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How well does the altitude function work on GPS units that have WAAS? Is it good enough to use for flying, or should I use a barometric pressure system instead?

I know the altitude reported by non-WAAS GPS units is quite laughable -- a friend logged the altitude reported by a non-WAAS GPS that was sitting on his windowsill and he reported he was getting nauseous from the reported ups and downs of over a 100 feet!

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I believe that with WAAS, there will less than 10 meters of error for most of the USA.

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I thought about using GPS for altitude control but came to the following conclusion... if the GPS unit is accurate to within 10 meters, then your actual position could be within 10 meters **all around you**. In other words, the GPS could report your position anywhere within a sphere that was 20 meter across. Imagine your airplane trying to stick to an altitude when the GPS reciever could report as much as a 20 meter difference at any point in time!

WAAS helps a lot... I have a little etrex that, on a good day, reports that it is getting 5 meter accuracy. Even then, the plane could waste a lot of energy moving up and down almost 10 meters.

I think a well designed barometric unit would work better.

P.S. Aren't we spoiled, we have these little devices that fit in our pocket and can nail our position on the face of the earth to within 10 meters, and we want <1 meter accuracy so we can play with our toys :rolleyes:

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yb2normal, my understanding is that the error is slowly varying with distance and time, so the plane won't be dancing up and down. However, I wanted people's real-world experiences with WAAS, not the FAA's glowing poopsheet, which is the reason I posted the question in the first place.

I guess I'll have to just try it and find out!

BTW, I've been considering using a pressure sensor anyway in order to determine airspeed, so I can account for wind. I would really like to know BEFORE I get into trouble that I'm flying downwind in a gale and won't have enough energy left to get back home if I don't turn around pretty quick!

--bluegill

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