Specifications

BATS AND PORTABLE BATS INSTALLATION
2-46
2.4 HYDROPHONE OFFSETS
The BATS computes range and bearing relative to the hydrophone, unless the system is instructed
to add or subtract corrections (offsets) which relate range and bearing to some other reference
point on the vessel. The hydrophone centerline, or zero bearing, is indicated by the index mark
(see Figure 2-1, 2-2 & 2-3), and is generally aligned so that it corresponds to the vessel heading.
It is difficult and time consuming to fabricate a mounting in which the hydrophone heading is
aligned to within a degree of the vessel centerline, unless the vessel is in drydock. If you have the
facilities to perform an exact mechanical alignment, or if the installation is to be permanent, con-
sider fabricating a mount that repeatedly returns the hydrophone to the same physical location
aligned to within 0.5 degrees of the vessel centerline.
Most operations requiring a portable tracking system will be on various vessels of opportunity, and
require a quick and temporary mounting. In this case align the hydrophone with the vessel center-
line as closely as possible. Use some variation of the following procedure to determine hydrophone
offsets.
2.4.1 BEARING OFFSET
The bearing offset must be determined before X, Y and Z offsets can be measured.
1. Measure the distance (d) from the vessel centerline to the hydrophone location as shown in
Figure 2-25.
NOTE: Almost all vessels have a centerline weld or plank that runs down the deck that can
be used for alignment purposes. If your vessel has a seamless deck, you have to
measure the vessel beam and determine the true centerline.
2. Weight a line to suspend the acoustic tracking source vertically in the water.
3. As far forward on the vessel as possible, hang the weighted acoustic beacon over the side at
the same distance from the vessel centerline as the hydrophone. (position B1 in Figure 2-
25.) It may be necessary to use a length of pipe or a boathook to extend the weighted line
the required distance. The beacon should be slightly deeper than the hydrophone.
4. Turn the System on and set it up to track the beacon in use. If you have aligned the mount-
ing bracket correctly, the system tracks the beacon at or very near a 0.0-degree bearing.
5. If you have designed a mounting that can rotate while in the vertical position, you can ro-
tate the hydrophone until you obtain the correct reading. If you cannot rotate the pipe it will
be necessary to enter a bearing correction at the time of operation. Bearing Offset can be
entered under the USBL HYDROPHONE PARAMETER menu.