Equipment Guide · Radar & ARPA
Marine radar is a stopwatch with an antenna: it times radio echoes to measure range, reads bearing from where the scanner points, and paints the result as a map of everything that reflects. ARPA then watches each echo move and turns its track history into a vector — course, speed, CPA and TCPA. Tap a step or press "Play" to follow the pulse from the magnetron to the display — then take the scope yourself in the simulator: you start the way collisions start, bearing steady and range closing, and it is your alteration that opens the CPA.
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Three plots, six minutes apart: 045° · 8.0 nm, then 045° · 6.2 nm, then 045° · 4.4 nm. The range has closed 1.8 nm every six minutes — a relative speed of 18 knots — and the bearing has not moved a degree. A bearing that will not change means the relative track points straight at you: CPA effectively zero, and from 4.4 nm at 18 kn the TCPA is about 15 minutes. You need no ARPA to see it — this is the oldest rule in collision avoidance, and it is exactly the geometry the machine computes for you. If instead the bearing draws steadily ahead or aft while the range closes, the target passes ahead or astern — the drift of the bearing is the first honest answer, minutes before any vector settles.
Two ships, live geometry. You are the centre of the scope; the target is acquired and tracked, her data on the right. The dashed line is her relative track across your screen and the circle on it is the CPA — where that track passes closest to you. You start the way collisions start: bearing steady, range closing. Alter course or speed and watch the CPA open — and note the tracker's honesty: after any change the vectors need a couple of minutes of steady track before their numbers are worth believing.
Scope legend — solid lines: true vectors (12-min run) · dashed line: relative track · circle: CPA point · rings every 4 nm · time runs at ×30.
CPA and TCPA here are computed from the tracker's smoothed vectors, the same way a real ARPA works — which is why the readout drifts for a while after any alteration (TRACK: SETTLING) before it tells the truth again. The hazard is taught by the geometry itself: hold your course against a steady bearing and the ranges simply run out.
She starts the way collisions start — bearing steady, range closing. Press Start, then alter course or speed and watch the CPA open.
Working principle — explained in our own plain words with our own schematic from general marine knowledge (not copied from any OEM manual). Conceptual; varies by ship/model. Reviewed and verified by a Master Mariner.