Imho for a 1-2m hover and slow flight you should rely on amplitude and not
time to measure attitude. This assumes that the surface is very smooth and
even. You would use only one transmitter in the center and four receivers
in the N,S,E,W positions (not corners). The transmitter works all the time
and you measure the amplitude of the four receivers. The difference in
sound level gives angle directly (front/back and left/right). Sound
amplitude decreases with an inverse square law with distance for this
configuration but maybe you can ignore the square part if you assume that
the deviation from horizontal is small most of the time. You will need to
look (deeply) into control theory to avoid making a pendulum out of your
vehicle. Imho maintaining horizontal flight is best handled by a pendulum
type attitude sensor for your application (slow flight of short duration)
(a damped pendulum hanging from a light-going potentiometer will work
probably). Use critical damping on the pendulum sensor (Q=1).

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