Electronic Navigation and Bridge Equipment: Radar, ECDIS, GPS, Echo Sounder, Compass and AIS
Domain 6 tests practical use and limitations of bridge electronics: what equipment displays, how to verify it, and when not to trust it.
Radar and ARPA
Radar shows relative motion; a steady bearing with decreasing range indicates risk of collision. ARPA gives CPA and TCPA, but trial manoeuvres are predictions, not guarantees. Radar horizon in nautical miles is about 1.23(√h₁ + √h₂), with heights in metres. Use short and long range scales and know blind and shadow sectors.
ECDIS, GPS and AIS
ECDIS must use a corrected ENC, a route-checked plan, the correct safety contour and safety depth, and the correct datum. GPS is subject to multipath, jamming and RAIM warnings; cross-check with visual or radar. AIS is a situational-awareness aid, not a collision-avoidance system; it can be absent, misconfigured or delayed.
Compass and echo sounder
Gyro error is found by comparing a celestial or terrestrial bearing. Magnetic compass error = variation + deviation. Echo sounder readings should be checked against the chart and draft; false echoes can come from aeration or an incorrect speed-of-sound setting.
Worked example
Radar antenna height 25 m and target height 9 m give a radar horizon of 1.23(5 + 3) = 9.84 NM. If the target is detected at 11 NM, anomalous propagation or a higher effective target height may explain it; the formula is not a hard detection limit.
Traps
Using AIS as the sole anti-collision tool; failing to route-check ECDIS; assuming GPS is always within accuracy; confusing relative and true motion; ignoring ARPA target swap and update lag; trusting an uncalibrated echo sounder.