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wassup with waas?

Old 08-07-2004, 06:01 PM
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Default wassup with waas?

Playing with my new Garmin 182. Noticed on the satellite search page when I first started up, the accuracy indicated 12 feet with waas off. When I turned waas on, accuracy went out to 30+ feet settling back down to 19. Now I know I'm probably being anal about this but I thought waas was supposed to make the unit more accurate. What gives?
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Old 08-07-2004, 08:17 PM
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Default RE: wassup with waas?

Ummm maybe I am missing something.

How did you measure this accuracy?

The only way I know of proving what you actually have is find an exact known coordinate on the ground
and move around with the gps and see what you get as an average. GPS doesn't have repeatable accuracy
so it will vary from time to time and day to day.

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Old 08-07-2004, 10:03 PM
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Default RE: wassup with waas?

On the startup (satellite acquisition) page there is a numerical indication for both accuracy (in feet) and dilution of position. Don't know the relative accuracy of these numbers. It just seems to me that accuracy should be at least somewhat better with waas enabled, no?
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Old 08-08-2004, 01:10 AM
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Default RE: wassup with waas?

Those numbers aren't an indication of accuracy but an "estimate" and that's quite different in the true meaning of the term accuracy.

Didn't notice any mention of your location? With a location then one could determine if in fact WAAS is practical at that location.

Cheers, Kerry.
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Old 08-08-2004, 08:29 AM
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Default RE: wassup with waas?

Was your unit reading the WASS satellite?
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Old 08-08-2004, 08:59 AM
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Default RE: wassup with waas?

How can I know the reliability of WAAS for a given location?
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Old 08-08-2004, 11:07 AM
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Default RE: wassup with waas?

WAAS is not always the best reception option depending on your location. Most modern day GPS units will use the best "mode" automatically if you don't change that manually. Nothing wrong with 12' accuracy IMO.
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Old 08-08-2004, 05:53 PM
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Default RE: wassup with waas?

I think that the following instructions can be logically used to test the ACTUAL accuracy of your unit.

Take your GPS out into an open playground or field where it can recieve as many satallites as posible. This will simulate an open ocean condition.
Walk over to a boulder or some other big heavy fixture that no one can move, and stay there for 20 seconds before you fix this location as a Go To point. Lower the scale on the unit and take 20 or paces away in a straight line from the go to point, and see if your GPS says you are about 60 feet away.
Go back to the rock to see if putting the GPS at exactly the same spot indicates that you are zero feet away, or if tells you that you are 5-10-15 feet away.
Turn the unit off and on again and then ask it to go back to the rock again to see if this also effects your previous readings.

Try it in a month later when there is a full moon to see if that effect its accuracy and then stand on your head while chewing gum to see waaas up with this application.

In shoirt, this methodology will tell you exactly how accurate your GPS is on flat and verticle dimensions.
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Old 08-08-2004, 06:22 PM
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Default RE: wassup with waas?

Remember GPS and WASS in particular were developed primarily for aviation use. The accuracy improvement using WAAS has a lot to do with the location of the ground WAAS station. Go here for more info http://gpsinformation.net/waasgps.htm


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Old 08-10-2004, 11:38 AM
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Default RE: wassup with waas?

OK, now for the explanation for what was actually happening.

When WAAS was switched on, not all satellites were receiving corrections. Your GPS was relying on the ones that were. These had a geometry which was not as good as some satellites that did not have corrections and were not being used.

As was stated earlier the "Accuracy" figure is actually what used to be know as "EPE" which stood for "Estimated Position Error". Since GPS units are now being sold to people that do not have any knowledge about satellite navigation, and no understanding about what EPE stood for, they got tired of answering questions and changed to designation to "Accuracy". This figure is nothing but a mathematical equation which takes into account the location of the satellites in the sky. If you believe this figure actually shows how accurate the unit is, then I have some coastal property in Florida for you, it's only a few miles from the beach.

Had you waited a little bit until all of the satellites were receiving WAAS corrections then the Accuracy figure would have then equaled the number of feet you had when WAAS was turned off. If you have a Garmin, the Accuracy figure is unaffected by WAAS except for which satellites are being used.
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Old 08-10-2004, 05:09 PM
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Default RE: wassup with waas?

Well if we knew actually where this receiver was located at the time then one can actually see if WAAS was viable or not in relation to either of the 2 satellites transmitting WAAS corrections. Many land based users generally have come to acknowledge that WAAS isn't all what it's cracked up to be, in actual fact even the FAA have acknowledged that it doesn't have the capabilities that it was designed for.

Cheers, Kerry.
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