GPS Fundamentals
GPS (Global Positioning System) determines your position by measuring the time it takes radio signals to arrive from multiple satellites simultaneously. A minimum of four satellites provides a 3D fix (lat, lon, altitude). Three satellites give a 2D fix. Accuracy is typically 3–5 meters with modern multi-constellation receivers.
WGS-84 Datum
GPS uses the World Geodetic System 1984 (WGS-84) as its reference datum — the mathematical model of Earth's shape. Most modern NOAA charts are also on WGS-84. If your chart uses a different datum (e.g., NAD-27), your GPS position may plot off by hundreds of meters. Always verify your GPS datum matches your chart datum.
Chart Datum Matters
When a GPS position plots on a paper chart, both must share the same horizontal datum. A GPS set to WGS-84 plotted on a NAD-27 chart can show errors of 50–300 meters — enough to run aground. Check the chart's title block for datum notes and configure GPS to match.
HDOP / PDOP
Horizontal Dilution of Precision (HDOP) and Position Dilution of Precision (PDOP) measure satellite geometry quality. Lower values are better. HDOP < 2 is excellent; > 5 is poor. Poor geometry (satellites clustered in one part of the sky) reduces accuracy even with strong signals.
Chartplotter Operation
A chartplotter combines GPS with electronic charts to display your vessel's position in real time. Understanding these core features is essential for both the exam and safe navigation.
Waypoint
A saved geographic coordinate (lat/lon). You can navigate point-to-point between waypoints or string them into a route.
Route
An ordered sequence of waypoints forming your planned passage. The chartplotter calculates bearing and distance for each leg.
Track
A recorded breadcrumb trail of your actual path over ground. Useful for retracing a passage or reviewing where you actually went versus where you planned.
Man Overboard (MOB)
A dedicated button that instantly marks current GPS position as a MOB waypoint and begins navigating back to it. Press immediately when someone goes overboard.
COG, HDG, SOG, and STW
| Term | Source | What It Measures | Current Effect |
|---|---|---|---|
| Heading (HDG) | Compass | Direction your bow points | Not affected by external forces |
| COG (Course Over Ground) | GPS | Direction you actually move | Wind (leeway) + current |
| SOG (Speed Over Ground) | GPS | Actual speed over earth | Favorable/adverse current |
| STW (Speed Through Water) | Log/impeller | Speed through the water mass | Not affected by current |
Leeway
Leeway is the sideways drift caused by wind pushing the vessel's hull and topsides. A vessel heading 090° in a strong northerly wind may make good a COG of 100°. The 10° difference is leeway. Sailboats and high-sided powerboats experience more leeway than low-profile vessels.
Current Effect on COG
A 2-knot beam current will push you off course even if you hold a steady compass heading. On GPS, this shows as COG diverging from HDG. To maintain a desired track, steer into the current — angle your bow upstream of your intended course line to compensate.
Cross Track Error (XTE)
XTE is the perpendicular distance from your current position to the active course line — the straight-line path between your last waypoint and your next waypoint.
- •XTE = 0: you are exactly on course
- •XTE = 0.1 nm: one-tenth mile off track
- •Positive XTE: right of course
- •Negative XTE: left of course
Practical Use
In a channel with 0.2 nm width, set an XTE alarm at 0.08 nm. If the alarm sounds, you're 0.08 nm off the centerline — with only 0.12 nm of margin to the channel edge. XTE is more useful than simple course deviation because it accounts for how far you are off the planned track spatially, not just angularly.
Electronic Chart Formats
RNC — Raster Nautical Chart
- •Digital scans of NOAA paper charts
- •Look identical to paper charts on screen
- •Not queryable — can't click a buoy to get info
- •Cannot support active grounding/collision alarms
- •NOAA discontinued production of new RNCs in 2024
- •Format: BSB (.kap files)
ENC — Electronic Navigational Chart
- •Objects stored as discrete vector data (points, lines, polygons)
- •Queryable: click any feature for full attributes
- •Supports automatic depth alarms and route checking
- •Layers can be turned on/off
- •Required for ECDIS on SOLAS vessels
- •Format: S-57 / S-100 (IHO standard)
AIS Integration
AIS (Automatic Identification System) uses VHF data link on channels 87B (161.975 MHz) and 88B (162.025 MHz) to broadcast vessel information. Class A transponders (required on commercial vessels >300 GT) transmit every 2–10 seconds underway. Class B (recreational/small commercial) transmit every 30 seconds.
