User Manual: Generic Section
Section: 13 Acoustic Navigation Principles
Revision: 00
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1.2
SHORT BASE LINE PRINCIPLES (REF. FIG 2)
A SBL system is can be fitted to a vessel such as a barge, semi-submersible or a large drilling vessel. A
number at least three or four acoustic transducers are fitted in a triangle or a square on the lower part of the
vessel. The distance between the transducers is made as large as is practical, and it is this distance that is
known as the base line.
The position of each transducer within a co-ordinate frame fixed to the vessel is determined by conventional
survey techniques or from the "as built" survey of the vessel.
If the distance from the transducers to the transponders is measured as described for LBL, then the position
of the transponders, within the vessel co-ordinate frame, can be computed. If redundant measurements are
made, a best estimate can be determined which is, statistically, more likely to be more accurate than the
basic position calculation.
It is impractical to transmit a signal from each SBL transducer in turn. So the systems that use transponders
transmit from one transducer but receive simultaneously on all transducers. The result is one distance (or
range) measurement and a number of range (or time) differences.
With an SBL system, the co-ordinate frame is fixed to the vessel. The vessel is subject to roll, pitch and yaw
motion. This movement can be overcome by using additional equipment such as a Vertical Reference Unit
(VRU) to measure roll and pitch and a Gyro-compass to measure yaw. The co-ordinates of the transponders
can then be calculated mathematically to remove the effect of the movement.
To estimate the position of the vessel in a "fixed" or "inertial" frame such as one fixed to the seabed. Then at
least one transponder should be in a fixed position on the seabed and used as a reference point. This
principle is used for Dynamic Positioning (DP) of vessels. If a transponder is fixed to the seabed and a
responder located on a Remotely Operated Vehicle (ROV), the system will calculate the position of the
vessel and the ROV in a seabed-fixed frame.
Summary of Contents for 8142-000-01
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