ultrasonic distance measurement using 89c51

The round trip time at one inch is roughly 148 microseconds at standard temperature and pressure, and I'm not going to bother getting the datasheet on the 89C51 - figure that one out yourself :)

Good article on the speed of sound at Wikipedia:

I assume you are going to use the echo principle which is why I specified the round trip time.

There are plenty of transducers around (google is your friend) and some offer complete range measurements.

Note that there are some issues to keep in mind (most of this is covered in basic echo [such as radar] theory) regarding resolution and range. For ultrasonics, the temperature also needs to be taken into account, so a decent thermal sensor is a must for accurate readings.

If the target is moving, you'll also experience doppler effects which may or may not be an issue (depends on the method of measurement and the specific transducer).

Cheers

PeteS