CHAPTER 6
LINK-4A
INTRODUCTION
The Link-4A system is a fully automatic, high-speed data transmission system used for aircraft control. The system provides controlling information to the aircraft, using radio transmission between the controlling ship and the controlled aircraft. The Carrier Aircraft Inertial Navigation System (CAINS) is also a part of the Link-4A system. The CAINS system is used to load alignment and way-point data into the aircraft on the flight deck or the hangar deck.
After completing this chapter, you should be able to:
Describe the operating modes of the Link-4A data terminal set.
Describe the functional operation of the Link-4A data terminal
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
hanger 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
sort of link40 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 6-2shows 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 s y n c  p r e a m b l e , the data bits, a n d  t h e transmitter un-key signal.Figure 6-2.—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.Followin
g 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 56time 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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last digit designates whether the message is an A or B
type. In most modes, both an A and a B type of
message are required to transmit all the necessary data
to the aircraft. The remaining data bit time slots
contain the various control commands.
Transmitter Un-key Signal
The last time slot (slot 70) is a 200-µsec period
allotted for transmitter turn-off time and does not
contain any data.
REPLY MESSAGE FORMAT
Reply messages are received during the 18-msec
receive cycle. The reply message contains a total of
56 time slots and occupies a period of 11.2 msec.
This 11.2-msec reply must be received during the
18-msec receive cycle. This allows for a maximum of
4.8 msec for transmission delay.
The reply message consists of a sync preamble,
42-data bit time slots, and a guard interval, as shown in figure 6-3. The sync preamble is identical to the
control message sync preamble. The information in
the 42-data time slots is divided into groups of digits
that identify the source and type of message, and the
message data. The last time slot is the guard interval
and it allows for transmitter turn-off time.
Figure 6-3.—The Link-4A reply message format.
TEST MESSAGES During Link-4A operations the controlling station sends test messages at periodic intervals to the data terminal set for testing the message processing and display circuitry of the aircraft
being controlled. The test messages also check the data terminal set and its interfaces. The two types of test messages are universal test message (UTM) and monitor control and reply messages (MCM/MRM).Universal Test Messages Universal test messages (UTMs) are Link-4A control messages that are always addressed to a particular universal address and contain fixed, specific information in each data field. The UTMs provide the controlled aircraft with a means to verify proper operation of the link.Monitor Control and Reply Messages Monitor control messages (MCMs) are Link-4A control messages that are sent to the data terminal set from the CDS computer to initiate internal testing of the data terminal set. After the data terminal set completes its self-check, the MCM is transmitted with the universal address. Depending on the equipment configuration of the aircraft, the MCM will either be rejected or processed as a UTM.The monitor reply message (MRM) is sent to the CDS computer upon the successful processing of the MCM. The MRM is effectively a return of the MCM data content which indicates that the internal and interface tests were successful.THE LINK-4A SYSTEM COMPONENTS The Link-4A system consists of the CDS computer, a data terminal set, a communications switchboard, and a UHF radio set.
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Figure 6-4.—The AN/SSW-1D/E data terminal set.
DATA TERMINAL SET AN/SSW-1D/E
The Link-4A data terminal set is the
AN/SSW-lD/E. The data terminal set performs the
following functions:
Provides overall Link-4A system timing
Converts parallel data from the CDS computer
into serial data for transmission to controlled
aircraft
Converts serial data received from controlled
aircraft into parallel data for input to the CDS
computer.
The current five versions of the AN/SSW-1 used
in shipboard Link-4A systems are the AN/SSW-1A,
1B, 1C, 1D, and 1E. The AN/SSW-1A, 1B, and 1C
are operationally and functionally identical, as are the
AN/SSW-1D and 1E. The major difference between
the two groupings of versions is the single-channel
capability of the AN/SSW-1A/B/C and the dual-
channel capability of the AN/SSW-1D/E. Each of the
dual channels is capable of the link operations of the
single channel AN/SSW-1. The dual-channel
AN/SSW-1D/E is also capable of transmitting CAINS
data. For purposes of this lesson, we use the
AN/SSW-1D/E.
The AN/SSW-1D/E shown in figure 6-4, consists
of the following eight major subassemblies: one
coordinate data transfer control, two digital-to-
digital converters, two monitor test panels, two
Figure 6-5.—The coordinate data transfer control assembly (AN/SSW-1D/E).
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