User Manual for the Type 8300 Compatt 6
(and Variants)
UM-8300-Compatt 6
Issue B3
A.1.3 Beacon Numbers
In principle, navigation can be achieved using just two transponder beacons fixed to the seabed.
However, with only two transponders, there would be ambiguity in the navigation solution because it
would not be possible to determine from the measurements which side of the baseline was the correct
transceiver position.
Also, when using only two beacons, the depth of the transceiver (equivalent to the transceiver’s
‘height’ above the beacons) must be assumed.
For these reasons, three beacons is the minimum that can be used to produce unambiguous results in
three dimensions. The use of five beacons provides sufficient redundancy to allow erroneous
measurements to be identified, and is useful for making checks on the quality of navigation.
A.1.4 Baseline Distance
The term ‘long baseline’ is used because, generally, the baseline distances are much greater for LBL
than for short baseline (SBL), and it is certainly much longer than for ultra-short baseline (USBL).
The longer baselines used by LBL systems mean that LBL is more accurate than SBL and USBL. LBL
also has the advantage of being able to position the vessel, or any other object, directly using a ‘fixed’
frame of reference. This removes most of the problems associated with vessel motion.
A.1.5 Calibration
It is necessary to calibrate the array of beacons on the seabed, and there are several techniques
available for doing this. The most appropriate method to use depends on the specific requirements of
the operation and on the available hardware.
With the continuing integration of LBL, SBL and USBL systems, intelligent transponder beacons that
can measure baselines directly, and satellite navigation systems, the calibration of seabed arrays has
become a quick and simple operation, completed automatically in some cases.
A.2 Short Baseline (SBL)
A.2.1 Hardware
Short baseline (SBL) acoustic position systems are usually fitted to a vessel such as a barge, a semi-
submersible, or a large drilling vessel.
In an SBL system there are at least three, and typically four, acoustic transducers fitted in a triangular
or rectangular array on the lower part of the vessel. The distances between transducers (the
‘baselines’) are made as large as practical on the vessel, and are typically at least 10 metres long.
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.
Appendix A – Acoustic Positioning Methods
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