Ship Navigation Equipment Complete Guide 2026

RADAR and ARPA

RADAR works by transmitting radio pulses and measuring how long the reflection takes to return, building a picture of everything solid within range — other vessels, land, buoys, and in poor visibility, weather. On its own, raw RADAR shows where targets are right now but not where they’re going. ARPA (Automatic Radar Plotting Aid) solves that by automatically tracking detected targets over time and computing their course, speed, CPA (closest point of approach), and TCPA (time to CPA) — numbers that would otherwise require manually plotting each target’s movement on a maneuvering board, target by target, call by call. In any situation with multiple vessels converging, ARPA’s automatic tracking is what makes real-time collision-risk assessment practically possible rather than a constant manual-plotting exercise.

RADAR does have real limitations officers need to hold in mind: small craft, particularly wooden or fiberglass boats with no radar reflector, can return a weak or intermittent echo; sea clutter in rough weather can mask genuine targets near the ship; and RADAR shadow behind large targets or in narrow channels with high banks can hide vessels entirely until they emerge from the shadow at close range.

GPS/GNSS

GPS (and the broader category of GNSS, which includes GLONASS, Galileo, and BeiDou alongside the US GPS constellation) gives continuous, automatic position fixing by triangulating signals from satellites in known orbits. It replaced celestial and terrestrial fixing as the primary method of determining position, and it feeds position data into essentially every other navigation and communication system on the bridge, from ECDIS to AIS to the ship’s log. Its main practical limitation is that it’s a single point of failure if not cross-checked — GPS signals can be degraded or lost due to atmospheric conditions, deliberate jamming (a real and increasing concern in certain high-risk regions), or equipment fault, which is exactly why officers are trained to maintain the ability to verify position independently rather than trusting a single GPS readout uncritically.

ECDIS

ECDIS (Electronic Chart Display and Information System) pulls together electronic navigational charts, real-time GPS position, RADAR overlay, and AIS target data into a single integrated display, and has been mandatory carriage on most SOLAS cargo ships and passenger vessels on a phased basis since 2012-2013. It lets officers see position, planned route, and surrounding traffic on one continuously updating chart rather than cross-referencing separate paper charts and instruments — a genuine improvement in situational awareness when used correctly, and a genuine hazard when officers over-trust it without applying the same critical cross-checking they’d apply to any single instrument.

Because of that risk, SOLAS specifically requires a backup navigation method even on fully ECDIS-equipped vessels — either an adequate folio of paper charts or a second, independent ECDIS system. Relying on one electronic system with no independent fallback is treated as an unacceptable single point of failure for safe navigation, and PSC inspections check for this backup capability as part of standard navigation equipment checks.

Why these three systems are taught together

None of RADAR, GPS, or ECDIS is fully reliable in isolation — RADAR shows what’s physically there but needs ARPA to interpret movement, GPS gives precise position but can fail silently, and ECDIS integrates everything but is only as good as the data feeding it and the officer’s willingness to cross-check rather than passively trust the display. Competent watchkeeping means treating all three as complementary checks on each other, not as three separate boxes to tick.

Chat with SailorGPT