4.13 Electronic Navigation: GPS, Chartplotters, Depth & AIS

Electronics Serve the Master — Not the Other Way Around

Q162 Navigation General — Near Coastal (50 questions, 70% pass) expects a Master Less Than 100 GRT to use modern bridge electronics the way a professional uses any tool: know the principle, the limitation, and the backup. GPS, chartplotters, depth sounders, and AIS keep small passenger vessels efficient and safer — until someone treats the screen as ground truth and drives a COI boat onto a charted rock that the eye and the paper chart still showed clearly.

This section is instruments and electronic navigation for the Master 100 level: not survey-grade GNSS theory, not full ECDIS type-approval law for SOLAS ships, but the knowledge that keeps you off the hard and answers the exam when it asks what can go wrong.

GPS / GNSS — How You Get a Position

GPS (Global Positioning System) is the U.S. satellite navigation system. GNSS is the broader term covering GPS plus other constellations (e.g., GLONASS, Galileo, BeiDou) that many modern receivers can use together. For exam and deck language, treat GPS/GNSS as the system that gives you:

• Position (latitude/longitude)
• SOG — speed over ground
• COG — course over ground
• Often UTC time and derived track/waypoint data

The receiver measures ranges to multiple satellites and solves for position. Accuracy under open sky with a healthy constellation is typically on the order of a few meters for civil receivers — good enough for coastal approach when used correctly, not a guarantee that the icon on the plotter is exactly on the pier bollard you think you see.

Limitations every master must name

Selective Availability (the old intentional GPS degradation policy) is historical; exams may still mention past accuracy limits, but modern focus is user-side errors: multipath, setup, datum, and believing the box when the eye disagrees.

Horizontal Datum — WGS 84 vs the Chart

Most GPS receivers default to the World Geodetic System 1984 (WGS 84) horizontal datum. Modern U.S. NOAA charts and ENCs are generally built for use with WGS 84 / NAD 83-compatible positions (NAD 83 is practically equivalent to WGS 84 for ordinary piloting). Problems arise when:

• An older chart or foreign product uses a different datum
• The receiver is set to a non-WGS 84 datum while the chart expects WGS 84 (or vice versa)
• Someone manually plots a GPS lat/long on a chart without checking the chart’s horizontal datum note in the title block

A datum error of tens or hundreds of meters is enough to put your electronic position in the channel when you are actually on the bank — or the reverse. Master rule: before trusting GPS against charted hazards, confirm receiver datum and chart datum agree. If they do not, convert or switch products; do not “split the difference” with hope.

Chartplotters and ECS

A chartplotter (or Electronic Chart System / ECS on many small vessels) overlays your GPS position on a digital chart (raster or vector/ENC-style display). Typical functions:

• Own-ship symbol, heading/COG vector, track history
• Waypoints, routes, and XTE (cross-track error) alarms
• Depth contours, ATON symbols, and AIS targets (when networked)
• Zoom, day/night palettes, and safety-depth settings on vector systems

Professional discipline:

1. Keep charts updated from authorized sources — an electronic chart is not automatically correct forever.
2. Know what layer you turned off (shallow depth, wrecks, text). Over-decluttering can hide the rock you need.
3. Set safety depth / contour appropriate to draft on vector displays.
4. Do not zoom out so far that the channel looks wide enough for two aircraft carriers when you are in a 40-foot boat.
5. Treat the magenta route line as a plan, not a COLREGS right-of-way or a guarantee of water.

ECS vs type-approved ECDIS: deep-sea SOLAS ships may carry type-approved ECDIS under carriage rules. Master 100 near-coastal work is almost always ECS/chartplotter territory. Exam language may say “electronic chart” or “chartplotter”; the safety message is the same — electronics assist; the master remains responsible.

Depth Sounder — Reading What the Transducer Says

An echo sounder / depth sounder transmits a pulse and measures return time from the bottom (or other targets). What you see on the display is depth related to the transducer, not automatically “depth under the keel” or “charted sounding + tide.”

