Lesson previewRange and Bearing as Evidence1 section shown
Preview lesson
Range and Bearing as Evidence
What you will learn
State exactly what one radar range and bearing measure, identify their reference and explain why a timed sequence is needed before inferring movement.
Range and bearing turn an echo into measured evidence. They do not supply identity, target course, closest approach or a safe passing result without reference checks and observations over time.
Radar range is distance from the radar origin to the selected part of an echo. The origin may not be the vessel's bow, GPS antenna or conning position. Radar bearing is direction from the selected EBL or cursor origin, expressed relative to the heading line or as a compass reference when the set and heading input support it.
One observation fixes an echo's measured position only at that moment. MCA guidance explicitly warns that a single range and bearing give no indication of another vessel's relative course or heading. A timed succession supports a movement trend, but target manoeuvre, own-ship movement and input errors still require continuing review.
Write the complete observation: time, range, bearing, bearing reference, scale, target identity or uncertainty and the point on the echo measured. That record can support plotting, position fixing or a clearing decision without turning a precise readout into more certainty than the sensor provides.
VRM and EBL workbench
Carry the measurement conditions with the number
Select a task to reveal the origin, bearing reference, visible working and release boundary needed for an auditable radar result.
1 · Observation
Name the origin, reference and echo edge
A distinct echo's nearest stable edge is 2.4 M from the centred radar origin on 042°T.
2 · Visible working
- 1.VRM: 2.4 M from the centred radar origin
- 2.EBL: 042°T, using a verified true-bearing reference
- 3.Point measured: nearest stable edge at 10:00
3 · Release boundary
Record 10:00 · 2.4 M · 042°T · centred origin · near edge. Repeat at a known interval before inferring movement.
Carry the reference, measured edge, arithmetic and safety boundary with the number.
Worked example
At 10:00 a distinct echo is 2.4 M at 042°T. At 10:03 it is 2.0 M at 042°T. Own course and speed have remained steady.
- 1Record both timed range-and-bearing observations with the same true-bearing reference and measured echo point.
- 2Notice decreasing range and unchanged measured compass bearing; treat this as possible collision risk requiring systematic assessment.
- 3Continue observations and complete the appropriate plotting or vector assessment rather than converting two samples into a guaranteed CPA.
Sense check: The second observation strengthens a trend; it does not predict a target manoeuvre after 10:03.
| Recorded evidence | Supports | Does not prove alone |
|---|---|---|
| 1.8 M to nearest echo edge | Separation from radar origin at that time | Target identity or future passing distance |
| 042°T EBL through a point target | True direction if heading alignment and reference are valid | Target course or intention |
| 070° relative bearing | Direction clockwise from own heading line | A chart bearing without conversion |
| Three timed observations | Relative movement trend through the measured points | That the target will not manoeuvre afterward |
Build a complete measurement
A new contact appears in a developing traffic situation and the display offers several numbers.
1. Identify the origin
Confirm whether range and EBL are centred on own radar or use an offset origin.
2. Name the reference
Record relative, magnetic or true bearing exactly as the set supplies it.
3. Choose the echo point
Use the nearest edge, centre or identifiable feature that matches the decision.
4. Time and repeat
Preserve the observation interval and continue systematic monitoring.
Sense check: A readout without origin, reference, echo point and time is not a reproducible measurement.
Common mistake or limitation
- Saying 'bearing 42' without stating relative, magnetic or true.
- Treating range as distance from the bow rather than from the radar origin.
- Using one observation to declare a target's course or CPA.
- Changing the point measured on a broad echo without recording it.
Recap
- Range, bearing, reference, echo point and time form one observation.
- A succession supports movement; one sample does not.
- Precise display digits remain conditional on set-up, inputs and target definition.
Optional quick check
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