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AIS Aid to Navigation (AtoN) Types: Physical (Real) | Virtual | Synthetic

An AIS AtoN is a digital aid to navigation that is broadcast by an authorized service provider using the AIS Message 21 (Aids-to-navigation report) and is displayed on navigation equipment such as the Electronic Chart Display Information System (ECDIS), radar, or an Integrated Navigation System (INS) (International Maritime Organization (IMO) MSC.1/Circ.1473).

The IMO, considering the mariner’s perspective, has recognized three types of AIS AtoN:

A Physical AIS AtoN (more commonly known as a Real AIS AtoN) is an AIS Message 21 transmitted from an AtoN that physically exists; and
A Virtual AIS AtoN is transmitted as a Message 21 for an AtoN that does not physically exist.
A Synthetic AIS ATON is a physical AtoN without an AIS transmitter, but for which AIS messages are broadcast from another AIS unit, usually land-based.

The International Association of Marine Aids to Navigation and Lighthouse (IALA), having assessed the possibilities of AIS technology according to AtoN service providers (national authorities), has proposed the addition of a third type: the Synthetic AIS AtoN.

In accordance with IALA Recommendation A-126, Synthetic AIS AtoN are divided into two types: monitored and predicted.

A Monitored Synthetic AIS AtoN is transmitted as a Message 21 from an AIS station that is located at a distance from the AtoN. The AtoN physically exists and there is a communication link between the AIS station and the AtoN. This communication link confirms the position and status of the AtoN. A Monitored Synthetic AIS AtoN ensures the integrity of the Message 21 and can be considered an alternative to the Real AIS AtoN.
A Predicted Synthetic AIS AtoN is transmitted as a Message 21 from an AIS station located at a distance from the AtoN. The AtoN physically exists, but it is not equipped with a monitoring device to confirm its position and status. A Predicted Synthetic AIS AtoN cannot ensure the integrity of the Message 21 (position and status), and therefore it is not recommended for use on a floating aid. The use of a Predicted Synthetic AIS AtoN on a fixed aid is acceptable, as the position will not change, but the status of the AtoN is not verified. .

Because information is transmitted using radio frequencies, each type of AIS AtoN must include Maritime Mobile Service Identity (MMSI) number, which is obtained from Innovation, Science and Economic Development Canada (ISED). As there is currently no recognized symbol for Synthetic AIS AtoN, the distinction between each type is made only by the MMSI number, the AIS AtoN coding parameters (virtual: yes/no), and the positioning mode.

**Table 2-1. Summary of MMSI numbers used and Virtual AIS AtoN coding parameters, and the positioning mode used to distinguish AIS AtoN (*IALA Recommendation A-126, Ed. 1.5, 2011*)**
Type of AIS AtoN	MMSI (ITU-R M.585-5)	AIS AtoN coding parameter Virtual (ITU-R M.1317-4)	Positioning Mode
Real	99MID1xxx	0	GPS
Synthetic	99MID1xxx	0	Surveyed
Virtual	99MID6xxx	1	Surveyed

A diamond shaped symbol is used to represent an AIS AtoN on the chart and radar systems that are interfaced with the AIS. Real and Synthetic AIS AtoN use a diamond shape with a solid line, while Virtual AIS AtoN use a dotted line (see Figure 2-1). The AIS AtoN information is displayed when you click the AIS AtoN symbol or move the cursor over it. The displayed description is the only way to distinguish a Real AIS AtoN from a Synthetic or Virtual AIS AtoN (see Figure 2-2).

Figure 2-1. Symbols for Real, Synthetic, and Virtual AIS AtoN (Source: IEC 62288 Ed.2) — Figure 2-1. Symbols for Real, Synthetic, and Virtual AIS AtoN
(*Source: IEC 62288 Ed.2*)

Figure 2-2. Comparison of information displayed for each type of AIS AtoN

**Table 2-2 Comparison of the types of AIS AtoN according to their association with a physical aid, as well as the origin of the AIS Message 21**
Type of AIS AtoN	AtoN physically exists	Transmission origin of AIS Message 21
Real	Yes	AIS unit installed on an aid
Monitored Synthetic	Yes	AIS shore station
Predicted Synthetic	Yes	AIS shore station or AIS AtoN unit
Virtual	No	AIS shore station or AIS AtoN unit

Our following products support AIS Virtual & Synthetic functionality:

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; see Table below
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 = GPS 2 = GLONASS 3 = Combined GPS/GLONASS 4 = Loran-C 5 = Chayka 6 = 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 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	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

CONTACT US FOR OUR NEW GENERATION AIS TEST EQUIPMENT (AITS-RV).

AIS Aid to Navigation (AtoN) Types: Physical (Real) | Virtual | Synthetic

Maritec Solutions | AIS AtoN Types