Chapter 13: Radar Navigation and ARPA

Radar principles and controls

A pulse is transmitted, and the elapsed time to the echo gives range; the antenna direction gives bearing. Range accuracy is high; bearing accuracy is limited by horizontal beam width. Pulse length sets range discrimination, and the antenna's vertical beam and height set the horizon. Tuning, gain, sea clutter (STC/anti-clutter sea) and rain clutter (FTC/differentiator) controls must be adjusted deliberately: excessive clutter suppression deletes small targets such as buoys, ice and small craft.

• Use range circles for navigation LOPs, not radar bearings, where possible
• Parallel indexing: a pre-drawn line at a set offset from a fixed object monitors cross-track error continuously
• Beware indirect echoes, side lobes, multiple echoes and blind sectors

Relative and true vectors, CPA and TCPA

In relative motion the own ship is fixed at the centre and the target's trail shows its motion relative to us — the direct indicator of collision risk. True vectors show each vessel's actual course and speed over ground, better for appraising a traffic pattern and for deducing the target's aspect. CPA is the closest point of approach along the relative-motion line, TCPA the time to reach it. A constant bearing with decreasing range means collision risk exists; a target whose relative trail passes through own ship has zero CPA.

ARPA capabilities and limits

ARPA acquires targets automatically or manually, tracks them, computes CPA/TCPA, raises guard-zone alarms and offers trial manoeuvre. Limitations matter: tracking lag of up to three minutes after a target manoeuvre, target swap in close quarters, and loss of track in clutter. Under COLREGS Rule 7, radar and ARPA data must be used properly — but scanty radar information must not be the basis for assuming risk does not exist.