A typical underwater sonar, although being far more complex than the ultrasonic devices used in this work, can take profit of those localization techniques accounting for the ultrasonic sensor limitations. Moreover, their basic behavior is shared with underwater sonar, which is a sensor vastly used in underwater and marine robotics. Their price and power consumption are better than those of laser scanners. However, ultrasonic range finders are still appealing in the mobile robotics community for several reasons. In general terms, standard ultrasonic range finders are only able to provide tenths of readings per second and have angular resolutions one or two orders of magnitude worse than laser scanners.
Other sensors, such as standard Time-of-Flight (TOF) ultrasonic range finders, do not have these properties. Most of these sensors are able to provide thousands of readings per second with a sub degree angular resolution. That is why localization strategies usually rely on accurate sensors such as laser range scanners. The quality of the pose estimates is, consequently, strongly related to the quality of the sensor measurements.
Exteroceptive sensor data can be correlated at subsequent robot positions to compute displacement estimates, based on initial guesses provided by proprioceptive sensors. A common approach to localize the robot is the use of exteroceptive sensors, such as range finders, measuring the external environment, combined with proprioceptive sensors, such as odometers. The process of estimating the robot pose with respect to such fixed reference frame is known as the localization problem. A crucial requirement for a mobile robot to accomplish useful long term missions is that the robot has some knowledge about its pose with respect to a fixed reference frame.
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