Electronic Navigational Equipment & Compasses – Key Vocabulary

These vocabulary flashcards cover the major electronic navigational instruments, compasses, key SOLAS regulations and technical terms discussed in the lecture.

Global Positioning System (GPS)

Satellite-based receiver that provides a ship’s real-time position, course and speed with high accuracy.

Differential GPS (DGPS)

Enhanced GPS using shore-based reference stations to improve positional accuracy for harbour and coastal navigation.

Integrated Bridge System (IBS)

Suite that combines DGPS, radar, AIS and electronic charts into one display for safer, more efficient navigation.

Automatic Identification System (AIS)

VHF transponder that exchanges vessel identity, position, course and speed data with other ships and shore stations.

AIS Class A

Full-function transponder required on all SOLAS ships ≥300 GT and all passenger ships on international voyages.

AIS Class B

Reduced-range transponder intended mainly for non-SOLAS craft such as pleasure vessels.

AIS 1 (Channel 87B)

Simplex ship-to-ship frequency 161.975 MHz used by AIS equipment.

AIS 2 (Channel 88B)

Duplex ship-to-shore frequency 162.025 MHz assigned to AIS.

SOLAS V/19

Regulation specifying carriage requirements and arrangement of navigational equipment on board ships.

Electronic Chart Display and Information System (ECDIS)

IMO-approved electronic chart system that can legally replace paper charts when fitted with appropriate back-up.

IMO Resolution A.817(19)

Performance standards that an ECDIS must meet to serve as a primary means of navigation.

Radar (Radio Detection And Ranging)

X-/S-band system that transmits electromagnetic pulses and displays echoes for detecting land, vessels and obstacles.

Magnetron

High-power oscillator in a marine radar that generates microwave pulses.

Pulse Repetition Frequency (PRF)

Number of radar pulses transmitted per second; determines maximum unambiguous range.

Automatic Radar Plotting Aid (ARPA)

Computer add-on to radar that automatically acquires targets and calculates CPA/TCPA for collision avoidance.

CPA (Closest Point of Approach)

Smallest distance two vessels will pass if their present courses and speeds remain unchanged.

Course Recorder

Graphic device that continuously plots gyro courses steered; provides legal evidence of a ship’s track.

Echo Sounder

Depth-finding instrument that measures time for a sound pulse to travel to the seabed and return.

Echo-sounding Formula

Depth = (Velocity × Time) ÷ 2; accounts for down-and-back travel of the pulse.

Gyro Compass

Non-magnetic compass that seeks true north by using a spinning rotor and Earth’s rotation.

Gyro Repeater

Remote indicator that displays the master gyro heading at steerage or wing stations.

Magnetic Compass

Direction device that aligns with Earth’s magnetic field; SOLAS requires a properly adjusted standard unit.

Compass Deviation Table

Record of residual magnetic errors for each heading that must be available near the magnetic compass.

Doppler Log

Speed log that determines vessel speed and distance by measuring Doppler shift of high-frequency acoustic pulses.

Electromagnetic Log

Speed log generating an electromagnetic field; induced voltage in seawater is proportional to speed through water.

Under-Keel Clearance

Vertical distance between the vessel’s keel and seabed, monitored using echo sounder and updated position data.

SOLAS Radar Requirement (≥300 GT)

Ships ≥300 GT must carry a 9 GHz radar and plotting aid; ≥500 GT need automatic tracking.

SOLAS Gyro Requirement (≥500 GT)

All ships of 500 GT and above must carry a gyro or alternative non-magnetic heading device.

SOLAS Speed-Log Requirement (≥50,000 GT)

Ships ≥50,000 GT must have two separate logs: one for speed through water and one for speed over ground in two axes.

IMO Resolution A.382(X)

States every ship must be fitted with a compensated magnetic compass and a deviation table.

Integrated Bridge Layout

Arrangement of bridge equipment (radar, ECDIS, gyro, log, VHF, autopilot) designed for ergonomic, SOLAS-compliant operation.

Pulse (Radar)

Short burst of microwave energy transmitted by radar to detect targets.

Waveguide

Hollow metal tube that carries microwave energy from a magnetron to the radar antenna.

Speed Over Ground (SOG)

Actual vessel speed relative to Earth’s surface, commonly derived from GPS or Doppler ground log.

Speed Through Water (STW)

Vessel speed relative to the surrounding water, measured by Doppler or electromagnetic log.

CPA/TCPA Alarm

ARPA alert indicating when a target will breach a preset closest approach distance within a given time.

ENC (Electronic Navigational Chart)

Vector chart data set officially issued for use in ECDIS and subject to weekly updating.

ETA Computation (ECDIS)

Automatic calculation of Estimated Time of Arrival along a planned route using speed and distance data.

Backup Arrangement (ECDIS)

Alternative navigation method (second ECDIS or paper charts) required by SOLAS V/20 to ensure safe navigation if primary fails.

VHF Marine Channels 87B & 88B

Radio frequencies reserved worldwide for AIS ship-ship and ship-shore data exchange.