Document another message 8 subtype.

1 files changed, 37 insertions, 2 deletions

diff --git a/www/AIVDM.txt b/www/AIVDM.txt
index a849ae0e..107d3666 100644
--- a/www/AIVDM.txt
+++ b/www/AIVDM.txt
@@ -455,7 +455,8 @@ set to use.
 The field breakdowns in this document have been checked against live
 decoded data rendered by known-good implementations for message types
 1-15, 18-21, and 24.  Described but unchecked are 16-17, 22-23, and 
-25-26.
+25-26. Also, the interpretation of IMO extension subtypes of messages
+6 and 8 has yet to be tested. 
 
 Bit lengths and length ranges are given because decoders should check
 them against the message type. Messages with correct checksums but the
@@ -1001,7 +1002,7 @@ DAC-FID pairs in use for type 8:
 |   1     | 11 |     | <<IMO236>> | Deprecated/In Use | Meteorological/Hydrological Data
 |   1     | 13 |     | <<IMO236>> | Deprecated | Fairway closed
 |   1     | 15 |     | <<IMO236>> | Deprecated | Extended ship and voyage
-|   1     | 17 |     | <<IMO236>> | In use     | VTS-Generated/Synthetic targets
+|   1     | 17 |     | <<IMO89>>  | In use     | VTS-Generated/Synthetic targets
 |   1     | 18 |     | <<IMO289>> | Standard   | Clearance time to enter port
 |   1     | 19 |     | <<IMO289>> | Standard   | Marine traffic signals
 |   1     | 20 |     | <<IMO289>> | Standard   | Berthing data
@@ -1039,6 +1040,40 @@ updates for tit in <<IMO289>>.
 Breakdowns of selected Message 8 subtypes from <<IMO289>> follow.
 (These are not yet implemented by GPSD.)
 
+==== VTS-Generated/Synthetic targets ====
+
+A message 8 subtype. DAC = 001 FID = 17. Variable length: 176-556 bits.
+This message is laid out identically in <<IMO236>> and <<IMO289>>, but
+both give the maximum length as 536. This is inconsistent with the actual
+sum of the field lengths at maximum extent, which is 556.  In <<INO236>>
+it is titled "Pseudo-AIS Targets".
+
+[frame="topbot",options="header"]
+|==============================================================================
+|Field   |Len    |Description          |Member      |Units
+|72-87   | 16    |DAC/FID              |            |DAC = 001 FID = 17
+|88-89   |  2    |Identifier type      |idtype      |0 = id is the MMSI number
+|        |       |                     |            |1 = id is the IMO number
+|        |       |                     |            |2 = id is the call sign
+|        |       |                     |            |3 = Other (default)
+|90-151  | 42    |Target identifier    |See below   |Target ID data. 
+|152-155 |  4    |Spare                |            |Not used
+|==============================================================================
+
+The interpretation of the target idenifier field depends on the ppreceding type
+key.  For type 2 and 3 it is 6-bit ASCII text, for 0 and 1 it is a big-endian
+binary integer. An unkbnown target is expressed by type 3 and the string
+"@@@@@@@".
+
+<<IMO289>> says: "When MMSI or IMO number is used, the least
+significant bit should equal bit zero of the Target Identifier." It is
+unclear how "bit zero" is to be interpreted, but it is not possible to
+reconcile interpreting it as the leading bit of the field with AIS
+big-endian encoding. Settling this awaits live testing.
+
+The trailing three fields may be repeated up to 3 times (for a total of 1 to 4
+field groups) to represent up to 4 targets.
+
 ==== Text description (broadcast) ====
 
 A message 8 subtype. DAC = 001 FID = 29. Variable length: 104-1028 bits.