AIS for Boats in Australia

AIS is one of the most useful safety technologies available to recreational mariners, and one of the most frequently misunderstood.

Three claims it does not support, stated plainly:

• AIS does not replace radar.
• AIS does not make you visible to every vessel.
• A quiet AIS screen does not prove the traffic picture is complete.

What it does extremely well is exchange identity and movement information between participating stations. That is a narrower capability than it appears and a genuinely transformative one — knowing that the ship four miles off is called Pacific Aria, is making 18.4 knots, and will pass 0.3 NM ahead in eleven minutes is information no other sensor aboard can give you.

#What AIS can provide

Depending on equipment class:

• MMSI — the vessel's digital identity
• vessel name and callsign
• position
• course over ground
• speed over ground
• heading, where available
• vessel dimensions
• vessel type
• voyage information on relevant Class A equipment — destination, ETA, draught, navigational status

Vessel type and dimensions matter more than people expect. A 300 m bulk carrier and a 12 m fishing boat showing the same CPA are different problems, and the type field tells you which of the collision regulations' special cases might apply.

#Receive-only AIS

A receive-only unit lets:

YOU see transmitting AIS vessels

but does not let those vessels see you through AIS.

AMSA distinguishes receive-only equipment and notes that it does not need an MMSI, because it does not transmit.

It is genuinely useful for awareness, for planning and for coastal cruising in light traffic, and it is better than nothing by a wide margin. But it is one-directional, and being visible is most of the value. A ship's bridge team watching a screen cannot give way to a target that is not there.

For serious offshore cruising, I would buy a transceiver instead.

#Class B

Class B is the normal recreational category. AMSA identifies it for non-SOLAS vessels including pleasure craft.

Modern Class B transceivers use different channel-access technologies, and that difference is the real purchasing decision.

#CSTDMA versus SOTDMA

CSTDMA — Carrier Sense Time Division Multiple Access. The unit listens for a free transmission slot and sends when it finds one. Simpler, cheaper, and entirely serviceable in quiet coastal water.

SOTDMA — Self-Organising Time Division Multiple Access. The unit reserves slots in an organised scheme.

Practical SOTDMA advantages can include:

• improved slot management, so you are less likely to be squeezed out when the channel is congested
• higher transmission power on many units
• more frequent reporting in relevant situations

The circumstances where those matter are the offshore ones: busy shipping approaches, distance from the receiving station, and closing situations where a stale position is worse than none.

For a new offshore cruising system in 2026, SOTDMA is generally the preferable direction where budget permits. For sheltered coastal use a good CSTDMA unit remains a sensible purchase. The full comparison, including Class A, is in Class A versus Class B AIS.

#Class A

Class A is principally associated with regulated commercial and SOLAS shipping. It reports more often, at higher power, with more voyage data.

Most private cruising yachts do not need Class A simply because they travel offshore. A yacht crossing an ocean is still a non-SOLAS pleasure craft, and Class A is specified and priced for a carriage requirement you do not have.

If you operate commercially, carry passengers for hire, or are subject to specific survey requirements, confirm your obligations with AMSA rather than inferring them.

#MMSI

MMSI is a nine-digit maritime identity used by systems such as:

• DSC radio
• AIS transceiver

AMSA manages Australian MMSI allocation.

Two disciplines:

Do not copy an old MMSI into new equipment without confirming it remains correct for the vessel. The number follows the vessel and its registration, not the box — and on many units it can only be entered a limited number of times.

Enter the static data properly. Name, callsign, vessel type, dimensions and antenna position. A target appearing with no name and wrong dimensions is markedly less useful to the bridge watching you, and getting the antenna offset wrong means your displayed position is offset from where your hull actually is. See marine VHF and DSC.

#CPA and TCPA

#CPA

Closest Point of Approach — the predicted minimum distance at which a target will pass.

#TCPA

Time to Closest Point of Approach — how long until that moment.

Example:

CPA:  0.2 NM
TCPA: 8 min

That target deserves attention.

The calculation assumes both vessels maintain approximately their current vectors. If either changes course or speed, the prediction changes — and that is not a defect, it is arithmetic.

