Stanley,

If you switch on the transmitter during 120ms, and the signal takes only between 0.5 and 2.9 ms to echo, how can you measure the echo? Between 0.5 and 2.9 ms after switching on your transmitter, you recieve the first echo. If you only start your timer after switching off your transmitter, you have been receiving an echo for at least 120 - 2.9 = 117.1 ms. When your timer starts, it should stop immediately as you are already receiving an echo (since 117.1 ms....).

regards

Patrick