A near miss at sea can raise difficult questions. How close did two vessels come? Which direction was each vessel traveling? Did either operator change course or speed? Was a developing collision risk visible early enough for action to be taken? Answering these questions often requires more than the recollections of the people involved.

Marine radar and the Automatic Identification System (AIS) are two important sources of information when reconstructing a vessel encounter. Both can help investigators understand traffic movements, but they work differently and provide different kinds of evidence. Radar detects targets using reflected radio energy, while AIS exchanges vessel information electronically. Neither system provides a complete account of an encounter on its own.

Understanding the strengths and limitations of each source helps vessel operators, marine professionals, and investigators assess what the available evidence can establish after a near miss.

How Marine Radar Records Vessel Movements

Marine radar transmits radio pulses and detects the energy reflected by objects within its operating range. Depending on the equipment, settings, environmental conditions, and target characteristics, radar may reveal nearby vessels, land, navigational markers, and other objects. The display can help a navigator assess relative positions, distances, and movement.

In a near-miss investigation, radar and AIS can provide complementary perspectives on the same encounter. Radar information may help establish where a target appeared relative to the vessel carrying the radar and how its position changed over time. If tracking data was recorded or retained, it may also support an assessment of the target's movement and the development of a potential collision situation.

Radar has an important advantage: it does not depend on another vessel transmitting AIS information to detect a radar-visible target. This can be useful when a vessel is not carrying AIS, its AIS transmitter is not operating, or its transmitted information is incomplete. However, radar does not identify every target perfectly, and a radar return alone may not reveal the target's name, destination, or intended maneuver.

What Radar Can Show After a Near Miss

Recorded radar images or tracking information may help investigators examine:

• The relative position of a target over time.

• Changes in the target's bearing and range.

• The apparent movement of vessels approaching or crossing one another.

• The presence of nearby traffic, coastlines, or navigational features.

• Whether a target's movement appears consistent with a reported course alteration.

The value of these observations depends on the recording method and available data. A screenshot of a radar display captures only a particular moment, while a sequence of retained images or track records may provide a more useful timeline. Some vessels store detailed radar data, while others retain limited information or no historical record at all.

Radar interpretation also requires care. Sea clutter, rain, target size, antenna position, blind sectors, range settings, and display configuration can influence what appears on the screen. A target may be difficult to distinguish from surrounding returns, and displayed tracks may depend on how the system processes observations.

What AIS Can Reveal About an Encounter

AIS is a maritime communication system that automatically exchanges information between appropriately equipped stations. Depending on the vessel's equipment and configuration, transmitted information can include vessel identity, position, speed over ground, course over ground, heading, navigational status, and other details.

Unlike radar returns, AIS messages are associated with transmitted data that can help identify a participating vessel. This makes AIS particularly useful when investigators need to connect a recorded position or movement to a specific vessel. Historical AIS records may also help establish a broader timeline of traffic in an area.

However, AIS information should not be treated as an exact representation of everything happening aboard a vessel. Some fields are entered manually, and certain values depend on sensor inputs or equipment configuration. Heading and course over ground are different measurements: heading describes the direction the vessel's bow points, while course over ground describes its direction of movement relative to the earth. Wind, current, and vessel motion can cause these values to differ.

Understanding AIS Data Limitations

AIS data can be incomplete or misleading for several reasons. A vessel may have an equipment fault, incorrect configuration, missing position information, or gaps in message reception. Some vessels may not be required to carry AIS, and others may not transmit the information an investigator expects to find.

Reception also varies with antenna placement, obstructions, distance, equipment performance, and the source used to collect the data. A historical AIS database may not contain every message transmitted during an encounter. The absence of a vessel from a particular dataset does not necessarily prove that it was absent from the area.

Investigators should therefore establish where the AIS data came from, what period it covers, and whether the original messages or only processed tracks are available. The collection method can affect the precision and completeness of the reconstruction.

Comparing Radar and AIS Evidence

Radar and AIS can answer overlapping but different questions. Radar may show a physical target's observed movement relative to the receiving vessel, while AIS may associate reported position and movement data with an identified transmitting vessel.

The following comparison illustrates their principal differences.

Neither source should automatically be treated as more authoritative in every situation. Their reliability depends on the specific question, the quality of the records, and how the systems were operating at the time.

A radar display may provide valuable relative-motion information, but its measurements and tracking behavior must be understood. AIS may provide vessel identification and reported movement data, but those messages also require validation. The strongest analysis often comes from comparing both sources rather than choosing one in isolation.

