Arc-transversal median algorithm: an approach to increasing ultrasonic sensor accuracy

This paper describes a new method for determining range information about a robot's surroundings using low resolution ultrasonic sensors. These sensors emit ultra-sound which bounces off of nearby objects and returns to the sensor. The time-of-flight for the sound to return to the sensor is the distance between the sensor and the object. A sonar arc represents the possible locations of the object. We model these locations with a simple uniform probability distribution on the sonar arc. We then introduce a new method to fuse sonar data to determine the actual obstacle location. This new method is termed the Arc-Transversal Median method because the robot determines the location of an object by intersecting one arc with other arcs whose angle-of-intersection exceeds a threshold and then taking the median of the intersection. The median is a robust estimator that is insensitive to noise because a few stray readings will not affect the median. We show via some simple geometric relationships, that this method can improve the accuracy of the sonar sensor by a specified amount, when certain assumptions were in place. Finally, experimental results on a real mobile robot verify this approach.