Marine Navigation Glossary
Every acronym on the spec sheet, defined once and cross-linked to the guide that explains the engineering behind it.
Marine electronics has an unusually dense vocabulary, much of it acronyms that appear on specification sheets without explanation. This glossary defines the terms used across the site, in plain language, with a link to the guide that covers each one properly.
A
- ACMAAustralian Communications and Media Authority
The Australian regulator for radio spectrum, including the maritime ship station class licence that covers eligible marine VHF operation. See marine VHF and DSC.
Used in Common questions, answered, Marine VHF and DSC in Australia, Trailer boat and small craft electronics
- AHRSAttitude and Heading Reference System
A sensor that fuses magnetometers, gyroscopes and accelerometers to produce heading, and usually rate of turn, pitch and roll. Sensor fusion makes it far more stable under motion than a bare magnetic compass sensor. Covered in heading sensors.
Used in Navigation redundancy that actually works, Heading sensors and satellite compasses, Marine autopilot guide
- AISAutomatic Identification System
A VHF-based system in which participating vessels broadcast identity and movement data — MMSI, name, position, course and speed over ground, and more — and receive the same from others. It is excellent for identity and early awareness of cooperating traffic and shows nothing that is not transmitting. See AIS for boats.
Used in Marine radar: Doppler, solid-state and open array, Marine GPS and GNSS explained, AIS for boats: Class B, SOTDMA and MMSI
- AIS AtoNAid to Navigation
A navigation aid represented in AIS. It may be physical (a real buoy transmitting), synthetic (a real object whose signal is transmitted from elsewhere) or virtual (a marked position with no physical buoy at all). Australian mariners should recognise these symbols before meeting one unexpectedly.
- AIS-SART
A search and rescue transmitter that broadcasts a distress position over AIS, so nearby AIS-equipped vessels see it. Distinct from an EPIRB, which alerts the satellite search and rescue system. See EPIRBs and safety equipment.
- AMSAAustralian Maritime Safety Authority
The national authority responsible for maritime safety, search and rescue coordination, Australian MMSI allocation and distress beacon registration.
Used in AIS for boats: Class B, SOTDMA and MMSI, Common questions, answered, Marine VHF and DSC in Australia
- Apparent wind
The wind measured on a moving vessel — the vector sum of true wind and the wind created by the boat's own motion. A sailing autopilot can steer to apparent or true wind angle.
Used in Marine GPS and GNSS explained, Sensor siting and installation, B&G marine electronics
- ARPA / MARPA
Automatic (or Mini Automatic) Radar Plotting Aid. Radar processing that tracks a target and estimates its course, speed, CPA and TCPA. Accuracy depends on own heading and speed, a stable radar return, and the target not manoeuvring. See marine radar.
- AusENC
The Australian Hydrographic Office's official vector electronic navigational chart service for relevant Australian domestic operations, updated on a cumulative and weekly basis. See marine charts Australia.
Used in Australian marine charts, AusENC, ZOC and S-100, Marine navigation systems: the complete overview, Sources and references
B
- Backbone
The trunk cable of an NMEA 2000 network, terminated at both ends, to which devices connect by short drop cables. See marine networks.
Used in Common questions, answered, Cruising sailing yacht navigation system, Marine electronics refit guide
- Beam width
The angular width of a radar antenna's transmitted beam. A narrower horizontal beam separates two targets at similar range but different bearings — one of the main reasons large open arrays retain an advantage over compact radomes.
Used in Marine radar: Doppler, solid-state and open array, Cruising catamaran electronics, Best marine radar 2026
- BeiDou
China's global satellite navigation constellation, one of several a modern multi-constellation GNSS receiver may track. See GPS and GNSS.
Used in Marine GPS and GNSS explained
C
- Chart datum
The vertical reference from which charted depths are measured, commonly Lowest Astronomical Tide on modern Australian charts. The predicted height of tide is added to charted depth to give actual water depth. See tides, currents and chart datum.
Used in Australian marine charts, AusENC, ZOC and S-100, Tides, currents and chart datum, Motor yacht navigation electronics
- CHIRP
A sonar transmission technique that sweeps through a range of frequencies rather than transmitting a single narrow frequency, improving target separation, bottom definition and sensitivity. See marine sonar.
