stanley,

your timing is correct, but I couldn't understand if you are using a pair of sensors (one pinging and the other listening to the echo) or one sensor that pings and listens to the echo (like the polaroid 6500).

if you are using one sensor you can't rely on it for distances under 1.3m, since after ping the sensor still vibrating mechanically "after" the sound is emmited.

there are ready made sensors made by devantech that can measure from 3cm to 6m. look at http://www.acroname.com/robotics/...SRF08.html

the datasheets can give you some ideas how to get that kind of precision http://www.acroname.com/robotics/...SRF04p.pdf

these sensors use a pair of transducers, one for ping the other for listening

hth

orlando