AIS and Radar for Collision Avoidance

Master Mariner Directory  |  2024

Collision avoidance at sea has two electronic layers: AIS, which identifies vessels that have transponders and are actively transmitting their position, and radar, which detects everything with a significant radar cross-section whether or not it carries any electronics at all. The critical thing to understand about both systems is their gaps. AIS tells you where equipped vessels are; it tells you nothing about the fishing boat that stripped its transponder, the warship running silent, or the container of timber that washed overboard in the last storm. Radar finds metal targets regardless — but requires interpretation, attention, and correctly adjusted controls to be useful.

The offshore sailor who relies on either system alone is operating with incomplete information. The one who understands how to use both together — and who maintains an actual watch rather than treating the chartplotter as a substitute for looking out — has the collision avoidance picture that conditions demand.

Class B AIS Transponders: What They Transmit

AIS (Automatic Identification System) is a VHF radio-based automatic position reporting system. Every vessel equipped with a Class B AIS transponder broadcasts a data packet every 30 seconds when the vessel is moving, or every 3 minutes at anchor, that contains: MMSI number, vessel name, call sign, vessel type, dimensions, GPS position, speed over ground (SOG), course over ground (COG), rate of turn, and navigational status (underway, at anchor, not under command, etc.).

Any AIS receiver — including the receiver built into most modern chartplotters — picks up these transmissions and displays each vessel as a target on the chart, complete with a vector showing its projected course and speed. At a glance, you can see every AIS-equipped vessel within VHF range (typically 20–40nm), their heading, speed, and projected track.

Class B is the recreational standard; Class A is mandatory for commercial vessels over 300 GT and all passenger vessels. The differences are primarily transmission power and update frequency: Class A transmits at 12.5 watts every 2–10 seconds depending on speed, while Class B transmits at 2 watts. Both are visible on all AIS receivers. The practical result is that a Class A commercial vessel (a tanker, container ship, bulk carrier) has more visible, more frequently updated AIS targets than a Class B recreational boat — which is appropriate, given that a 300-meter ship traveling at 18 knots closes distance several times faster than a 12-knot sailboat.

CPA and TCPA: The Alarms That Matter

CPA (Closest Point of Approach) and TCPA (Time to Closest Point of Approach) are the two alarm parameters that determine whether an AIS target requires attention. CPA is the minimum distance between your vessel and the target if both maintain current course and speed. TCPA is how many minutes until that closest point occurs.

The alarm thresholds most commonly used by offshore sailors: CPA of 0.5 to 1.0 nautical miles, TCPA of 20 to 30 minutes. When any AIS target is projected to pass within 1nm in the next 30 minutes, the chartplotter sounds an alarm. At that point, you have time to assess the situation — look out the companionway, verify the target's behavior, consult the radar, and decide whether a course alteration is appropriate. Setting CPA at 0.25nm and TCPA at 10 minutes gives you far less margin; setting them at 5nm and 60 minutes produces constant false alarms on busy shipping lanes.

The single most important discipline with AIS alarms: never silence the alarm without looking at the target. The alarm is the system telling you something requires attention. The appropriate response is to evaluate and respond, not to hit "dismiss" reflexively.

What AIS Misses

AIS's gaps are significant and frequently misunderstood by sailors who have become accustomed to seeing vessel traffic on their chartplotter. The categories of vessels not visible on AIS:

Fishing vessels under 300 GT: In international waters, AIS is only mandatory for vessels over 300 gross tons and passenger vessels on international voyages. The vast majority of commercial fishing vessels — trawlers, longliners, trap fishermen — are not required to carry AIS and many do not. A fleet of 40 trawlers working a fishing ground may show as zero targets on your chartplotter while being entirely visible on radar.

Naval and military vessels: Warships routinely operate AIS in receive-only mode or turn it off entirely in operational situations. In areas with naval exercises or military operations, the radar display may tell a dramatically different story than the AIS display.

Recreational vessels under 8 meters: AIS is not required for pleasure vessels below 8 meters (approximately 26 feet). This covers a substantial fraction of day sailors, powerboats, and small charter vessels. In popular coastal areas, most of the traffic visible visually has no AIS signature at all.

