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List: gpsd-commit-watch
Subject: [Gpsd-commit-watch] r5593 - trunk/www
From: esr at BerliOS <esr () mail ! berlios ! de>
Date: 2009-05-27 11:22:36
Message-ID: 200905271122.n4RBMaGJ029006 () sheep ! berlios ! de
[Download RAW message or body]
Author: esr
Date: 2009-05-27 13:22:34 +0200 (Wed, 27 May 2009)
New Revision: 5593
Modified:
trunk/www/AIVDM.txt
Log:
Describe meessage type 21. Add section on improving this document.
Modified: trunk/www/AIVDM.txt
===================================================================
--- trunk/www/AIVDM.txt 2009-05-27 09:52:12 UTC (rev 5592)
+++ trunk/www/AIVDM.txt 2009-05-27 11:22:34 UTC (rev 5593)
@@ -69,6 +69,26 @@
extended AIVDM to me by email. He also communicated some critical
information from <<IEC-PAS>>.
+== Improving This Document ==
+
+To avoid copyright difficulties, I rely only on freely redistributable
+public documents and paraphrased reports from people who have seen the
+relevant proprietary standards. If you are such a person, please help
+by reporting the following to be included in future versions of this
+document:
+
+* The meaning of the type field in the Aid-To-Navigation message (type
+ 21).
+
+* How the end of the Name Extension field is detected or computed in
+ Type 21 messages.
+
+* Field offsets, types, and descriptions for messages 23-26 not
+ described in <<IALA>>.
+
+* Sample sentences of types 6-8, 10-18, and 20-26 together with
+ decoded dumps of the field values.
+
== Open-Source Implementations ==
The http://gpsd.berlios.de/[GPSD project] ships an AIVDM/AIVDO
@@ -350,11 +370,15 @@
come from <<Schwehr>>, who reports they were added in Version 3 of
<<ITU1371>>.
+The field breakdowns in this document have been checked against
+live decoded data rendered by known-good implementations
+for message types 1-5, 9, 18, and 19.
+
=== Types 1, 2 and 3: Position Report Class A ===
Type 1, 2 and 3 messages share a common reporting structure for
-navigational information; we'll call it the Common Navigation
-Block. This is the information likely to be of interest for decoding
+navigational information; we'll call it the Common Navigation Block
+(CNB). This is the information likely to be of interest for decoding
software.
.Common Navigation Block
@@ -721,8 +745,8 @@
38-49 12 Altitude Unsigned integer: See below
50-59 10 SOG Unsigned integer: See below
60-60 1 Position Accuracy See below
-61-88 28 Longitude Minutes/10000 (see below)
-89-115 27 Latitude Minutes/10000 (see below)
+61-88 28 Longitude Minutes/10000 (as in CNB)
+89-115 27 Latitude Minutes/10000 (as in CNB)
116-127 12 Course Over Ground (COG) Relative to true north to 0.1 degree
128-133 6 Time Stamp Unsigned integer: UTC second.
134-141 8 Regional reserved
@@ -811,7 +835,7 @@
A less detailed report for vessels using Class B transponders. Omits
navigational status and rate of turn. Fields are encoded as in the
-common navigation block. 168 bits.
+common navigation block. 168 bits total.
It is reported that ITU 1371-3 introduces bit-by-bit interpretations
for the Regional Reserved field in this message; I don't yet know what
@@ -826,8 +850,8 @@
38-45 8 Regional Reserved
46-55 10 Speed Over Ground (SOG) As in common navigation block
56-56 1 Position Accuracy See below
-57-84 28 Longitude Minutes/10000 (see below)
-85-111 27 Latitude Minutes/10000 (see below)
+57-84 28 Longitude Minutes/10000 (as in CNB)
+85-111 27 Latitude Minutes/10000 (as in CNB)
112-123 12 Course Over Ground (COG) Relative to true north to 0.1 degree
124-132 9 True Heading (HDG) 0 to 359 degrees, 511 = not available
133-138 6 Time Stamp Second of UTC timestamp.
@@ -840,8 +864,9 @@
A more detailed report for vessels using Class B transponders. Omits
navigational status and rate of turn. Fields are encoded as in the
-common navigation block. Note that until just before the reserved field
-at bit 139 this is identical to message 18. 312 bits.
