I've built a powered UAV already and am considering building an unpowered
research UAV. On the powered one, I did use GPS for altitude... not real
accurate, but good enough. I never actually measured it, but I think with
WAAS it should be good to about 10m.

BTW, two problems with cell phone control. First of all, it *is* illegal to
fly a UAV in the national airspace without a specific letter of
authorization from your local controlling authority (ATC center, usually)
issued 30 days in advance, both for safety and security reasons, and so that
your local FSS can issue NOTAMs for the area you'll be operating in. You
also have to have a transponder on board so ATC can route traffic around
you. The Association for Unmanned Vehicle Systems International (AUVSI,
www.auvsi.com) has an FAA liason currently fighting for looser restrictions,
but with no real results yet. While you can probably get away with flying an
amateur UAV (under constant control, and within sight) in the national
airspace, under the auspices that it's just a souped-up RC. I did it, and
lots of others have, too. Don't blame me if the Feds show up on your
doorstep! :O)

Secondly, under cell control, you couldn't go over 1000 feet AGL or so,
since the cell transmitters radiate mostly horizontally. This is one reason
why cell phones are frequently flakey in aircraft (besides the fact that
it's mostly illegal to use a cell phone in an aircraft). Cell coverage in
the air is even flakeyer (is that a word? :O) than on the ground, which says
a lot for the reliability of such a control system.

All that said, I think people *have* tried using cells in their UAVs for
basic, nonessential telemetry. Google around. You're sure to find a couple,
I know I saw at least one in my surfing over the last year or two.
Best of luck!

- Robert

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