Equipment failures and transponder issues: AIS transponders fail, lose GPS fix, have dead batteries, or are turned off by operators who don't understand their obligation. Any vessel's AIS can go dark. Never assume that because you don't see a target on AIS, there is nothing there.

Radar: Seeing What AIS Misses

Marine radar works by transmitting microwave pulses (typically at 9 GHz in the X-band, or 3 GHz in the S-band) and detecting the reflections returned by objects. Anything with a significant metal surface reflects radar energy — vessels of all types, land masses, heavy rain, and floating debris. Radar does not care whether a target has electronics aboard; it cares whether the target returns a detectable reflection.

For offshore collision avoidance, radar and AIS are complementary in an essential way: AIS tells you who those electronic targets are and gives you their course and speed data cleanly; radar confirms they are physically there and reveals everything else. The offshore watch-keeping discipline at night or in reduced visibility: check both. A target on radar that has no corresponding AIS signature is a vessel that may have no electronics, may have its AIS off, or may be debris. Any of those possibilities requires the same response — evaluate the risk and alter course or speed as needed.

The major radar manufacturers for recreational offshore sailing are Simrad, Garmin, and Furuno. All three produce solid equipment; the differences between them at the recreational price point are primarily in integration with their respective chartplotter ecosystems, software interface, and beamwidth performance. A 4-kilowatt open-array radar provides better target discrimination at distance than a radome unit of the same power; for offshore work where detecting small targets at range matters, open-array is generally preferred.

Radar Controls: Gain, Sea Clutter, and Rain Clutter

A radar with incorrectly set controls is nearly useless — it either shows nothing or shows so much noise that actual targets are buried. The three primary adjustments:

Gain is overall sensitivity. Too low: real targets disappear. Too high: the screen fills with random noise returns. The correct setting shows a light, even speckle of background noise across the display, with actual targets appearing as clearly brighter returns. Most modern radars have auto-gain; use it as a starting point but learn to adjust manually in varying conditions.

Sea Clutter (STC — Sensitivity Time Control) suppresses the return from nearby wave surfaces, which in any sea state can overwhelm the display within a few miles of the vessel. Set too high and you suppress the clutter but also suppress nearby vessel returns — the most dangerous range. Set at the minimum level that removes the wave return from center of screen, and no higher.

Rain Clutter (FTC — Fast Time Constant) reduces the return from precipitation. Heavy rain shows as a smear on radar and can mask vessels within it. The FTC filter removes some of that smear, though it also reduces target strength. Use it when rain is clearly present; turn it off in clear conditions.

The practical skill is adjusting these three controls as conditions change. A squall that was 10 miles out and clear on radar becomes a rain-smeared patch as it passes over you, and the fishing vessel running ahead of it may disappear unless you adjust. This is the hands-on element of radar watch-keeping that chartplotter automation cannot replace.

MARPA: Tracking Non-AIS Targets

MARPA (Mini Automatic Radar Plotting Aid) is the radar target tracking function available on most modern recreational chartplotter-radar systems. When you manually acquire a radar target with MARPA, the system calculates that target's CPA and TCPA based on radar data alone — no AIS required. It draws a vector on the radar display showing the target's projected course and sounds an alarm if it approaches your set CPA threshold.

MARPA is the tool that gives you AIS-equivalent collision avoidance information for non-AIS targets. Acquire the fishing vessel fleet on radar, let MARPA calculate their CPA, and you have the same alert framework for those targets that AIS provides for equipped vessels. The limitation: MARPA requires a stable GPS input to function accurately, works best on targets at moderate range (not immediately adjacent), and requires an initial manual acquisition step for each target. Fully automatic ARPA (as on commercial vessels) tracks targets without manual acquisition; recreational MARPA is semi-automatic.

In practice, MARPA is most valuable in two specific situations: fishing fleet passage at night, where multiple non-AIS targets require monitoring, and restricted visibility in shipping lanes, where you want collision avoidance data for everything in the vicinity regardless of their electronics status.

The Radar Reflector Debate

A fiberglass sailboat is nearly invisible to radar. The hull is non-metallic; the standing rigging, mast, and boom return some signal, but a 40-foot sailboat may appear as barely a blip at 3 miles on a commercial vessel's radar — exactly the range at which an 18-knot container ship needs to be making avoidance decisions. The radar reflector is the solution. The debate is about which one actually works.

