Table of Contents
Advertisement
Operat or 's
Man
ual
Automatic
Identifac
tion Syste
m
An A
IS
report
fits
into one or
several
ofthese
2,250 times lots, which are selected
automatically based
on data link
traffi
c
and
proj ections
of
future
actions
by
other
stations
currently on
the network
.
Whe
n a ship first
en
ters the ce
ll
of
another
ship , it
takes
an unoccupied
timeslot.
The
AIS
stations
continually
synchronize
thei
r slot
selections
with each other.
Timeslots and
time-out
periods are selected on a
rand omized basis. When a
statio
n
changes
its
slot
assignment,
it ann
ou nce
s
to
all
other
stations on the
chan
nel
its new
location and tim
e-out for
that
locat
ion.
Each
station
continually
upd ates its internal
"slot map
"
to reflect changes
in
occupied slo
ts and
time-
outs. Special
provisions
are
made
for automatic
conflict
resolution
in the event
two
stations end up in
the sam e
times
lot, to
en
sure that stations always choose
unoccupied
slots.
In
situa
tions
of
high tra ffic
density it
may be necessary
to
reduce
the
number
of
ships
in a communication
cell,
as
described
above.
This
enab les time slots
used
by weak
stations
far
away,
to be
used
also
by
a sta
tion
nearby.
The AIS
system
applies very specific rules on how this
reoccupation
of
timeslots
is
done.
Th
e
key to
the
SOTDMA
scheme
is
the
availability ofa
highly
accurate
standard
time
reference,
to which all ofthe stations can synchronize their
time
slot
assign-
ments,
in order to avoid
overlap.
This
time
reference is suppl ied
by the
precise
timing
signal
in the GPS satellite
message.
Thu
s,
GPS
plays
it
critical role in
AIS,
providing the
universal
time
reference
as well
as positioning data for each
ship.
AIS 1
A!S 2
(.
I~enli~"
• Po'1i
l:0'l
• S;Jt!ed
O.-e GrtH..
..,d
.. CcI".Jr:;c
o~
Gr!Jun
d
•
He.
d
H~
~
•
Rate
or
Ti.Jn,
•
N.;r"'
~
l j
c n
S!J
lu::;;
• llml?
S
Ia;Yl(l
138
Transpon der
transm ission us ing ti
m e
slots.
Version
1.5
Advertisement
Table of Contents
