Two bands, two jobs
X-band (9.4 GHz, 3 cm) gives sharp short-range detail — perfect for navigating a busy harbour or spotting a small buoy. S-band (3 GHz, 10 cm) sees through rain, snow and sea clutter — preferred for open-ocean watch. Most ships over 3,000 tons carry both.
ARPA — automatic radar plotting aid
The computer tracks every echo, calculates its course and speed, and projects whether it will pass close enough to count as a collision risk. The officer on watch sees CPA (closest point of approach) and TCPA (time to it) on every target — and gets an alarm if either crosses a threshold.
AIS overlay
Automatic Identification System transponders broadcast each ship's name, type, course and speed. The radar overlays this on the screen — so the unknown blip becomes 'Maersk Edinburgh, 366 m container, CPA 0.4 NM in 12 minutes'.
Sea clutter and rain modes
Waves return energy too. Modern radars use STC (sensitivity time control) to suppress nearby sea echoes, FTC (fast time constant) for rain, and pulse-compression to keep range resolution. A novice mate often blames the radar; the experienced one tweaks the gain.