Notes relating to Table 73:

(1)     When using Fig. 41 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 (see Table 73).

 
This message should be transmitted immediately after any parameter value was changed.
Note on AtoN within AIS:
The competent international body for aids-to-navigation, IALA, defines an AtoN as: “a device or system external to vessels designed and operated to enhance safe and efficient navigation of vessels and/or vessel traffic.” (IALA Navguide, Edition 2010).
The IALA Navguide stipulates: “A floating aid to navigation, which is out of position, adrift or during the night is unlighted, may itself become a danger to navigation. When a floating aid is out of position or malfunctioning, navigational warnings must be given.” Therefore, a station, which transmits Message 21, could also transmit safety related broadcast message (Message 14) upon detecting that the floating AtoN has gone out of position or is malfunctioning, at the competent authority’s discretion.

 
TABLE 74
The nature and type of aids to navigation can be indicated with 32 different codes

	Code	Definition
	0	Default, Type of AtoN not specified
	1	Reference point
	2	RACON
	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.