Related Navigational Instruments and Their Use
This domain tests the instruments feeding the compass and the position: echo sounder, logs, GNSS, AIS, autopilot, ECDIS, sextant and chronometer. The mark usually turns on what each instrument measures versus what the navigator assumes it measures.
Underwater and speed sensors
- Echo sounder: pulse travel time, 1500 m/s nominal, referenced to the transducer, with range scales and shallow-water alarms; performance suffers in aerated water and soft mud.
- EM log: Faraday induction in seawater, needs conductivity, gives speed through water and is affected by fouling and salinity.
- Doppler log: frequency shift from particles, water track or bottom track, accuracy around 1 percent; bottom track gives ground speed.
- Pitometer log: pressure difference, simple but sensitive to fouling and openings.
Position, heading and control
- GNSS gives position, SOG and COG over the ground; it is not a heading sensor, and COG is erratic at low speed.
- AIS broadcasts static, voyage and dynamic data, mostly GNSS and gyro derived; it is not an anti-collision instrument and may be absent or wrong.
- Autopilot takes heading from the gyro, rudder feedback from the steering gear, and applies course keeping; it needs adaptation to weather and speed.
- ECDIS combines chart, position and alarms; datum, safety contour and scale settings must be correct.
- Sextant and chronometer remain the independent cross-check: index error for the sextant, chronometer error and rate for time.
Worked example
A vessel steams 090 degrees at 12.0 kn through the water in a current setting 000 at 3.0 kn. Ground vector: east 12.0, north 3.0. SOG = square root of (144 + 9) = 12.37 kn. COG = 090 + arctan(3.0/12.0) = 090 + 14.0 = 104.0 degrees. The GNSS shows about 12.4 kn and 104 degrees while log and gyro show 12.0 kn and 090 degrees; both are correct because they measure different quantities.
Traps
- Taking GNSS COG as heading, especially at low speed or when stationary.
- Plotting a through-water log speed without allowing for current and leeway.
- Misreading an echo sounder range scale or a second trace as true depth.
- Treating AIS as collision avoidance, or assuming its data are complete and correct.