You are on watch at 0200 in restricted visibility. The master has been called but is not yet on the bridge. You have two targets on ARPA — one with a CPA of 0.2 nm closing from ahead, and another you have just acquired manually on the starboard quarter. You need to trust your radar picture. Can you?
Radar Modes — Know Which You Are In
The choice of presentation affects everything you read from the display.
- North-Up Stabilised (True Motion or Relative Motion): Vessel's heading changes on screen; chart overlay is intuitive. Most used for pilotage.
- Head-Up Unstabilised: Own ship always points to top; picture rotates with every helm movement — smearing occurs. Acceptable for quick acquisition; poor for ARPA accuracy.
- Course-Up Stabilised: Set course held at top; resets on course change. Good compromise underway.
For ARPA to produce reliable vectors, the radar must be gyro-stabilised. An unstabilised Head-Up display degrades ARPA target history and vector accuracy.
True vs Relative Vectors
- Relative vector: Shows where a target appears to be going relative to own ship. CPA/TCPA read directly. Fast to read. Does not reveal the target's actual course and speed.
- True vector: Shows each target's actual course and speed over ground. Immediately reveals if a contact is a vessel underway, at anchor, or a buoy. Use both; switch between them to build the full picture.
Common Radar Errors — Identify and Allow For
| Error | Cause | Effect |
|---|---|---|
| Side lobes | Antenna design | False echoes either side of a strong target |
| Multiple echoes | Signal bouncing between own ship's structure and a target | Repeated echoes on same bearing |
| Indirect (false) echoes | Reflection off own superstructure | Echo on wrong bearing |
| Blind/shadow sectors | Obstruction of beam by mast, funnel | Targets not detected; sector marked on radar record |
| Range and bearing discrimination | Pulse length, beamwidth | Two close targets merge into one |
| Radar clutter (sea/rain) | Returns from waves or precipitation | Targets obscured; reduce sensitivity carefully, never to zero |
Always know your set's declared blind sectors. These are vessel-specific and should be recorded onboard.
ARPA — Acquisition, Settling Time, and Limitations
- Manual acquisition gives you control; auto-acquisition may miss slow or small targets.
- After acquisition, allow the ARPA time to settle before relying on vectors — typically several minutes depending on system. Early vectors are unreliable.
- ARPA is only as good as its inputs: speed log errors (current set, leeway) corrupt true vector calculations. Check inputs regularly.
- A guard zone or ring alarm supplements watch-keeping but does not replace it.
Parallel Indexing
Parallel indexing is the primary technique for maintaining a safe track in confined waters or restricted visibility using radar.
- Identify a fixed, discrete radar-conspicuous object on a beam or near-beam bearing.
- On the radar, measure the desired perpendicular distance that object should maintain from own ship's track (the index range).
- Draw a line parallel to the intended track at that distance — this is the index line.
- As you proceed, keep the target's echo running along the index line. Drift toward or away from the line shows cross-track error immediately, before it appears on GPS.
Parallel indexing works in any display mode but is most intuitive in North-Up Stabilised. It does not require GPS and provides an independent cross-check of position.