Reconstructing the Timeline of a Near Miss

A useful investigation begins by establishing a timeline. Investigators need to identify when the vessels first came into relevant proximity, how their relative movements developed, when course or speed changes occurred, and when the closest approach took place.

Radar and AIS records can be aligned with other evidence, including voyage data recorder information, electronic chart system records, bridge audio, VHF radio communications, engine data, and witness statements. Each source may use a different time reference or recording interval, so timestamps should be checked before the records are compared.

For example, AIS reports may be received at different intervals depending on vessel movement and system conditions, while radar tracking updates may follow the equipment's scan cycle. A course change recorded by one source may appear at a different point in another source because of reporting intervals, processing, or timing differences. These differences should be investigated rather than automatically interpreted as contradictions.

Time synchronization is especially important when estimating the closest point of approach or determining how much time an operator had to respond. Analysts should account for the precision and uncertainty of the underlying data and avoid presenting estimates as exact measurements when the records do not support that level of certainty.

What Radar and AIS Cannot Establish Alone

A near-miss reconstruction is not simply a matter of plotting two vessel tracks. The technical record may reveal positions and movement patterns, but important questions about navigation decisions and the surrounding circumstances often require additional evidence.

For example, a change in course does not necessarily reveal why the operator made the change. The maneuver could have been intended to avoid another vessel, respond to a navigational hazard, follow a planned route, or address another operational need. Likewise, a vessel's recorded position may not establish what the operator could see from the bridge or which visual and audible cues were available at the time.

Radar and AIS alone may not establish:

• What the bridge team could actually see.

• Whether a lookout identified the developing risk.

• Which communications took place unless separately recorded.

• Why a particular maneuver was selected.

• Whether the operator correctly interpreted the developing situation.

• Whether equipment, environmental conditions, or human factors contributed to the encounter.

These questions may require bridge recordings, navigation logs, weather information, visibility conditions, equipment settings, witness accounts, and other contemporaneous evidence. Conclusions should reflect the limits of the material available.

Preserving Evidence After a Near Miss

When a near miss may lead to a safety review, insurance question, or legal dispute, relevant records should be preserved promptly. Some onboard systems overwrite older information, and external data services may retain historical records only for limited periods.

Vessel operators should follow applicable reporting and preservation procedures. Depending on the circumstances, useful records may include radar recordings or screenshots, AIS data, voyage data recorder information, electronic chart histories, bridge logs, VHF communications, relevant photographs, and written accounts from crew members.

Original files should be retained where possible, along with information about the equipment, collection method, timestamps, and any processing performed. If data is exported into a spreadsheet, image, or track format, the original source should remain available so that the transformation can be reviewed.

A clear record of who collected the evidence and when it was obtained can help establish its provenance. Investigators should also document gaps, limitations, and any uncertainty in the data. Preserving only selected screenshots or a simplified track may omit details needed to understand the sequence of events.

The Role of Independent Marine Investigation

When the circumstances of a near miss remain disputed, an independent marine investigation may help organize and evaluate the available evidence. Depending on the assignment, the work may involve examining radar and AIS records, comparing timestamps, assessing relative motion, reviewing navigation procedures, and correlating electronic data with other records.

The investigator should distinguish between directly recorded information, calculated estimates, and interpretations. For instance, a recorded AIS position is a reported data point, while a reconstructed track between reports involves additional analysis. Similarly, a radar target track may be affected by the equipment's tracking behavior and should be interpreted in the context of its configuration and limitations.

A sound report explains the methods used, identifies gaps in the evidence, and states which conclusions are supported by the available information. Technical analysis can help clarify how an encounter developed, but determinations of legal responsibility may require broader consideration of applicable navigation rules, witness evidence, and the complete circumstances.

Conclusion

Marine radar and AIS provide valuable but different perspectives on a near miss at sea. Radar can help reveal the movement of radar-visible targets, while AIS can provide vessel identity and transmitted navigation information. When the records are available and properly interpreted, they can help reconstruct relative movements and establish a timeline of an encounter.

Neither system tells the whole story. Recording limitations, incomplete data, timing differences, sensor performance, and the absence of information about operator decisions can all affect the conclusions that can reasonably be drawn.

Preserving original records and comparing them with other reliable evidence creates a stronger foundation for understanding what happened. A careful, evidence-based approach helps distinguish what the data demonstrates from what remains uncertain.