Used in Marine sonar: CHIRP, side scan and live sonar, Depth sounders and transducer selection, Trailer boat and small craft electronics
- COGCourse Over Ground
The direction the vessel is actually travelling over the earth, derived from successive GNSS positions. Not the same as heading, because current and leeway push the boat sideways. See GPS and GNSS.
Used in Marine GPS and GNSS explained, Night and poor-visibility navigation, How to tune marine radar
- COLREGs
The International Regulations for Preventing Collisions at Sea. They define lookout obligations, safe speed, risk of collision, give-way and stand-on responsibilities, and conduct in restricted visibility. Electronics change how early you see a situation, not who must act. See collision avoidance.
Used in Collision avoidance with radar and AIS, Sources and references
- Cospas-Sarsat
The international satellite system that detects and locates 406 MHz distress beacon transmissions and routes alerts to the responsible rescue coordination centre.
Used in EPIRBs, beacons and safety equipment, Satellite communications for boats
- CPAClosest Point of Approach
The predicted minimum distance at which a target will pass, assuming both vessels maintain their current vectors. A prediction, not a fact — it changes the moment either vessel alters course or speed. See AIS.
Used in Marine radar: Doppler, solid-state and open array, AIS for boats: Class B, SOTDMA and MMSI, Collision avoidance with radar and AIS
- CSTDMACarrier Sense Time Division Multiple Access
The channel access method used by older Class B AIS equipment, in which the unit listens for a free slot before transmitting. Compare SOTDMA.
Used in AIS for boats: Class B, SOTDMA and MMSI, Common questions, answered, Class A vs Class B AIS
D
- Deviation
The local magnetic error at a compass or magnetic sensor caused by the vessel's own magnetic fields. Automatic calibration models deviation; it cannot rescue a sensor mounted beside a major magnetic field.
Used in Marine GPS and GNSS explained, Heading sensors and satellite compasses, Cruising catamaran electronics
- Device instance
A configuration value that distinguishes multiple identical devices on an NMEA 2000 network — for example, port and starboard engines, or multiple fuel tanks. Incorrect instances are a common cause of mislabelled data on twin-engine boats.
Used in Marine electronics refit guide, NMEA 2000, NMEA 0183 and marine Ethernet, Cruising catamaran electronics
- Doppler radar
Radar that examines frequency shift caused by relative motion to identify approaching and receding targets. Marketed as VelocityTrack, MotionScope, Raymarine Doppler and Furuno Target Analyzer. It reduces interpretation workload; it does not replace knowledge of the COLREGs. See marine radar.
Used in Marine radar: Doppler, solid-state and open array, AIS for boats: Class B, SOTDMA and MMSI, Cruising sailing yacht navigation system
- Drying height
A height on a chart showing how far an object — a sandbank or rock — rises above chart datum, usually underlined. The height of tide is subtracted from it, not added.
Used in Australian marine charts, AusENC, ZOC and S-100, Tides, currents and chart datum
- DSCDigital Selective Calling
Digital signalling added to marine radio. A DSC distress alert can carry MMSI, position, time and distress category, communicating critical information before any voice MAYDAY. See marine VHF and DSC.
Used in Marine GPS and GNSS explained, AIS for boats: Class B, SOTDMA and MMSI, Common questions, answered
E
- ENCElectronic Navigational Chart
An official vector chart conforming to hydrographic office standards, as distinct from a raster scan or a commercial recreational chart product.
Used in Marine radar: Doppler, solid-state and open array, Marine GPS and GNSS explained, AIS for boats: Class B, SOTDMA and MMSI
- EPIRBEmergency Position Indicating Radio Beacon
A 406 MHz distress beacon registered to a vessel, alerting the Cospas-Sarsat satellite system with a coded identity and, on modern units, a GNSS position. AMSA manages Australian registration. See EPIRBs and safety equipment.
Used in Common questions, answered, Cruising sailing yacht navigation system, Navigation redundancy that actually works
- Ethernetmarine
Manufacturer high-bandwidth networking used for radar imagery, detailed sonar and video — Garmin BlueNet, Raymarine RayNet, Navico and Furuno networks. This is why radar generally cannot be mixed across MFD brands. See marine networks.