Two consequences worth internalising. First, a CPA you improved by altering course is a plan, not an outcome; watch the target until the situation is genuinely resolved. Second, a vessel altering slowly can hold an apparently safe CPA while steadily worsening the real situation.

See collision avoidance for how this feeds into an actual decision.

#AIS alarm fatigue

An alarm profile suitable offshore may be unbearable in Sydney Harbour.

The failure mode is predictable: offshore thresholds left running through busy water, the crew gets flooded, and the alarms are switched off permanently. A silenced alarm is worse than none, because it creates false confidence.

Configure alarms for the operating environment. Useful profiles:

• harbour — very tight thresholds or alarms off, relying on visual watch
• coastal — moderate
• offshore — generous CPA and TCPA, because you want early warning
• poor visibility — the most sensitive, because radar and AIS are doing the lookout's work

Do not permanently disable alarms because the defaults were noisy. Change the profile instead.

#AIS and radar are complementary

This is the central point of the whole page.

AIS shows cooperative transmitters. Only vessels that are transmitting, only while they transmit, only what they choose to send.

Radar shows physical reflective targets. Whatever returns energy, regardless of cooperation.

Radar may detect:

• a non-AIS fishing boat
• a yacht with no transceiver
• a buoy
• rain and squalls
• the coastline

AIS may provide:

• vessel name
• MMSI
• a transmitted vector and vessel type

Neither list contains the other. The strongest offshore picture uses both, which is why every vessel recommendation on this site pairs a transmitting AIS with Doppler radar — see marine radar.

#AIS aids to navigation

AIS can represent navigation aids as:

• physical — a real buoy carrying a transmitter
• synthetic — a real object whose signal is transmitted from elsewhere
• virtual — a marked position with no physical buoy at all

A virtual AIS AtoN can mark a hazard, a new wreck or a temporary exclusion without anyone laying a buoy. Australian mariners should understand these symbols before encountering them unexpectedly, because a mark on the screen with nothing visible in the water is otherwise alarming rather than informative.

#Antenna options

#Dedicated AIS antenna

Advantages: physical separation, independent hardware, two failure paths.

Disadvantages: an extra antenna and cable run.

#Active splitter

Lets AIS and VHF share one antenna.

Advantages: fewer antennas, and AIS gets the benefit of masthead height — which matters, because AIS is in the VHF band and therefore largely line-of-sight.

Disadvantages: introduces a shared component whose failure affects both systems.

For offshore sailing, weigh failure separation as heavily as installation convenience. Neither answer is wrong; it should be a deliberate choice rather than whatever the installer had in the van. See navigation redundancy.

#Integrated VHF and AIS

Garmin launched Signal VHF 400 in May 2026 with built-in Class B AIS, and other manufacturers offer integrated products.

#Advantages

• fewer boxes
• a cleaner helm
• simpler integration, and one fewer GNSS feed to configure

#Disadvantages

• a common hardware failure removes two functions
• upgrade cycles become linked — replacing the radio means replacing the AIS

On an offshore boat this is a genuine trade rather than an obvious win. Make it deliberately.

#Internet AIS

Web-based vessel tracking can be delayed or incomplete, because it depends on shore and satellite receiving stations rather than on what your own antenna hears.

It is useful for:

• passage planning
• shoreside tracking by family
• general awareness of traffic patterns

It is not a substitute for direct onboard AIS for collision avoidance. A target you can see on a website and not on your own receiver is a target you cannot rely on.

#Offshore recommendation

For a new offshore cruiser:

• a modern Class B SOTDMA transceiver
• NMEA 2000 integration, so targets appear on the chart
• a correct MMSI and complete static data
• a good antenna installation, dedicated or via an approved splitter, chosen deliberately
• sensible CPA and TCPA alarm profiles for each environment
• radar as an independent sensor, because AIS cannot see what does not transmit

#References

• AMSA AIS: www.amsa.gov.au/safety-navigation/navigation-systems/types-automatic-identification-systems
• AMSA MMSI: beacons.amsa.gov.au/mmsi/