+common navigation block and the Type 5 message. Note that until just
+before the reserved field at bit 139 this is identical to message 18.
+312 bits total.
In practice, the information in the ship name and dimension fields is
not reliable, as it has to be hand-entered by humans rather than
@@ -856,8 +881,8 @@
38-45 8 Regional Reserved
46-55 10 Speed Over Ground (SOG) As in common navigation block
56-56 1 Position Accuracy See below
-57-84 28 Longitude Minutes/10000 (see below)
-85-111 27 Latitude Minutes/10000 (see below)
+57-84 28 Longitude Minutes/10000 (as in CNB)
+85-111 27 Latitude Minutes/10000 (as in CNB)
112-123 12 Course Over Ground (COG) Relative to true north to 0.1 degree
124-132 9 True Heading (HDG) 0 to 359 degrees, 511 = not available
133-138 6 Time Stamp Second of UTC timestamp.
@@ -875,6 +900,68 @@
307-311 5 Spare Unused, should be zero
--------------------------------------------------------------------------------
+=== Type 21: Aid-to-Navigation Report ===
+
+Identification and location message to be emitted by aids to
+navigation such as buoys and lighthouses.
+
+This message is unusual in that it varies in length depending on the
+presence and size of the Name Extension field. May vary between 272
+and 360 bits.
+
+`--------`---`------------------------`-----------------------------------------
+Field Len Description Units
+--------------------------------------------------------------------------------
+0-5 6 Message Type Unsigned integer: 19
+6-7 2 Repeat Indicator As in Common Navigation Block
+8-37 30 MMSI Unsigned integer: 9 decimal digits
+38-42 5 Aid type See Below
+43-162 120 Name Name of the aid to navigation
+163-163 1 Position Accuracy As in common navigation block
+164-191 28 Longitude Minutes/10000 (as in CNB)
+192-218 27 Latitude Minutes/10000 (as in CNB)
+219-227 9 Dimension to Bow Unsigned integer: Meters
+228-236 9 Dimension to Stern Unsigned integer: Meters
+237-242 6 Dimension to Port Unsigned integer: Meters
+243-248 6 Dimension to Starboard Unsigned integer: Meters
+249-252 4 Type of EPFD As in Message Type 4
+253-258 6 UTC Second As in Message Type 5
+259-259 1 Off-Position Indicator See Below
+260-267 8 Regional reserved
+268-268 1 RAIM flag As for common navigation block
+269-269 1 Virtual-aid flag See Below
+270-270 1 Assigned-mode flag See <<IALA>> for details
+271-271 1 Spare
+272-360 88 Name Extension See Below
+--------------------------------------------------------------------------------
+
+According to [[IALA]], the aid type field has values 1-15 for fixed
+and 16-31 for floating aids to navigation. I have not yet found a
+public description of the type codes.
+
+The name field is 6-bit ASCII. If this field is full (has no trailing @
+characters) the decoder should interpret the Name Extension field
+later in the message and concatenate it to this one to obtain the full name.
+
+<<IALA>> describes bits 219-248 As "Dimension/Reference for Position",
+implying that it is vessel dimensions as in message type 5.
+
+The Off-Position Indicator is for floating Aids-to-Navigation only: 0
+= on position; 1 = off position. Only valid if UTC second is equal to
+or below 59.
+
+The Virtual Aid flag is interpreted as follows: 0 = default = real Aid
+to Navigation at indicated position; 1 = virtual Aide to
+Navigation simulated by nearby AIS station.
+
+If present, the Name Extension consists of packed six-bit ASCII
+characters followed by 0-6 bytes of padding to an 8-bit boundary. The
+<<IALA>> description says "This parameter should be omitted when no
+more than 20 characters for the name of the A-to-N are needed in
+total. Only the required number of characters should be transmitted,
+i.e. no @-character should be used.", but does not specify how to
+detect end-of-field.
+
== Local extensions ==
Some regional authorities extend the AIS message set.
@@ -1003,4 +1090,5 @@
Version 1.6 corrects some minor errors in the interpretation of Type
5 messages.
-Version 1.7 adds description for Type 10, 11, and 19 messages.
+Version 1.7 adds descriptions for Type 10, 11, 19, and 21 messages,
+and the new section on Improving This Document.
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