Used in Common questions, answered, Marine electronics refit guide, Marine chartplotter buying guide
F
- Fairing block
A shaped block fitted under a through-hull transducer so its face presents square to the water flow on a hull with deadrise. See depth sounders and transducers.
Used in Marine electronics refit guide, Depth sounders and transducer selection, What it costs, and where to spend
- Forward-looking sonar
Sonar that attempts to show underwater terrain ahead of the vessel. Useful in unfamiliar anchorages, on a rising seabed and in poorly surveyed approaches, with range heavily dependent on depth, geometry and acoustic conditions. It is not underwater radar.
Used in Cruising sailing yacht navigation system, Marine sonar: CHIRP, side scan and live sonar, Depth sounders and transducer selection
G
- Galileo
The European Union's global satellite navigation constellation.
Used in Marine GPS and GNSS explained
- GLONASS
Russia's global satellite navigation constellation.
Used in Marine GPS and GNSS explained
- GNSSGlobal Navigation Satellite System
The umbrella term for satellite positioning systems — GPS, Galileo, GLONASS, BeiDou and QZSS. A multi-constellation receiver gains improved satellite availability and geometry rather than merely more logos. See GPS and GNSS.
Used in Marine radar: Doppler, solid-state and open array, Marine GPS and GNSS explained, AIS for boats: Class B, SOTDMA and MMSI
- GNSS compass
A heading sensor using two or more antennas separated by a known baseline, deriving orientation from satellite carrier-phase differences. Stable at zero speed and immune to local magnetic deviation. Also called a satellite compass.
Used in Marine GPS and GNSS explained, Common questions, answered, Cruising sailing yacht navigation system
- GPSGlobal Positioning System
The United States satellite navigation constellation — one GNSS among several, though the name is commonly used generically.
Used in Marine radar: Doppler, solid-state and open array, Marine GPS and GNSS explained, Common questions, answered
- GRIB
A compact binary format for gridded numerical weather model output — wind, pressure, waves, rain and sometimes current. Small enough to download over satellite, and raw model output with no forecaster judgement applied. See weather routing.
Used in Weather data and routing offshore, Tides, currents and chart datum, Satellite communications for boats
H
- HDOP / PDOP
Horizontal and Position Dilution of Precision: numerical expressions of how favourable the satellite geometry is. Widely spread satellites give a stronger solution than clustered ones.
- Heading
The direction the bow points. Distinct from course over ground, and required for radar overlay, MARPA, true wind and layline calculations. See heading sensors.
Used in Marine radar: Doppler, solid-state and open array, Marine GPS and GNSS explained, AIS for boats: Class B, SOTDMA and MMSI
I
- In-hull transducer
A transducer mounted inside the hull, radiating through the laminate, avoiding a hull penetration. Works only through suitable solid construction and typically sacrifices some performance.
L
- LATLowest Astronomical Tide
The lowest tide level predicted under average meteorological conditions over a long period. The chart datum used by modern Australian charts, which makes charted depths conservative.
Used in Marine radar: Doppler, solid-state and open array, Marine GPS and GNSS explained, AIS for boats: Class B, SOTDMA and MMSI
- Layline
The course on which a sailing yacht can reach a windward mark or destination on one tack without further tacking. Calculated from wind data, heading and boat performance — which is why heading and wind sensor quality matter to sailing navigation.
Used in Marine GPS and GNSS explained, Cruising sailing yacht navigation system, Marine chartplotter buying guide
- Leeway
The sideways drift of a vessel caused by wind pressure, contributing to the difference between heading and course over ground.
Used in Marine GPS and GNSS explained, Heading sensors and satellite compasses
- LiveScope / live sonar
Sonar providing near-real-time imagery of fish and structure. Garmin launched LiveScope 2 in July 2026 with improved resolution, noise reduction and coverage. Transformative for some fishing techniques; not essential navigation equipment for bluewater cruising.
M
- Magnetron
The high-peak-power transmitting valve used in traditional marine radar. Proven and capable at long range, but requires warm-up, uses more power and ages. Compare solid-state radar.
