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Fire Controlman Volume 06-Digital Communications | Next
Monitor Control and Reply Messages |
CARRIER AIRCRAFT INERTIAL
NAVIGATIONAL SYSTEM (CAINS)
The CAINS system is used to load alignment and
way-point data into aircraft on the flight deck or the
hangar deck.
Aircraft alignment data consists of
longitude, latitude, and ship’s velocity data from the
ship’s inertial navigation system. Way-point data is a
set of predetermined geographical points loaded into
the aircraft’s navigation computer.
Way points
provide the aircraft with destination or target
information.
When the CAINS system is used, data can be
loaded into the aircraft by either a hard-wired system
or RF radio transmission. The hard-wired insertion of
data is accomplished when the aircraft is connected to
a deck edge outlet box (DEOB). The pulse amplifiers
of the AN/SSW-1D/E can provide outputs for up to
40 of these DEOBs. After the initial data is loaded,
the aircraft is disconnected from the DEOB, but it
continues to receive alignment data until the launch.
Then the aircraft system reverts to its original tactical
condition.
LINK-4A MESSAGE FORMATS
The following are the three types of messages
used in the Link-4A system: control messages, reply
messages, and test messages. These messages use
two basic formats. Control messages are transmitted
from the controlling ship to the aircraft. Reply
messages are transmitted from the aircraft to the
control station.
The timing for Link-4A communications is
determined from the duration of the transmit and
receive cycles. The standard CDS control messages
are 14 msec in duration, while the receive cycle for
reply messages is 18 msec in duration. The CAINS
system does not use reply messages; therefore, a
2-msec receive cycle is substituted to allow time for
the Link-4A data terminal set to initialize the next
message. Thus we have the following two timing
cycles: 14/18 (control message 14 msec/receive cycle
18 msec) and 14/2 (control message 14 msec/receive
cycle 2 msec).
CONTROL MESSAGE FORMAT
Control messages are assembled and transmitted
during the 14-msec transmit frame. Figure 4-2
shows the standard structure of a Link-4A control
message. During the transmit frame, the transmit key
signal and the control message pulse train are sent to
the radio set transmitter. The transmit frame is
divided into seventy 200-µsec time slots that contain
the sync preamble,
the data bits, and the
transmitter un-key signal.
Figure 4-2.—The Link-4A control message format.
Sync Preamble
The sync preamble is made up of the first 13 time
slots of the control message. The first eight time slots
each contain one cycle of a square wave, consisting of
100 µsec in the “0” state and 100 µsec in the “ 1” state.
These eight time slots are known as the sync burst.
Following the sync burst are four time slots in the “0”
state, called the guard interval. The guard interval
indicates the changeover to the 200-µsec data signals.
Time slot 13 is the start bit and is always a “ 1.”
Data Bits
The Link-4A message data is contained in the 56
time slots (slots 14 through 69) that follow the sync
preamble. Each time slot contains one data bit. The
first 13 bits of this data is a binary number that
indicates the address of the particular aircraft. Only
the aircraft with this preassigned address will
recognize the message and act on the message data.
Following the address is a five digit label that
designates the type of data contained in the message.
The labels correspond to the modes of operation. The
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