Ultrasonic sensors are widely used as external sensors for mobile robots,
because they are simple to build and use, and are low cost.
Generally in the pulse-echo method,
the distance to the target can be accurately measured,
but accurate measurement of the reflecting target direction is difficult.
To realize navigation of a mobile robot in a complicated environment
like a doorway or surface tracking of the wall,
the resolution of the lateral direction
and surface angle of reflecting target are very important.
In this paper, we propose a method which can find the direction of the target
by analyzing the frequency spectrum of the echo
in the pulse-echo method.
The new method is based on the consideration of
the physical properties of the ultrasonic transducer and its environment.
Using this method, we will be able to determine
the direction angle to the reflecting object surface from the sensor,
using only a single receiver in the pulse-echo-method.
We present experimental results to show the potential for practical applicaion.