A wall is one of the most important objects in an indoor environment
for mobile robot localization and environment sensing.
In a conventional ultrasonic sensor for a mobile robot,
the range to a wall is obtained by the pulse-echo method,
but the orientation angle of the walls can not be measured.
We developed an ultrasonic sensor system containing one transmitter
and multiple receivers to measure the inclination angle of the walls
and named it the RADIAN sensor.
In this paper we firstly present a way to measure the angles of walls with
the RADIAN sensor system.
Second, we describe an extension of the RADIAN sensor system
to measure multiple walls and objects.
And finally, we present an experiment which uses an extended
RADIAN sensor.
The experiment was successful, and demonstrated the feasibility of this
extended method.