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Identification
System
AfS Communications Scheme
Operator
's Manual
AIS
messages
mu
st
be updated
and
rc
transm
iu
cd
ev
ery few seconds at
a
mini
-
mum
, si
nce the
use fuln
ess
o
f
the
data decays rapi
dly
as a
function
o
f ti
me
.
To
accomm
odate thi
s high
upd
ate requiremen t,
A
IS
ut
ilizes a unique se
lf-orga nizi ng
time-di
vision
multipl
e
acce ss
(SOT DMA) data
communications scheme,
whi
ch
uses
th
e preci
se timing dat a
in
the
G
PS signals
to sy nchro
nize
multipl e
data
transm
issions
from
man y users
on a sing
le
narro wb and cha nne l.
a
exchanges data
with
@
@exchanges data
with
e
and @
@
exchanges
data
with
@
Each ship
broad cast
s its
AI
S
messages a
nd
rece ive s messages
from
a
ll
s
hi ps
within VHF
rad io range
.
The area
in which AIS mes
sages
can
be
received
is
ca
lled
the
ship
' s
"ce ll" .
Each
ship is in this
way
in
the eenter of its own
communication
cell.
The
practical
size
o
f
the ce
ll
can
be varied accord ing to
the
traffic den
sity
o
n th
e
AIS cha nnel. If th
e
num
ber o
f AIS
messages
beg ins
to
ove
rlo
ad
the
networ
k.
the
ship
'
s
AIS system
ca n
au
tomatically
shrink its cell
by ignoring weaker stations
furth
er
away
in
favor
o
f
those
nearby.
Under
the
SOTDMA prot
oco
l,
each
minute
of time is di
vided
into
2,250 timesl ots.
Version 1.5
137
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