Automatic Idenfication System Message 21
The following is a detailed description of the Automatic Identification System (AIS) Message 21. For basic information on additional AIS messages, consult the list of AIS messages.
| Parameter | Bits | Description |
|---|---|---|
| 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 (AtoNs) | 5 | 0 = not available = default; refer to appropriate definition set up by IALA; |
| Name of aids to navigation (AtoN) | 120 | Maximum 20 characters 6-bit ASCII“@@@@@@@@@@@@@@@@@@@@” = 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 (<10m)0 = low (>10m)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) |
| Dimension/reference for position | 30 | Reference point for reported position; also indicates the dimension of an AtoN. |
| Type of electronic position fixing device | 4 | 0 = Undefined (default) 1 = GPS2 = GLONASS3 = Combined GPS/GLONASS4 = Loran-C5 = Chayka6 = Integrated Navigation System 7 = surveyed. For fixed AtoN and Virtual AIS AtoN, the charted position should be used. The accurate position enhances its function as a radar reference target8 = Galileo9-14 = not used15 = 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 | 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, 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 |
| Virtual AtoN flag | 1 | 0 = default = real AtoN at indicated position;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 |
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