Lesson previewThree Different Evidence Streams1 section shown
Preview lesson
Three Different Evidence Streams
What you will learn
Separate radar measurement, AIS reporting, automatic radar tracking and chart overlay before combining them into one traffic picture.
A modern display can make different sensors look like one answer. Safe interpretation keeps the source, reference, age and failure mode attached to every symbol and number.
Radar measures returned energy in range and bearing from own ship. AIS receives reports created from another station's sensors and configured fields. MARPA or target tracking derives motion from selected radar echoes. A radar/chart overlay transfers radar information into a chart and position/heading reference. Agreement can strengthen a picture, but it does not make the sources independent or error-free.
An integrated display may associate radar and AIS data that meet equipment-specific position and motion criteria, then show one symbol or one preferred data source. One tidy symbol therefore does not prove that every source agrees. The operator must still know whether range, bearing, CPA, identity or vector came from radar, AIS or a combined presentation.
Start with raw observations and source labels, then compare movement, age, reference and fixed-feature alignment. Investigate disagreement rather than choosing the most convenient answer. Preserve lookout, safe speed, systematic radar observation and time to act while the cause remains unresolved.
Evidence-association lab
Diagnose the source chain before trusting the combined symbol
Select a display case. Observation, source check, discriminating check and stop boundary stay together.
1 · Observation
One symbol can contain more than one source
A radar echo and activated AIS target sit close together with coherent motion. The display presents one preferred target symbol and one data panel.
2 · Source check
Inspect association status and target priority. Identity may be AIS while range, bearing or the alternate forecast remains available from radar tracking.
3 · Discriminating check
Compare raw echo movement, AIS age and reference, dimensions and independent lookout over time instead of accepting symbol proximity.
4 · Stop boundary
Report a plausible association with named sources. Meeting thresholds is not proof that every field is correct or that the target has a particular intention.
Keep source, observation, discriminating check and stop boundary together before releasing an identity or collision-risk conclusion.
Worked example
An integrated display shows one activated symbol beside a ferry name and one CPA value, while the raw echo can still be seen underneath.
- 1Open or inspect the target data source: identity and ground motion may be AIS while the echo and radar-relative track are separate evidence.
- 2Check association status, target priority, data age and whether raw radar movement agrees over known times.
- 3Release only the conclusions supported by each source; retain uncertainty if identity or motion association is not stable.
Sense check: If one symbol is being treated as proof that radar, AIS and tracking all agree, the source chain has been lost.
| Evidence stream | Direct contribution | Leading dependency |
|---|---|---|
| Raw radar | Measured echo range and bearing | Picture quality, controls, geometry and heading reference |
| AIS | Reported identity, position and motion fields | Target sensors, manual entries, transmission and reception |
| MARPA / target tracking | Derived radar-target motion and forecast | Echo continuity, acquisition and own sensors |
| Radar/chart overlay | Spatial comparison with charted features | Position, heading, datum, timing, alignment and chart display |
Read source before value
A busy approach display offers names, vectors, alarms and a radar overlay on one screen.
1. Name the source
Identify whether the evidence is raw radar, radar tracking, AIS or transferred overlay data.
2. Check the reference
Record true/relative, sea/ground, vector time, position source and target priority.
3. Compare independently
Use repeated raw observations, fixed features and lookout rather than symbol proximity alone.
4. Keep a stop rule
If association or alignment is doubtful, do not release identity, passing distance or safe-water claims.
Sense check: A compact display is useful only when its hidden source choices remain visible to the operator.
Common mistake or limitation
- Treating a combined display as one independent sensor.
- Reading an associated AIS label as proof that the nearest radar echo has that identity.
- Comparing vectors without checking source, mode, stabilisation and time.
Recap
- Each evidence stream observes or derives something different.
- Association can reduce clutter while hiding source disagreement.
- Labels and forecasts need source, reference, age and quality.
- Unresolved disagreement increases caution rather than certainty.
Optional quick check
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