Radar Observer · ARPA — Relative Vectors and Risk
With ARPA set to RELATIVE vectors, a target poses a collision risk when its vector:
- AIs the longest on the screen
- BPoints away from own ship
- CPoints directly at (or very close to) own ship's position at the screen center✓ Correct
- DIs colored green
Explanation
A relative vector is the path the target will trace relative to own ship; if it passes through or near own ship at the center, the CPA is small and risk exists. The vector's length (for the set vector time) shows how far/soon. This is the quickest visual check for danger.
Authority: IMO Performance Standards for ARPA
Practice the full Radar Observer bank
Free spaced-repetition quizzing across 2550 USCG exam questions — it schedules your reviews so the ones you miss come back until they stick.
Related Radar Observer questions
- Display — Motion
On a relative-motion radar display, your own ship is:
- Collision Avoidance — CPA
The closest point of approach (CPA) of a radar target is:
- Collision Avoidance — TCPA
TCPA on a radar/ARPA readout represents the:
- Controls — EBL
The electronic bearing line (EBL) on a radar is used to:
- Controls — VRM
The variable range marker (VRM) on a radar measures:
- Controls — Clutter
The anti-clutter sea (STC / sensitivity-time control) is adjusted to: