ultrasonic distance meter + atmel

Hi ,
I am currently doing research for my final year project. Part of my project need me to construct a ultrasonic sensor . Most common available ultrasonic sensor is from PARALLAX PING.

Did anyone have experience programming this kind of sensor with microcontroller ? i wish to write own code for sensor triggering and distance calculation. but the problem i face is that its datasheet never specify the relations of distance versus respond time.

Can anyone help me on this .. Thank you !