altitude sensors anyone?

[reversed_rocker]

for the collegiate first pilot program we would really like a way to be able to tell the altitude of our quadcopter. since i dont to trust GPS signals inside a building and i don't know of any altimeters that work at this low of an altitude, does anyone know of a good distance measuring sensor?

We're looking at measuring distances of up to 10m and to be accurate within about 6 inches or so. all of the sonic range finders i'm familiar with do not have a large enough range. the rules state that we're not allowed to have lasers of any kind, so that throws out laser range finders.