aircraft. Data is initially entered via a hard-wired
deck edge outlet box and updated by UHF radio until
the aircraft is launched. At launch, the aircraft’s on-
board computer reverts to tactical operation.
LINK-4A MESSAGE FORMATS— The three
types of messages used in the Link-4A system are
control messages, reply messages, and test messages.
CONTROL MESSAGE FORMAT— Control
messages consist of seventy 200-µsec time slots that
contain a 13-time slot sync preamble, 56 time slots of
data bits, and a one time slot transmitter un-key
signal.
Control messages are sent from the
controlling station to the aircraft and are transmitted
during the 14-msec transmit frame.
REPLY
MESSAGE
FORMAT— Reply
messages consist of 56 time slots that contain the sync
preamble, 42 data bits, and the transmitter un-key
signal. Reply messages are sent from the aircraft to
the controlling station in response to a control
message.
Reply messages are received by the
controlling station during the 18-msec receive frame.
The additional time required for reply messages is to
compensate for time delays encountered during
transmission of the control and reply messages.
TEST MESSAGES— The two types of test
messages used in the Link-4A system are the
universal test message and monitor control/reply test
message. Test messages are generated periodically
during Link-4A operations to verify proper operation.
Universal test messages are sent to the controlled
aircraft, using a universal address and verify proper
operation of the controlled aircraft’s system.
Monitor control messages are sent from the CDS
computer to the data terminal set and cause the data
terminal set to initiate a self-test. When the data
terminal set successfully completes the self-test, the
monitor control message is sent back to the CDS
computer as a monitor reply message.
DATA TERMINAL SET AN/SSW-1D/E— The
most common Link-4A data terminal set is some
variation of the AN/SSW-1(). The AN/SSW-1D and
AN/SSW-1E provide dual channel operation. The
data terminal set provides the overall Link-4A system
timing, converts parallel data into serial data for
transmission, and converts received serial data into
parallel data for input to the CDS computer.
LINK MONITOR SYSTEM (LMS-4)— The
LMS-4 is a stand-alone system that allows the
technician to monitor Link-4A operations and perform
readiness checks on the Link-4A system.
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