User Manual: Generic Section
Section: 13 Acoustic Navigation Principles
Revision: 00
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ACOUSTIC NAVIGATION SYSTEM PRINCIPLES
1.1
LONG BASE LINE PRINCIPLES (REF. FIG 1)
A Long-Base-Line (LBL) system has two parts or segments.
The first segment comprises a number of acoustic transponders moored in fixed locations on the seabed.
The positions of the transponders are described in a co-ordinate frame fixed to the seabed. The distances
between them form the “baselines” used by the system.
The second segment comprises an acoustic transducer on a transceiver which is temporarily installed on the
vessel, or on a Tow Fish. The distance from a transducer to a transponder can be measured by causing the
transducer to transmit a short acoustic signal which the transponder detects and causes it to transmit an
acoustic signal in response. The time from the transmission of the first signal to the reception of the second
is measured. As sound travels through the water at a fixed speed, the distance between the transducer and
the transponder can be estimated.
The process is repeated for the remaining transponders and the position of the vessel relative to the array of
transponders is then calculated or estimated.
Navigation can be achieved using just two seabed transponders but in doing this there is a possible doubt as
to which side of the baseline (a line drawn between the transponders) the vessel may be on, also the depth
or height of the transducer has to be assumed.
Three transponders is the minimum required for certain navigation in three dimensions. Four is the minimum
number required for some degree of redundancy - which is useful for checks on the quality of navigation.
LBL baselines are much larger, so an LBL system is very accurate compared with the Short Base Line (SBL)
and Ultra Short Base Line (USBL) versions and also has the advantage of positioning the vessel (or any
other object) directly in a "fixed" or an inertial frame. This removes most of the problems associated with
vessel motion.
The array of seabed transponders needs to be calibrated and there are several techniques to do this. The
applicable technique depends on the requirements of the task or job and the available hardware.
With the continuing integration of LBL, SBL and USBL systems, intelligent transponders (that measure
baselines directly) and satellite navigation systems, the calibration of seabed arrays is becoming a quick and
simple operation. The operator will be free to choose the techniques applicable to the task requirements.
Summary of Contents for 8142-000-01
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