Radar and its setting up
Radar measures the range and bearing of targets and the coastline, and its usefulness depends on setting it up for the conditions — gain, sea and rain clutter, and the right range scale and band (X-band for definition and small targets, S-band for penetrating rain and clutter). The command officer knows the radar's limitations: false echoes, blind arcs, minimum range, and the effect of weather on detection, and does not leave the controls on automatic when the returns are difficult.
ARPA and target assessment
Assessing a target with ARPA
- Acquire and track the target so ARPA computes its motion.
- Read the CPA (closest point of approach) and TCPA (time to it) to judge the risk.
- Read the target's true course and speed and its aspect to understand the encounter.
- Use the trial-manoeuvre function to preview a proposed alteration, remembering the delay after any real manoeuvre.
- Apply the IRPCS — the aids inform, the rules decide.
WARNING — Aids inform; the rules and the lookout decide ARPA gives the CPA, TCPA and the target's motion, but Rule 7 warns against assumptions on scanty information and no rule lets you hand the decision to a box. Confirm with radar and eyes, then act under the collision regulations.
TIP — Match the band to the weather X-band gives sharper definition and better small-target detection in clear conditions; S-band earns its place in rain and heavy sea clutter when X-band goes blind. Use both, and know which to trust in the conditions.