Transducer offset (critical exam and dock skill)

• If the transducer is mounted on the hull bottom, the raw reading is roughly depth of water under the transducer.
• Many units allow a keel offset / transducer offset so the display shows depth below keel (or below waterline, depending on setup).
• If offset is wrong or zero when it should not be, you may think you have 6 feet under the keel when you have 6 feet under the transducer and only 4 feet under a deep skeg or prop.

Master checklist:

1. Know where the transducer is mounted and whether display is below transducer, below keel, or below waterline.
2. Verify offset after haul-out, transducer replacement, or software reset.
3. Cross-check with known depth at the berth (charted depth + tide − draft logic from tides chapter).
4. Remember aeration, weeds, soft mud, and steep slopes can give false or lost echoes — especially at speed in a chop.
5. A depth sounder does not replace a lookout for floating debris or a missing buoy; it reports depth/echoes, not the full navigational picture.

AIS — Classes and Collision Awareness

Automatic Identification System (AIS) broadcasts and receives vessel identity and navigation data over VHF-linked frequencies so equipped vessels (and shore stations) can see each other on plotters and AIS displays.

AIS commonly provides name/MMSI, position, COG/SOG, heading (if available), ship type, dimensions, destination/nav status (as entered). Use it to:

• Acquire targets early in traffic
• Call a vessel by name on VHF when appropriate
• Build a mental traffic picture with radar and visual lookout

Hard limits:

• Not every vessel carries AIS (kayaks, many small craft, some workboats, failures, deliberately off).
• Static data can be wrong (incorrect draft, status “underway” while anchored, wrong ship type).
• AIS is not a substitute for Rule 5 lookout, Rule 7 risk of collision, or radar when fitted and appropriate.
• Do not assume CPA from AIS alone without verifying motion by eye and, if available, radar.

Cross-Check: Electronic, Visual ATON, and Paper/Backup

Professional near-coastal navigation is layered:

1. Electronic fix (GPS/plotter) — continuous position and track
2. Visual ATON and landmarks — confirm you are where the channel marks say you are
3. Depth — water under the hull matches the plan
4. Paper chart or independent backup — second source if the plotter freezes, loses power, or lies
5. Radar/AIS (next section and above) — traffic and range context

If GPS says you are on the range but the front and rear range lights are open, believe the ranges. If the plotter shows mid-channel but the green can is on your starboard bow when returning, stop trusting the magenta line until you sort datum, chart update, or antenna error. Exam and real-world answer: electronics do not override visual ATON and good piloting sense.

Navigation Safety Regulations (33 CFR 164) — Awareness

33 CFR Part 164 (Navigation Safety Regulations) sets equipment, charts, and navigation procedure requirements primarily for certain commercial vessels (classic focus includes larger self-propelled vessels on U.S. navigable waters — thresholds and applicability are regulatory text you apply by vessel type/tonnage/route). For Master 100 study:

• Know that federal rules can require charts/publications, compass, depth sounding, and other navigation gear for regulated vessels, and impose voyage planning / navigation safety expectations.
• Your Certificate of Inspection (COI) and vessel-specific regulations may impose carriage and operational requirements even when you are below the big-ship Part 164 cutoffs discussed in unlimited contexts.
• Exam items at this level test concepts: carry and use adequate navigation information, maintain a proper watch, and do not navigate blindly on electronics alone — not memorizing every gross-tonnage applicability footnote of a VLCC.

If a question invokes Navigation Safety Regulations on a smaller commercial vessel, choose the answer that emphasizes required navigational equipment, current charts/pubs, and safe navigation practice, not “GPS alone is always enough.”

Electronic navigation pocket card

1. GPS/GNSS → position, COG, SOG; watch multipath, sky view, quality.
2. Match WGS 84 (typical GPS) to chart datum.
3. Chartplotter/ECS = tool; update, set depths, keep layers honest.
4. Depth sounder = under transducer unless offset is set correctly.
5. AIS A/B = awareness; not all vessels, not a COLREGS substitute.
6. Cross-check electronics with visual ATON, depth, and backup charts.
7. 33 CFR 164 awareness: regulated navigation safety culture — equipment + practice, not screens alone.

Own the failure modes and Q162 electronics items become common sense — the same common sense that keeps a loaded head boat off the spoil bank when the plotter icon looks perfect and the bottom is coming up fast.