Used in Marine radar: Doppler, solid-state and open array, Best marine radar 2026, Radome vs open array
- MFDMultifunction Display
A display that integrates chart, radar, AIS, sonar, autopilot, instruments, engine data, cameras and more, as distinct from a basic chartplotter. Most premium 2026 products are MFDs. See chartplotters.
Used in Marine GPS and GNSS explained, Common questions, answered, Australian marine charts, AusENC, ZOC and S-100
- MMSIMaritime Mobile Service Identity
A nine-digit digital maritime identity used by DSC-capable VHF, AIS transceivers and MF/HF DSC equipment. AMSA manages Australian allocation. Never copy an old MMSI into new equipment without confirming it remains correct for the vessel.
Used in AIS for boats: Class B, SOTDMA and MMSI, Common questions, answered, Marine VHF and DSC in Australia
- MOBMan Overboard
Both the event and the chartplotter function that marks the position at the moment the button is pressed. The mark is where the person was; an AIS MOB device transmitting the person's own position is far more useful because it updates.
Used in Marine VHF and DSC in Australia, Weather data and routing offshore, EPIRBs, beacons and safety equipment
N
- NMEA 0183
The older serial marine data standard, still common on legacy GPS, autopilots, VHF, AIS and instruments. Gateways can preserve useful legacy equipment through a refit.
Used in Marine electronics refit guide, NMEA 2000, NMEA 0183 and marine Ethernet, Sources and references
- NMEA 2000
The CAN-based marine network standard carrying position, heading, wind, depth, AIS, engine and tank data between devices. Requires a terminated backbone with drops, and correct device instances. See marine networks.
Used in Marine GPS and GNSS explained, AIS for boats: Class B, SOTDMA and MMSI, Common questions, answered
- NMEA OneNet
The newer high-bandwidth marine Ethernet standard, which NMEA describes as supporting 100 Mbps to 10 Gbps using IPv6. Designed for far richer data than NMEA 2000.
- Notices to Mariners
Official notifications of changes affecting navigation — shoals, wrecks, obstructions, pilotage changes, new chart editions. Published for Australian waters by the Australian Hydrographic Office and worth reviewing before a significant passage.
Used in Common questions, answered, Australian marine charts, AusENC, ZOC and S-100, Passage planning, end to end
O
- Open array
A radar antenna with an exposed rotating bar, giving a narrower beam, better angular target separation and stronger long-range performance than a radome. Best suited to larger motor yachts and sportfishers. See best marine radar.
Used in Marine radar: Doppler, solid-state and open array, Cruising sailing yacht navigation system, Marine chartplotter buying guide
P
- PGNParameter Group Number
The identifier for a particular message type on an NMEA 2000 network. Useful when diagnosing whether a device is actually transmitting the data another device expects.
- PLBPersonal Locator Beacon
A 406 MHz distress beacon registered to a person rather than a vessel, small enough to be carried on the body.
Used in Trailer boat and small craft electronics, EPIRBs, beacons and safety equipment, Offshore fishing electronics
- Polars
A performance model describing a sailing yacht's expected speed at each true wind angle and speed. The input weather routing software optimises against — and manufacturer polars are usually optimistic for a cruising crew.
Used in Cruising sailing yacht navigation system, Marine chartplotter buying guide, Weather data and routing offshore
Q
- QZSS
Japan's regional satellite navigation system, tracked by many modern multi-constellation receivers and useful in the western Pacific.
Used in Marine GPS and GNSS explained
R
- Radome
An enclosed radar antenna in a compact dome. Lower windage, lower weight and easier to mount than an open array, which makes it the normal choice for sailing yachts.
Used in Marine radar: Doppler, solid-state and open array, Cruising sailing yacht navigation system, Cruising catamaran electronics
- Rate of turn
The rate at which heading is changing, output by many AHRS sensors and used by autopilots and radar processing.
Used in Heading sensors and satellite compasses, NMEA 2000, NMEA 0183 and marine Ethernet
- Rudder reference
A sensor reporting actual rudder angle to an autopilot computer, improving control quality where fitted.
