Underwater Robots - Gianluca Antonelli.pdf

8 1. Introduction
Fig. 1.5. Pig, manned vehicle used by the Royal Italian Navy during the second
world war inthe Mediterranean Sea. The thruster and the group rudder/stern can
be observed in the bottom left angle of the photo
on an AUV inorder to implement sensor fusion algorithms and obtain an
estimation more reliable than by using asingle sensor.
Among the possible methods is the use of baseline acoustics, those rely
on the use of transmitters/receivers mounted on the vehicle and onknown
locations needed for triangulations. In same cases one single module can be
used and mounted onasurface vehicle the position of which isacquired by
means of aGPS. In case of partially structured environments, suchasharbors,
transmitters/receivers at known positions can be easily deployed. In absence
of baseline acoustics there is the need tomeasure atime derivatives of the
vehicle position such asthe acceleration with the IMU or the velocity with
aGround Speed sonar fused with the vehicle orientation measurements. It
is well known in estimation theory, however, that the time integration of a
measurementleads to the Brownian motion,or random walk,i.e., astochastic
model whose variance grows linearly with the elapsed time. After some time,
thus, the estimation isuseless and areset of the error is necessary by, e.g.,
surfacing the vehicle and measuring its real position with aGPS. Finally,
when the vehicle uses sonar or video-cameras itcan measure several time its
relative position with respect to afixed feature; this information can be used