Passive reflectors — octahedral aluminum shapes hung in the rigging — have been the traditional choice for decades. The Firdell Blipper, a teardrop-shaped passive reflector, is among the most tested and commonly found. Independent tests by the UK's RYA and Southampton Solent University found that most passive octahedral reflectors in their standard "catch-rain" mounting position perform significantly worse than their manufacturers claim, and that their effective radar cross-section at realistic ranges is marginal. The Firdell Blipper performed better than most in independent testing, but still showed significant variability with heel angle — a reflector hung in the rigging tilts as the boat heels, which can move the reflecting corner geometry out of the optimal position.

Active radar enhancers — the Echomax Active-X being the most commonly tested — receive the incoming radar signal and retransmit it amplified, dramatically increasing the apparent radar cross-section. In independent testing, the Echomax Active consistently outperformed passive reflectors by a factor of 5–10 in radar cross-section at typical target ranges. The trade-off is power consumption (12V, approximately 0.15A) and the possibility of equipment failure. For offshore passages, an active enhancer backed by a passive reflector as insurance is the belt-and-suspenders approach that test data supports.

Keeping a Radar Watch

Technology does not replace the watch-keeper. The appropriate use of radar and AIS is as decision-support tools for a human who is actively attending to the collision avoidance picture, not as substitutes for maintaining a watch. COLREGS (the International Regulations for Preventing Collisions at Sea) require that every vessel maintain a proper lookout by all available means at all times. AIS and radar are two of those means; visual observation, listening on VHF, and human judgment are others.

At night offshore, the practical watch-keeping discipline: brief radar scan every 5–10 minutes, AIS display check with attention to any new targets within the CPA envelope, visual scan of the horizon. At anchor in a busy anchorage or channel, radar on standby (not transmitting) is generally appropriate courtesy; in open water underway, radar should be on and transmitting. The 15 minutes of warm-up time that older magnetron radars required has been replaced by instant-on solid-state radars in most current equipment, removing the excuse for keeping radar on standby during offshore passage-making.

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Frequently Asked Questions

How often does a Class B AIS transponder transmit a position update? A Class B AIS transponder transmits a position update every 30 seconds when the vessel is underway, compared to Class A transponders on commercial ships which transmit every 2 to 10 seconds depending on speed. The 30-second interval is generally adequate for collision avoidance purposes given the closing speeds involved between a sailing vessel and a ship.

What CPA and TCPA settings are appropriate for AIS collision avoidance on a cruising sailboat? A Closest Point of Approach alarm set at 0.5 to 1 nautical mile and a Time to Closest Point of Approach alarm set at 20 to 30 minutes provides adequate warning for a cruising sailboat to take avoiding action before a conflict develops. In congested waters such as shipping lanes, tighter settings of 0.5nm CPA and 15 minutes TCPA allow earlier awareness of converging traffic.

What is MARPA and why is it useful for tracking non-AIS targets? MARPA, or Mini Automatic Radar Plotting Aid, is a chartplotter feature that allows the navigator to manually tag a radar return and have the plotter calculate that target's speed, course, CPA, and TCPA over time. It is particularly valuable for tracking fishing vessels, small craft, and objects that do not carry AIS transponders, which would otherwise be invisible on an AIS display.

Why is a radar reflector still necessary on a yacht with an AIS transponder? A radar reflector is necessary because not all ships monitor AIS closely, watchkeepers may rely primarily on their own radar display, and AIS transponders do not make a vessel more visible on radar. A fibreglass yacht presents a poor radar cross-section compared to a metal vessel of similar size, and an active radar reflector such as the Echomax Active-X increases radar visibility by five to ten times compared to a passive reflector.

What is the Firdell Blipper radar reflector and how does it compare to active alternatives? The Firdell Blipper is a passive octahedral radar reflector widely used on yachts as a lower-cost alternative to active radar enhancers. While the Blipper improves radar cross-section compared to no reflector, active radar reflectors such as the Echomax Active-X respond to incoming radar pulses with amplified returns, producing a significantly stronger and more consistent radar signature than any passive reflector in all sea states.