AIS and VHF in Collision Avoidance

What AIS is and how it works

In short: Read Bridge Equipment: SOLAS V/19 and Risk of Collision and Action to Avoid Collision first. AIS (Automatic Identification System) is a VHF radio system: every fitted ship broadcasts who she is, where she is and what she is doing, and receives the same from others.

• AIS = Automatic Identification System. It is one of the items on the SOLAS V/19 carriage list.
• Frequency: the VHF maritime band, on two dedicated channels: AIS 1 = 161.975 MHz and AIS 2 = 162.025 MHz (MGN 324 (M+F) Amd 2, 4.1). A shore station can switch ships to other channels where these are not available (IMO A.1106(29), para 8).
• Range: VHF range, typically 20–30 NM at sea, depending on antenna height. Because it is radio, AIS can “see” round bends and behind islands if the land is not too high (IMO A.1106(29), para 10).
• It works on its own: the unit transmits continuously and automatically, with no action by the OOW. A shore station can “poll” a ship to get more frequent reports — it can raise the reporting rate, never lower it (IMO A.1106(29), para 11).
• Purpose: identify ships, help target tracking, help search and rescue, cut down verbal ship reporting and improve situational awareness (IMO A.1106(29), para 4).

What feeds the AIS

• VHF antenna — one VHF transmitter, two multi-channel VHF receivers and one channel 70 receiver for channel management (IMO A.1106(29), Appendix 1).
• GNSS (GPS) — position, time (UTC), and usually COG and SOG. The AIS also has its own GNSS receiver for timing. See GNSS errors and DOP.
• Gyro — heading.
• Rate of turn (ROT) indicator, or ROT derived from the gyro.
• Speed log and other sensors, through the interfaces to heading and speed devices.
• Links out to radar/ARPA, ECDIS and the integrated bridge, plus a built-in integrity test (BIIT) and a minimum keyboard and display (MKD).

SOTDMA — how many ships share two channels

• SOTDMA = Self-Organising Time Division Multiple Access.
• Time on the channel is cut into short slots. A Class A AIS pre-allocates (books) its future transmission slots and tells everyone, so other stations avoid them. No shore station is needed to organise it — the ships organise themselves.
• That is why the system can carry a great many ships at once, with priority for Class A. Class B units send less often, or only when free slots are available (IMO A.1106(29), para 9).

AIS classes and who must carry AIS

In short: Class A is the full SOLAS unit. Class B is the cheaper unit for craft outside SOLAS. Big and passenger ships must carry Class A; many small craft carry nothing.

• Carriage (SOLAS V/19.2.4): all ships of 300 GT and upwards on international voyages; cargo ships of 500 GT and upwards not on international voyages; all passenger ships, whatever their size. Fitting was phased in from 1 July 2002 to 1 July 2008 (IMO A.1106(29), para 3).
• Not fitted: warships, naval auxiliaries and government ships are not required to carry AIS. Small craft such as leisure craft and fishing boats may not have it, and some shore stations (including some VTS centres) have none (IMO A.1106(29), paras 3 and 32).
• Class A: meets the IMO carriage standard. It sends the full set of data, at the fastest rates, and has priority on the channel.
• Class B: does not fully meet the IMO standard; used on ships outside the SOLAS carriage rules (for example many yachts). Reports less often. Two types: “SO” (self-organised, like Class A) and “CS” (carrier-sense: it listens and transmits only in a free slot) (IMO A.1106(29), para 5 and Table 3).
• On the bridge: a 25 m motor yacht with Class B looks very different on your display from a 3,000 GT ship with Class A — slower updates, and often no heading (see Limitations).

AIS data and reporting intervals

In short: A ship sends three kinds of data — static, dynamic and voyage-related — plus short safety messages. Static and voyage data go every 6 min; dynamic data go every 2 s to 3 min, faster when the ship is fast or turning.

Static data — set on installation, sent every 6 min

• MMSI (Maritime Mobile Service Identity)
• Name and call sign
• IMO number
• Type of ship (chosen from a list)
• Length and beam (principal dimensions)
• Location of the position-fixing (EPFS/GNSS) antenna on board, and the type of positioning system
• Changes only if the ship changes name, MMSI, antenna position or type (IMO A.1106(29), para 12).

Dynamic data — from the sensors, sent every 2 s to 3 min

• Position, with accuracy indication (about 10 m) and integrity status
• Time stamp in UTC
• COG (course over ground) and SOG (speed over ground)
• Heading (true, from the gyro)
• Rate of turn (ROT)
• Navigational status — the only dynamic item the OOW enters by hand (underway by engines, at anchor, NUC, restricted in ability to manoeuvre, moored, constrained by draught, aground, engaged in fishing, underway by sail).
• Some revision lists put navigational status under voyage data instead; IMO A.1106(29) Table 1 lists it as dynamic, entered manually.

