AIS own-vessel messages

The simulator transmits its own vessel as an AIS class A station: a position report (ITU-R M.1371 message type 1, 2 or 3) and a static and voyage data report (message type 5). Both are sent as VDO sentences, the form an AIS transponder uses for the own vessel, and optionally also as VDM sentences, the form used for received targets, for consumers that ignore VDO. The four registry entries are independent, so any combination can be enabled. The design is recorded in ADR 0013.

Registry id	Formatter	Message	Default	Default period
VDO-POS	VDO	Position report, type 1 (or 2, 3)	on	2 s
VDO-STATIC	VDO	Static and voyage data, type 5	on	30 s
VDM-POS	VDM	The same position report framed as received	off	2 s
VDM-STATIC	VDM	The same static data framed as received	off	30 s

The default talker is AI.

Sentence framing

!AIVDO,1,1,,A,13SsIh@vA11dWJ`Eg0R1nAKh0000,0*34
!AIVDO,2,1,6,A,53SsIhH00001<TmP000plD61<TmDh5@u:1P0000U1P43340Ht4PAAjCP@000,0*72
!AIVDO,2,2,6,A,00000000000,2*20

#	Field
1	Total number of sentences carrying this message
2	Number of this sentence, from 1
3	Sequential message id, 0 to 9, shared by the sentences of one message; empty for a single-sentence message. Derived from the simulated clock (seconds modulo 10).
4	Radio channel, always A
5	Payload: the message bits in groups of six, each written as one character (0-W, `-w)
6	Number of fill bits added to complete the last character, 0 to 5

The payload is split so that every sentence stays within 82 bytes: at most 60 payload characters per sentence. A position report (168 bits, 28 characters) needs one sentence, a static data report (424 bits, 71 characters) needs two.

Text fields use the six-bit ASCII alphabet (@, A-Z, [\]^_, space, ! to ? including digits). Lower-case letters are sent upper-case, characters outside the alphabet become ?, and fields are padded with @.

Position report (message types 1, 2 and 3)

Bits	Field	Value sent
6	Message type	position_report_type from the profile: 1 scheduled, 2 assigned, 3 in response to interrogation; anything else is sent as 1
2	Repeat indicator	0
30	MMSI	mmsi
4	Navigational status	navigation_status: 0 under way using engine, 8 under way sailing, ...
8	Rate of turn	4.733 × √(rate in °/min) with the rate's sign, rounded; ±126 for 708 °/min and faster. The simulated rate counts as a turn indicator, so ±127 (no turn indicator) is never sent; −128 (not available) when the rate is not a number
10	Speed over ground	tenths of a knot; 1022 for 102.2 kn and above; 1023 (not available) without a fix
1	Position accuracy	1 with a differential fix, 0 otherwise
28	Longitude	1/10000 minutes, east positive; 181° (not available) without a fix
27	Latitude	1/10000 minutes, north positive; 91° (not available) without a fix
12	Course over ground	tenths of a degree; 3600 (not available) without a fix
9	True heading	whole degrees, normalised to 0 to 359 (−10° is sent as 350); 511 (not available) when the heading is not a number
6	Time stamp	UTC second of the simulated clock
2	Manoeuvre indicator	0 (not available)
3	Spare	0
1	RAIM flag	0
19	Radio status	0

Static and voyage data (message type 5)

Bits	Field	Value sent
6	Message type	5
2	Repeat indicator	0
30	MMSI	mmsi
2	AIS version	2 (ITU-R M.1371-5, the edition whose layouts are implemented)
30	IMO number	imo_number, 0 for none
42	Call sign	call_sign, 7 characters
120	Vessel name	name, 20 characters
8	Type of ship and cargo	ship_type, 0 to 255 (37 pleasure craft, 36 sailing, 70 cargo, 30 fishing)
9	Dimension to bow	dimension_to_bow_m, whole metres, at most 511
9	Dimension to stern	dimension_to_stern_m
6	Dimension to port	dimension_to_port_m, at most 63
6	Dimension to starboard	dimension_to_starboard_m
4	Position fix device	1 (GPS)
4, 5, 5, 6	ETA month, day, hour, minute	0, 0, 24, 60 (not available)
8	Maximum static draught	draught_m in tenths of a metre, at most 25.5
120	Destination	destination, 20 characters, empty when not set
1	DTE	0 (data terminal ready)
1	Spare	0

Defaults

The built-in profile transmits MMSI 239000001, name NMEA SIMULATOR X, call sign SIMX, ship type 37, dimensions 12/4/3/3 m and a draught of 1.8 m. Every value is set in the simulation.seed.ais object of the profile and on the Vessel tab of the settings dialog.

Checking the output

The repository ships tools/check_ais_stream.py, which decodes the stream with the third-party pyais library and checks the MMSI, name and call sign; CI runs it on every platform. The golden lines above are also the fixture tests/fixtures/ais/own_vessel.nmea that the same check reads.

pip install pyais
nmeasim run --stdout --quiet --duration 3 | python3 tools/check_ais_stream.py --mmsi 239000001