18.3.1 All vessels should be fitted with an echo
sounder, or other means, to measure the available depth of water.
See section 25.6.3 for requirements for
dedicated pilot vessels.
18.3.2 A vessel which operates more than 20 miles
from land (Area Category 0, 1 or 2) should be provided with:-
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.1 A receiver for a global navigation satellite
system or a terrestrial radionavigation system, or other means suitable
for use at all times throughout the intended voyage to establish and
update the vessel's position at all times.
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.2 A distance measuring log; except that this
need not be provided where the navigational aid in Section 18.3.2.1
provides reliable distance measurements in the area of operation of
the vessel.