This section should give an introduction
to general use of a is use of a is
information in collision avoidance use
of a is by vessel traffic services VTS
use of a is in search-and-rescue the IMO
display requirement for a is is minimum
keyboard and display mkd that is
presentation of target information in
alpha numeric format only for most
operators this type of information is
probably a very limited value since this
type of display is likely to still
remain the cheapest for some years ahead
it is reasonable to assume that most
ships will be equipped with this type of
a is display the question is therefore
of how much use
will this type of the ISB to the officer
on watch until more experience with a is
has been gained this question will
remain unanswered call reg rule 7 risk
of collision states that every vessel
shall use all available means
appropriate to the prevailing
circumstances and conditions to
determine if risk of collisions exists
if there is any doubt such risks shall
be deemed to exist the call regs obliged
ships to apply all available means to
detect the danger of collision and to
take preventive measures one of these
means especially during reduced
visibility is ship-borne radar another
aid now available is a is a is
broadcasts the identity position heading
course over ground speed over ground and
certain other relevant ship data at an
update rate dependent upon the ship's
speed or rate of turning during course
alterations the AIS performance
surpasses ship-borne radar in
respects a is aims to achieve a
positional accuracy of better than 10
meters when associated with differential
global navigation satellite system gnss
corrections
this compares favorably with radar whose
accuracy is a function of frequency
pulse repetition rate and antenna being
with and which often achieves a
positional accuracy of 30 to 50 meters
due to higher positional accuracy and
less need for plot filtering the
position and changes of course over
ground can be presented with less delay
than by radar the AIS provide
supplementary information about other
vessels that is not readily available
from radar such as identity heading
course over ground co ji speed over
ground sog rate of turn our OT and
navigational status AIS is also less
sensitive than radar to- weather effects
such as heavy rain snow etc a is also
has the capability to see around bends
and corners as long as a is provides
more accurate and complete information
the passing distance between vessels can
be determined with greater accuracy and
reliability from the navigational status
information available any maneuvering
restrictions on a vessel become evident
immediately and can be taken into
account since a is provides information
different from that provided by the
shipboard radar it is strongly
recommended that a is and radar be used
together in order to enhance the
available information AIS can also
assist in the identification of targets
by name or call sign and by ship type
and navigational status thus reducing
the requirement for verbal information
exchange
this clearly shows a potential situation
shortly after a target vessel be has
executed a course change to starboard in
order to obtain safe CPA with own ship a
the ARPA time filters used to stabilize
the RP vector may cause an offset
between the ARPA and a is this is why
it's important for the officer on watch
always to remember that the ARPA is an
early warning device a study made by the
German marine board indicated that the
so-called radar assisted collisions
occurred and restricted visibility when
radar provided insufficient incomplete
or ambiguous data
the study concluded that many of these
collisions could have been avoided if
the navigators involved had been able to
access timely and dynamic information
position heading speed and rate of turn
etc on the other vessel involved a is in
the ship-to-ship application can now
provide such dynamic information
accurately and at high update rates when
target information is available on the
ships involved hopefully AIS will
contribute to a reduction and collisions
at sea
however if the navigators don't take
early actions whenever possible we may
soon see another member in the family of
assisted collisions namely a is assisted
collisions at present radar is the main
sensor of the VTS for detecting a ship
the VTS radar has almost the same errors
and range and bearing resolution as all
other radars although it has a known
position and is North oriented without
the errors of the compass involved the
limitations of the ARPA radar to track
ships due to target swap to land beacons
bridges and other ships makes the
tracking facility in the ARPA VTS rather
limited
vessel traffic services are currently
limited to tracking ships in the bay
area or on approaches from the sea to
harbor there is a requirement for
improving the VTS so that it will be
able to cover areas where radar coverage
is almost impossible to achieve identify
radar echoes on the VTS radar
automatically interrogate ships for
information regarding type of cargo
track with high update rate know which
port ship is bound for know the size and
draft two ships in the vicinity detect a
change in a ship's heading almost in
real time the only a is solution that
could solve all the requirements of the
VTS is a high update rate broadcast a is
with interrogation capabilities in the
near future
hey is as a part of aids to navigation
will be available to the users the
establishment of a global a is in
general or a land-based a is reception
network in particular may actually
mitigate the need for BTS in many areas
the cost of installing and maintaining
an AIS network is negligible compared to
that of ETS radar Network marine rescue
and coordination centers operations
would be much more efficient if they had
every rescue craft fitted with a is to
quickly determine which ship is closest
to a distressed situation during a
search all crafts could be tracked
enabling the marine Rescue Coordination
Center mrcc to monitor their progress to
direct available resources efficiently
and determine whether search coverage is
without gaps