Navigation Center
United States Coast Guard
U.S. Department of Homeland Security

AIS Aids to Navigation (ATON) Report (Message 21)

This message is used by an aids to navigation (AtoN) AIS station. It is generally transmitted autonomously at a rate of once every three minutes and should not occupy more than two slots.




Message ID 6 Identifier for Message 21
Repeat indicator 2 Used by the repeater to indicate how many times a message has been repeated.
ID 30 MMSI number
Type of aids-to-navigation 5 0 = not available = default; refer to appropriate definition set up by IALA; see Table below
Name of Aids-to-Navigation 120 Maximum 20 characters 6-bit ASCII, as defined in Table 47
“@@@@@@@@@@@@@@@@@@@@” = not available = default.
The name of the AtoN may be extended by the parameter “Name of Aid-to-Navigation Extension” below
Position accuracy 1 1 = high (<10 m)
0 = low (>10 m)
0 = default

Longitude 28 Longitude in 1/10 000 min of position of an AtoN (±180°, East = positive, West = negative
181 = (6791AC0h) = not available = default)
Latitude 27 Latitude in 1/10 000 min of an AtoN (±90°, North = positive, South = negative
91 = (3412140h) = not available = default)
reference for position
30 Reference point for reported position; also indicates the dimension of an AtoN.  See Figure below.
Type of electronic position fixing device 4 0 = Undefined (default)
1 = GPS
3 = Combined GPS/GLONASS
4 = Loran-C
5 = Chayka
6 = Integrated Navigation System
7 = surveyed. For fixed AtoN and virtual AtoN, the charted position should be used. The accurate position enhances its function as a radar reference target
8 = Galileo
9-14 = not used
15 = internal GNSS
Time stamp 6 UTC second when the report was generated by the EPFS (0-59 or 60) if time stamp is not available, which should also be the default value or 61 if positioning system is in manual input mode or 62 if electronic position fixing system operates in estimated (dead reckoning) mode or 63 if the positioning system is inoperative)
Off-position indicator 1 For floating AtoN, only: 0 = on position; 1 =  off position.
NOTE 1 – This flag should only be considered valid by receiving station, if the AtoN is a floating aid, and if time stamp is equal to or below 59. For floating AtoN the guard zone parameters should be set on installation
AtoN status 8 Reserved for the indication of the AtoN status see {IEC 62288, Annex L}
00000000 = default
RAIM-flag 1 RAIM (Receiver autonomous integrity monitoring) flag of electronic position fixing device; 0 = RAIM not in use = default; 1 = RAIM in use see Table 50
AtoN flag
1 0 = default = real AtoN at indicated position; {For synthetic AtoN, also set Repeat Indicator field to 1}
1 = virtual AtoN, does not physically exist.
Assigned mode flag 1 0 = Station operating in autonomous and continuous mode = default
1 = Station operating in assigned mode
Spare 1 Spare. Not used. Should be set to zero. Reserved for future use
Name of Aid-to-Navigation Extension 0, 6, 12, 18, 24, 30, 36, ... 84 This parameter of up to 14 additional 6-bit-ASCII characters for a
2-slot message may be combined with the parameter “Name of Aid-to-Navigation” at the end of that parameter, when more than 20 characters are needed for the name of the AtoN. 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
Spare 0, 2, 4, or 6 Spare. Used only when parameter “Name of Aid-to-Navigation Extension” is used. Should be set to zero. The number of spare bits should be adjusted in order to observe byte boundaries
Number of bits 272-360 Occupies two slots

Reference point for reported position

Image Depicting Reference Points for AIS Reported Positions



Bit fields

Distance (m)

A 9 Bit 21 – Bit 29 0-511
511 = 511 m or greater
B 9 Bit 12 – Bit 20 0-511
511 = 511 m or greater
C 6 Bit 6 – Bit 11 0-63
63 = 63 m or greater
D 6 Bit 0 – Bit 5 0-63
63 = 63 m or greater

(1)     When using this figure for AtoN the following should be observed:

  • For fixed Aids-to-Navigation, virtual AtoN, and for off-shore structures, the orientation established by the dimension A should point to true north.
  • For floating aids larger than 2 m * 2 m the dimensions of the AtoN should always be given approximated to a circle, i.e. the dimensions should always be as follows A = B = C = D ≠ 0. (This is due to the fact that the orientation of the floating Aid to Navigation is not transmitted. The reference point for reported position is in the centre of the circle.)
  • A = B = C = D = 1 should indicate objects (fixed or floating) smaller than or equal to 2 m * 2 m. (The reference point for reported position is in the centre of the circle.)
  • Floating off shore structures that are not fixed, such as rigs, should be considered as Code 31 type from Table 74 AtoN. These structures should have their “Dimension/reference for position” parameter as determined above in Note (1).
  • For fixed off shore structures, Code 3 type from Table 74, should have their “Dimension/reference for position” parameter as determined above in Note (1). Hence, all off shore AtoN and structures have the dimension determined in the same manner and the actual dimensions are contained in Message 21.

(2)     When transmitting virtual AtoN information, i.e. the virtual/pseudo AtoN Target Flag is set to one (1), the dimensions should be set to A=B=C=D=0 (default). This should also be the case, when transmitting “reference point” information.

Type of aids to navigation



Definition (Type of aid to navigation)

  0 Default, Type of AtoN not specified
  1 Reference point
  3 Fixed structures off-shore, such as oil platforms, wind farms.
(NOTE 1 – This code should identify an obstruction that is fitted with an AtoN AIS station)
  4 Emergency Wreck Marking Buoy
Fixed AtoN 5 Light, without sectors
  6 Light, with sectors
  7 Leading Light Front
  8 Leading Light Rear
  9 Beacon, Cardinal N
  10 Beacon, Cardinal E
  11 Beacon, Cardinal S
  12 Beacon, Cardinal W
  13 Beacon, Port hand
  14 Beacon, Starboard hand
  15 Beacon, Preferred Channel port hand
  16 Beacon, Preferred Channel starboard hand
  17 Beacon, Isolated danger
  18 Beacon, Safe water
  19 Beacon, Special mark
Floating AtoN 20 Cardinal Mark N
  21 Cardinal Mark E
  22 Cardinal Mark S
  23 Cardinal Mark W
  24 Port hand Mark
  25 Starboard hand Mark
  26 Preferred Channel Port hand
  27 Preferred Channel Starboard hand
  28 Isolated danger
  29 Safe Water
  30 Special Mark
  31 Light Vessel/LANBY/Rigs

NOTE 1 – The types of aids to navigation listed above are based on the IALA Maritime Buoyage System, where applicable.

NOTE 2 – There is potential for confusion when deciding whether an aid is lighted or unlighted. Competent authorities may wish to use the regional/local section of the message to indicate this.



(Source : International Telecommunications Union Recommendation ITU-R M.1371-5)