450 and my wife's 550 never give the exact same readings. Most of the time they're close, quite often very close, but never exactly the same. It's just how GPS works. GPS heavily depends on timing, and the integrated clock of an affordable receiver is never as accurate as it would need to be. So it uses the timing data that comes from the GPS system itself, or rather, it tries to. It tries as best as it can, and can get it almost perfectly right... Just never really perfectly right. This is where the differences come from.

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I have a Dakota 20 and my wife also has one. They are set up the same way. How come if they are sitting next to each other the coordinates are a little different?




The GPS signal you are using is coming from an object 12,000 miles above your head moving at ~7000 miles an hour. It's signal is constantly changing because it's position is constantly changing. In those 12,000 miles the signal travels, it can be slightly distorted by objects, including but not limited to: airplanes, other satellites, space junk, clouds, etc. Any of these could distort or slow down the signal. Therefore, the fact that we can get 15 feet accuracy is incredible in itself. So there are a lot of variables that can affect the reported position. They will get slightly different signals from the satellites. They will interfere with one another. They may be averaging slightly different time periods. They may be "seeing" different satellites.

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