Used in Marine autopilot guide, Commissioning and sea trials
S
- S-100
The IHO framework for a new generation of hydrographic data products. The Australian Hydrographic Office says it is focused on selected products from 2026: S-101 ENC, S-102 Bathymetric Surfaces, S-104 Water Level Information and S-111 Surface Currents. See marine charts Australia.
Used in Common questions, answered, Australian marine charts, AusENC, ZOC and S-100, Tides, currents and chart datum
- Safe speed
The speed at which a vessel can take proper and effective action to avoid collision and stop within an appropriate distance, given visibility, traffic, manoeuvrability, sea state and radar capability. A COLREGs requirement, not a guideline.
Used in Collision avoidance with radar and AIS, Night and poor-visibility navigation
- Satellite compass
See GNSS compass.
Used in Marine GPS and GNSS explained, Common questions, answered, Heading sensors and satellite compasses
- Scope
The ratio of anchor rode deployed to maximum expected water depth. Calculated on depth at high water, not the depth on arrival. See anchoring electronics.
Used in Marine radar: Doppler, solid-state and open array, Common questions, answered, Marine sonar: CHIRP, side scan and live sonar
- Sea clutter
Radar returns from wave surfaces. Aggressive clutter suppression also suppresses real weak targets, so learning the radar's honest appearance in different sea states matters.
Used in Marine radar: Doppler, solid-state and open array, Collision avoidance with radar and AIS, How to tune marine radar
- Side scan / side imaging
Sonar looking outward from the vessel, used to find reef edges, wrecks, bait, rock and bottom transitions. A major fishing tool, rarely a core cruising requirement.
- SOGSpeed Over Ground
Speed relative to the earth, derived from GNSS. Differs from speed through water when there is current.
- Solid-state radar
Radar using semiconductor transmitters with pulse compression rather than a magnetron. Fast startup, lower power, excellent close-range performance, Doppler capability and no magnetron to replace — the logical default for new recreational installations.
Used in Marine radar: Doppler, solid-state and open array, Marine navigation systems: the complete overview, Raymarine marine electronics
- SOTDMASelf-Organising Time Division Multiple Access
The channel access method used by higher-performance Class B AIS, which can bring improved slot management, higher transmission power and more frequent reporting than CSTDMA. Generally the preferable direction for a new offshore system where budget permits. See AIS.
Used in AIS for boats: Class B, SOTDMA and MMSI, Common questions, answered, Cruising sailing yacht navigation system
- Speed through water
Speed relative to the water, measured by a paddlewheel or ultrasonic sensor. Required for true wind calculation and for detecting current when compared against speed over ground.
Used in Marine GPS and GNSS explained, Cruising sailing yacht navigation system, NMEA 2000, NMEA 0183 and marine Ethernet
- SplitterAIS antenna
An approved active device letting AIS and VHF share one antenna. Reduces antenna count and can give AIS masthead height, at the cost of introducing a shared component. See AIS.
Used in AIS for boats: Class B, SOTDMA and MMSI, Marine VHF and DSC in Australia, Marine electronics refit guide
- Squat
The increase in a vessel's effective draft when moving, more pronounced in shallow water. One of the error terms that a proper under-keel clearance margin absorbs.
Used in Tides, currents and chart datum, Passage planning, end to end, Pre-departure checklist
T
- TCPATime to Closest Point of Approach
The predicted time until a target reaches its closest point of approach. Read together with CPA, and equally dependent on both vessels maintaining their current vectors.
Used in Marine radar: Doppler, solid-state and open array, AIS for boats: Class B, SOTDMA and MMSI, Collision avoidance with radar and AIS
- Terminator
A resistor fitted at each end of an NMEA 2000 backbone. Exactly two are required; a missing or extra terminator causes intermittent faults easily mistaken for equipment failure.
Used in Marine electronics refit guide, NMEA 2000, NMEA 0183 and marine Ethernet, Cruising catamaran electronics
- Thermocline
A layer of rapid temperature change in the water column, often visible as a sonar return and relevant to where fish hold.
- Through-hull transducer
A transducer mounted through the hull, usually the best performing option when correctly sited and faired. See depth sounders and transducers.