Trap: CPA and TCPA are not transmitted. Your own AIS display (or radar) calculates them from the target’s broadcast position, course and speed (IMO A.1106(29), para 7).

Voyage-related data — entered by hand, sent every 6 min

• Draught (maximum for the voyage)
• Hazardous cargo type (DG, HS, MP) — quantities not needed
• Destination and ETA (IMO recommends the UN/LOCODE; the destination field can hold both the port of departure and the next port of call)
• Route plan (waypoints), at the Master’s discretion
• Many units also have a field for persons on board (POB); it is not in the IMO A.1106(29) table.

Short safety-related messages

• Free-text messages to one ship (by MMSI) or to all ships, e.g. “iceberg sighted” or “buoy off station”. Up to 158 characters, but keep them short (IMO A.1106(29), para 15).
• They help meet SOLAS V/31 (danger messages: report dangerous ice, derelicts and other dangers “by all the means” at your disposal).
• They are extra: they do not replace any GMDSS requirement (para 17). AIS-SART alerts also arrive as special AIS messages — see EPIRB, SART and DSC alerts.

Reporting intervals (IMO A.1106(29), Tables 2 and 3)

• Memory hook for Class A: “2 to 10 seconds underway, 3 minutes at anchor, 6 minutes for static”.

Keeping your own AIS data right

In short: Wrong data from your AIS can mislead every ship around you. Enter voyage data at the start of each voyage, update it when anything changes, and check it regularly.

• Enter by hand, at the start of the voyage and whenever it changes: draught, hazardous cargo, departure/destination and ETA, route plan, the correct navigational status, and safety messages (IMO A.1106(29), para 23; MGN 324 (M+F) Amd 2, 4.6). Put it on the navigating officer’s checklist.
• Navigational status follows the lights and shapes: when you change them, change the AIS too. Example: after a steering failure you show NUC lights or shapes — set AIS to NUC at the same time (see machinery failure).
• Check static data whenever there is a reason, and at least once per voyage or once a month, whichever is shorter. Change it only on the Master’s authority (para 24).
• Check dynamic data regularly: position (on WGS 84), SOG and sensor information — more often in coastal waters (paras 25 and 27).
• The BIIT raises an alarm and stops transmission if the unit fails, but it cannot check whether the sensor data are right (paras 26–27).
• Easiest outside check: ask another ship or a shore station what they see from you (MGN 324 (M+F) Amd 2, 4.3).

Switching AIS off

• AIS should always be on when underway or at anchor (IMO A.1106(29), para 22; SOLAS V/19.2.4).
• The Master may switch it off if keeping it on would compromise the ship’s safety or security (e.g. piracy threat).
• Then: record it in the logbook with the reason; report it to the competent authority if in a mandatory ship reporting system (unless that adds to the danger); switch on again as soon as the danger has passed. Stored data are kept; the unit transmits again after a 2-minute start-up.
• UK: if it has been off, turn it on within 100 NM of the UK coast (MGN 324 (M+F) Amd 2, 4.2).

AIS on the MKD, radar and ECDIS

In short: The basic AIS screen (MKD) is a three-line text display. AIS is far more useful overlaid on the radar or ECDIS — but treat that overlay with care.

• MKD = Minimum Keyboard Display: lets you enter data by hand, shows at least 3 lines of alphanumeric data (bearing, range and name of a selected ship) and must have an alarm (IMO A.1106(29), paras 26 and 29). Scroll sideways for more data on that ship, down for other ships.
• MGN 324 (M+F) Amd 2, 4.4: a bare 3-line MKD misses most of the benefit of AIS — the graphic overlay on radar.
• Radars and ECDIS installed on or after 1 July 2008 can show AIS overlays (MGN 324, 4.9).
• Target types on a graphic display: sleeping (just a symbol), activated (vector, heading, ROT), selected (full data with CPA/TCPA), dangerous (inside your CPA/TCPA limits — alarm) and lost (signal stopped — alarm at last position) (IMO A.1106(29), para 30).
• AIS aids to navigation: a physical AIS AtoN is an open diamond; a virtual AtoN (no real buoy there) is an open diamond with a crosshair (MGN 324, section 5). A new wreck may get one — see emergency wreck marking buoy. On ECDIS, check AIS targets against the chart and radar.
• Pilot plug: the Pilot Connector Socket and a power outlet should be near the main conning position (MGN 324, 4.5).
• How the radar merges a tracked echo and an AIS report into one target is covered in target tracking and AIS association. Key point here: a fused target shown with AIS priority is for awareness only — be ready to switch to the radar-tracked target and its CPA/TCPA (MGN 324, 4.10).

