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Appendix B - Digital interface
LRF – Long Range Function
This sentence is used in both long-range interrogation requests and
long-range interrogation replies. The LRF-sentence is the second
sentence of the long-range interrogation request pair, LRI and LRF
(See the LRI-sentence.). The LRF-sentence is also the first sentence of
the long-range interrogation reply. The minimum reply consists of a
LRF-sentence followed by a LR1-sentence. The LR2-sentence and/or
the LR3-sentence follow the LR1-sentence, if information provided in
these sentences is requested in the interrogation. When the AIS creates
the LRF-sentence for the long-range interrogation reply, fields 1, 2, 3,
and 4 should remain as received in the interrogation; and field 5
(Function Reply Status) and a new checksum are added to the LRF
reply sentence.
NOTE 1
This is used to bind the contents of the LRI and LRF sentences
together. The LRF sentence shall immediately follow the LRI
sentence and use the same sequence number. The requestor process
shall increment the sequence number each time a LRI/LRF pair is
created. After 9 is used, the process shall begin again from 0. The
Long-range interrogation is not valid if the LRI and LRF sequence
numbers are different.
NOTE 2
The Function request field uses alphabetic characters based upon IMO
Resolution A.851(20) to request specific information items. Specific
information items are requested by including their function
identification character in this string of characters. The order in which
the characters appear in the string is not important. All characters are
upper case. Information items will not be provided if they are not
specifically requested – even if available to the AIS. The IMO
Resolution defines the use of all characters from A to Z, but not all of
the defined information is available from the AIS. The following is a
list of the function identification characters with the information they
request:
A = Ship's: name, call sign, and IMO number
B = Date and time of message composition
C = Position
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