Used in Marine electronics refit guide, Depth sounders and transducer selection, Trailer boat and small craft electronics
- Tidal gate
A location where passage is only practical or safe within a window around slack water or a favourable stream — a narrow entrance, a bar or a strait. Tidal gates dominate passage timing. See tides, currents and chart datum.
Used in Tides, currents and chart datum, Passage planning, end to end, Pre-departure checklist
- Transducer
The device that converts electrical energy into sound and returning echoes back into electrical signals. It determines frequency, beam angle, power, sensitivity and mounting geometry, and therefore sets the ceiling on sonar performance.
Used in Common questions, answered, Cruising sailing yacht navigation system, Marine sonar: CHIRP, side scan and live sonar
- True wind
The actual wind over the water, computed from apparent wind, boat speed and heading. Errors in any input propagate into true wind, and from there into laylines and performance data.
Used in Marine radar: Doppler, solid-state and open array, Marine GPS and GNSS explained, Cruising sailing yacht navigation system
U
- Under-keel clearance
The water remaining between the deepest part of the vessel and the seabed. Equal to charted depth plus height of tide minus draft minus safety margin, with the margin absorbing chart, tide, swell, squat and sounder-offset error.
Used in Australian marine charts, AusENC, ZOC and S-100, Depth sounders and transducer selection, Tides, currents and chart datum
V
- VelocityTrack / MotionScope / Target Analyzer
Manufacturer names — Navico, Garmin and Furuno respectively — for Doppler radar processing that highlights approaching and receding targets.
- VHFVery High Frequency
The primary local marine communication band. Largely line-of-sight, so antenna height dominates practical range. See marine VHF and DSC.
Used in Marine GPS and GNSS explained, AIS for boats: Class B, SOTDMA and MMSI, Common questions, answered
- Voltage drop
The loss of voltage along a cable run under load, proportional to current, round-trip length and conductor resistance. It sizes marine cable, not the fuse rating. See marine DC power.
Used in NMEA 2000, NMEA 0183 and marine Ethernet, Cruising catamaran electronics, Trailer boat and small craft electronics
W
- Waypoint
A stored position used in a route. A waypoint should exist for a reason — a course change, a hazard clearance, a decision point — rather than to trace a coastline precisely. See passage planning.
Used in Marine GPS and GNSS explained, Marine chartplotter buying guide, Marine autopilot guide
- WGS 84
The geodetic reference frame used by modern marine navigation. Coordinates from old charts or historical cruising notes may use a different datum and should be verified before use.
Used in Marine GPS and GNSS explained
- Windvane
A mechanical self-steering system using wind pressure and water flow rather than electrical power. A genuine autopilot backup for offshore sailing precisely because it shares no failure domain with the vessel's electrical system.
Used in Cruising sailing yacht navigation system, Navigation redundancy that actually works, Marine autopilot guide
X
- XTECross Track Error
The perpendicular distance between the vessel and the intended track between waypoints. A configurable alarm threshold, and one of the core monitoring parameters in a passage plan.
Used in Marine GPS and GNSS explained, NMEA 2000, NMEA 0183 and marine Ethernet, Collision avoidance with radar and AIS
Z
- ZOCZone of Confidence
The Australian Hydrographic Office's description of the quality of source hydrographic data, in categories A1, A2, B, C, D and U, accounting for horizontal accuracy, depth accuracy, seabed coverage and survey method. A depth in a low-confidence area should not be treated like a depth from a modern full-coverage survey. See marine charts Australia.
Used in Common questions, answered, Australian marine charts, AusENC, ZOC and S-100
Where these terms are explained in depth
The glossary defines; these guides explain the engineering behind the definition.
Foundations
Marine navigation systems: the complete overview
The system-level guide: eight questions every navigation system has to answer, why sensors matter more than screens, and how to design a boat where technologies cover one another's blind spots.
Read the guideReference
Common questions, answered
The questions that come up most often, answered directly, each linked to the guide that covers it in depth.
Read the guideFoundations
Marine GPS and GNSS explained
What a GNSS receiver actually knows, why display precision is not accuracy, how course over ground differs from heading, and how to build position redundancy that survives a network failure.
Read the guide