Marine Network and Navigation Terms – Complete and Easy to Understand Glossary

Modern boating is more digital than ever. GPS, AIS, radar, sonar, networks, apps and sensors all need to work together – but all these terms can easily feel like a jungle.

In this guide, we'll break down everything you need to know about your boat's electronics and marine networks – from NMEA 0183 and NMEA 2000 to autopilot, heading, current, galvanic corrosion and digital connections. Clear explanations, concrete examples and practical tips make it easy to understand how everything actually works on board.

Marine networks – the structure that ties all electronics together

NMEA 0183 – the older faithful servant

A serial format where each device sends data to another via its own wires. The information is sent as “sentences”, such as GPS position, speed, course or depth.

Commonly found in:
– Older plotters
– AIS receiver
– VHF with DSC
– Early autopilots
– Echo sounder

Think like this:
Like connecting a printer directly with USB → one cable per device.
If you want to send multiple data types, more cables are required → messy.

NMEA 2000 – today's standard

A modern bus network where all devices share the same data backbone.

Available in: modern plotters, engine instruments, AIS, wind sensors, autopilots, tank sensors, etc.

Advantages:
✔ One cable harness for everything
✔ Fast and stable data traffic
✔ Easy to expand
✔ Less cable clutter

Think like this:
Like a home network – all devices talk through the same “router”.

Backbone – the backbone of the network

The main cable where both data and power flow.

Important:
– Terminator at each end
– Only one power supply
– All units are connected via T-connectors

Terminators – small resistors with big significance

Sits at each end of the backbone and electrically closes the network.

Without them, you risk:
✘ No device found
✘ Autopilot problems
✘ Unstable data
✘ “Dangling” network

PGN – data types in NMEA 2000

PGN (Parameter Group Number) defines what kind of data is sent.

Examples: wind data, GPS position, engine data, tank level, battery status.

Think of PGN as a file format:
JPG = image
MP3 = audio
PGN 128259 = speed through water

Raymarine's SeaTalk & SeaTalkNG

– SeaTalk : older proprietary system
– SeaTalkNG : newer variant, compatible with NMEA 2000 via adapter

When upgrading, the right cabling and converters are often needed.

Multiplexers – the data traffic police

Combines or splits multiple NMEA 0183 streams. Example: GPS + AIS → chartplotter, or GPS → both autopilot and VHF.

Gateway – the translator between 0183 and 2000

Allows old and new systems to talk to each other.
Example: AIS in 0183 → plotter in N2K.

Navigation & communication – everything that helps you find and be seen

GPS – the global standard

Provides position, speed, course and time. Used in chartplotters, apps, VHF/DSC and autopilots.

GNSS – more modern and more accurate

Includes GPS, Galileo (EU), GLONASS, BeiDou.
Provides faster and more reliable position, especially in archipelagos.

AIS – see and be seen

Transmits and receives information via VHF: position, course, speed, MMSI, CPA etc.
Invaluable in darkness, fog and traffic.

Radar – sees what the eyes cannot see

Works independently of light and visibility. Shows land, boats, buoys – even in darkness and fog.

Echosounder – depth below the keel

Provides depth, bottom profile and sometimes fish. Important for safety when anchoring and basic navigation.

Heading – the course that the autopilot actually follows

Heading is one of the most misunderstood concepts on board. That's why it gets its own, extra clear section here. In short: heading is the direction the bow of the boat is pointing at the moment , and it is this direction that the autopilot uses to steer.

When you start connecting wind sensors, radar, autopilot and plotter, it quickly becomes important to understand the difference between magnetic heading and true heading .

Magnetic heading – course according to the compass

Magnetic heading is the course that the onboard measuring instrument directly reads. It comes from:

• magnetic compass
• fluxgate compass
• some gyro/IMU sensors

It is the direction the boat is physically pointing in relation to the magnetic north pole .

Affected by:

• Deviation – magnetism from things in the boat (cables, speakers, steel, tools, etc.)
• Variation – the difference between magnetic north and true north (varies depending on where in the world you are)

Advantages:

✔ always works, even without GPS

Disadvantages:

✘ can be several degrees of error if deviation/variation is not corrected

True heading – course in relation to true (geographic) north

True heading is the corrected course and is used when navigation systems need the highest accuracy.

True heading = magnetic heading + deviation + variation

This is the course in relation to the Earth's geographic north – exactly the course that nautical charts are based on and that advanced systems use.

Used by:

• advanced autopilots (for precise route following)
• radar overlay on plotter
• navigation software
• wind calculations on sailboats (true wind will be wrong without correct heading)

The difference in practice

A realistic example:

• Magnetic heading: 045°N
• Deviation: +2°
• Variation: +7°

→ True heading = 045 + 2 + 7 = 054°T

So: the compass says 045°, but your boat is actually pointing towards 054° in relation to geographic north.

Why does this matter?

The autopilot

An autopilot that is going to follow a route often needs true heading in order to:

• correct for current and wind
• keep exact course line between waypoints
• do the right thing when turning (XTE and next-track)

With incorrect heading, the autopilot easily drifts off course.

Radar overlay

For the radar and map to be exactly on top of each other , true heading is required.
Otherwise, the radar echo will be shifted to the side.

Wind data

True wind is calculated using heading as input value.
Wrong heading = wrong wind → wrong trim, wrong autopilot mode.

COG, SOG & STW – three courses, three speeds

✔ COG – Course Over Ground
GPS-measured actual course over the Earth's surface. Affected by wind and current.

✔ SOG – Speed Over Ground
Speed over land – important for ETA.

✔ STW – Speed Through Water
Speed through the water – good for sailing and trimming.

Compare SOG and STW → you get current strength.

Wind data – Apparent vs True

– Apparent wind : wind you feel on board
– True wind : actual wind relative to the Earth

Engine 5 knots into the wind → apparent increases
Sailing downwind → apparent decreases sharply

On-board digital connections

• Ethernet – radar, sonar, map sharing
• Wi-Fi – apps, tablets, remote viewing
• Bluetooth – sensors and remote controls

Electricity, corrosion & safety

Galvanic corrosion

Small DC currents in the water eat away metal: gears, propellers, shafts.

Galvanic isolator

Cheap protection against currents via shore power.

Isolation transformer

Best protection – isolates the entire electrical system from shore.

Summary – what this guide gives you

You can now:
✅ Understand how NMEA systems are connected
✅ Connect AIS, radar, autopilot and transducers correctly
✅ Understand heading, COG, SOG & true wind
✅ Avoid common network problems
✅ Understand the difference between magnetic and true heading
✅ Troubleshoot faster and install smarter

Results:
– safer navigation
– easier installation
– less cable chaos
– better instrument performance