Limitations of AIS in collision avoidance

In short: AIS supports, but never replaces, the look-out, radar and the COLREG. It only shows ships that carry AIS, have it switched on, and feed it correct data.

• Can AIS be used to apply the COLREG on its own? No. It relies on the other ship’s equipment and data entry. It can, however, greatly improve situational awareness.
• AIS is “an additional source of navigational information”: it supports radar target tracking and VTS, and does not remove the OOW’s duty to comply with the COLREG, especially Rule 7 (IMO A.1106(29), para 41). Never use it as the sole source (para 42).
• Rule 5 and Rule 7(c): AIS does not replace a proper look-out by all available means; relying on it alone risks assumptions on scanty information (MGN 324 (M+F) Amd 2, 4.11.2).
• Over ground, not through the water: AIS gives COG/SOG. The aspect you need for the COLREG comes from course and speed through the water — in a strong tide the two differ (MGN 324, 4.11.1).
• Data quality: received data are only as good as the data sent. Wrong input or badly set-up sensors give wrong targets (IMO A.1106(29), paras 35–39). Equipment made before 2003 may give poorer COG/SOG (MGN 324, 4.7).
• Position mismatch: the AIS position comes from the target’s GNSS and its antenna position, so it may not sit exactly on the radar echo (MGN 324, 4.11.4).
• No heading: ships under 500 GT that need no gyro are unlikely to transmit heading (MGN 324, 4.11.5).
• Ghosts: synthetic targets (e.g. sent by VTS for ships without AIS) and virtual AtoN can appear anywhere. Take special care when an AIS target has no radar echo (MGN 324, 4.11.6; IMO A.1106(29), para 45).

AIS compared with radar

• For the radar side of the decision — plotting, CPA/TCPA, trial manoeuvre — see collision avoidance with radar.

VHF and collision avoidance — MGN 324

In short: Do not agree passing arrangements on VHF (“let’s go green to green”). Apply the COLREG. VHF can occasionally help — for example in pilotage waters — but the COLREG stay uppermost.

• Current notice: MGN 324 (M+F) Amendment 2 (22 March 2022), Navigation: watchkeeping safety – use of VHF radio and AIS. It replaced Amendment 1.

Using VHF properly (MGN 324, section 2)

• Channel 16 (156.8 MHz): distress, urgency, very brief safety messages, and calling to move to a working channel.
• Channel 70 (156.525 MHz): DSC only — never voice.
• Channel 13: worldwide navigation safety channel, mainly ship-to-ship.
• UK routine ship-to-ship: channels 6, 8, 72 and 77.
• Port operations and VTS channels: only for operational, ship movement and safety traffic.
• Identify your ship (name or call sign) before every transmission. Each VHF station must be under the control of a certificate holder.
• More: IMO Resolution A.954(23) (Proper use of VHF channels at sea) and Admiralty List of Radio Signals Vol. 5.

Why VHF agreements cause collisions (MGN 324, section 3)

• Who am I talking to? At night, in restricted visibility or with more than two ships about, you can rarely be sure the ship answering is the ship you mean.
• Language: misunderstanding and vague messages.
• Time: calling takes time that should go to applying the COLREG — and even a “successful” call can lead to a non-COLREG manoeuvre that causes the collision.
• AIS showing the ship’s name does not remove these dangers (MGN 324, 4.11.3).
• Real cases quoted by the MGN:
  • 1995: the court found the VHF talk probably contributed, by distracting officers from the radar, and advised superintendents to forbid VHF passing agreements.
  • 2002: in fog, a call for a red-to-red pass — the caller actually meant green to green. Neither ship watched the other’s action; collision.
  • 2006: a starboard-to-starboard pass agreed with the wrong ship, needing a non-COLREG turn; collision.
  • 2014: open sea, little traffic; officers relied only on VHF to agree a non-COLREG manoeuvre, and a non-native language confused the relieving officer; collision.
• Exception: VHF may sometimes help, e.g. in pilotage waters, but understand the risks and follow the COLREG as far as possible (MGN 324, 3.8).

“Never” or “rarely”? What to say

• A common revision line says “never use VHF to ask for green to green”. The MGN itself is softer: VHF use for collision avoidance is discouraged and “sometimes” justified, with the COLREG uppermost. Both lead to the same bridge practice.
• Safe oral answer: “I do not agree manoeuvres on VHF. I apply the COLREG.” If you add anything, say VHF may only support an action that already complies with the rules.
• On the bridge: if the other ship does not seem to act, use the COLREG tools — the doubt signal of at least five short and rapid blasts (COLREG Rule 34(d)), and your own early, large action.