MMSI
Maritime Mobile Service Identity — a unique 9-digit number that identifies a vessel on AIS and DSC radio. Vessel name, call sign, type, and dimensions are transmitted alongside position data.
CPA
Closest Point of Approach — the minimum distance between your vessel and a tracked target if both maintain current course and speed. Low CPA values (< 0.5 nm in busy waters) warrant attention.
TCPA
Time to Closest Point of Approach — minutes until CPA occurs. Combined with CPA, it tells you both how close and how soon. A CPA of 0.2 nm in 12 minutes demands immediate assessment.
Critical Exam Point: COLREGS Trump AIS
AIS is a collision avoidance aid, not a collision avoidance system. You cannot legally transfer your duty under COLREGS to an AIS alarm. Rule 5 requires a proper lookout by sight and hearing at all times. Many vessels do not carry AIS — fishing vessels under 65 ft, recreational boats, and some foreign vessels. An AIS clear screen does not mean no traffic. COLREGS Rules 16 and 17 (give-way and stand-on obligations) apply regardless of what AIS shows.
Radar Basics
Marine radar transmits microwave pulses and measures the time for echoes to return. Range is calculated from time-of-flight; bearing from antenna rotation angle. Radar is essential in restricted visibility and complements GPS by detecting targets that may not be on charts or AIS.
| Feature | Function |
|---|---|
| Range Rings | Concentric circles at fixed distances from the vessel. Used to estimate range to a target. |
| VRM (Variable Range Marker) | Adjustable ring used to measure the range to a specific target precisely. |
| EBL (Electronic Bearing Line) | Rotatable line from the vessel used to measure bearing to a target. |
| MARPA | Mini Automatic Radar Plotting Aid. Tracks selected targets and calculates CPA/TCPA automatically — similar function to AIS without requiring a transponder on the target. |
| Gain | Radar receiver sensitivity. Too low = miss targets; too high = sea clutter. Adjust for conditions. |
| Rain/Sea Clutter (FTC/STC) | Filters that suppress clutter from rain or wave returns. Use carefully — they can also suppress real targets. |
Radar in Restricted Visibility
COLREGS Rule 7 states you must use all available means — including radar — to determine if risk of collision exists. When using radar:
- •Take systematic radar observations (radar plotting)
- •Detect targets early, at maximum range
- •Reduce speed (Rule 6 — safe speed)
- •Sound fog signals (Rule 35)
Radar Limitations
- •Blind sectors from mast, superstructure
- •Rain and sea clutter can mask targets
- •Small fiberglass vessels return weak echoes
- •Fixed range minimum: targets within 50–75 m may not appear
- •Radar cannot identify vessel type or intentions
Dead Reckoning — GPS Backup Navigation
Dead reckoning (DR) is the foundational backup when GPS is unavailable. It requires only a compass, a clock, and knowledge of your speed. DR accuracy degrades over time as errors accumulate from compass deviation, current, and leeway. Always update your DR position when a reliable fix is available.
DR Formula
Plot the resulting distance along your compass course from the last known position.
Record last known GPS fix
Note position, time, course, and speed
Track elapsed time
Use ship's clock; note minutes since last fix
Calculate distance run
Distance = Speed (kts) × Time (hrs)
Plot DR position
From last fix, advance along compass course by calculated distance
Account for leeway/current
Adjust DR for known set/drift if in current-affected waters
Update when fix obtained
Cross-check DR against visual bearing, depth sounding, or landmark
Worked Example
Last GPS fix at 1400: position 34°12.5'N / 119°48.2'W. Course 270°, speed 8 kts. GPS fails. What is DR position at 1445?
Elapsed time = 45 minutes = 0.75 hours
Distance = 8 kts × 0.75 hrs = 6.0 nautical miles
Course 270° = due West. Longitude increases westward (larger number).
6.0 nm west at 34°N ≈ 6.0 nm ÷ cos(34°) ≈ 7.2' of longitude
DR position: 34°12.5'N / 119°55.4'W
Note: this ignores any current or leeway. In practice, apply known set/drift and leeway corrections.
GPS Failure Scenarios & Backup Navigation
GPS is reliable but not infallible. Power failure, antenna damage, signal interference, or satellite outages can all cause GPS loss. A licensed captain must be prepared to navigate without electronics.
Immediate Response Protocol
Note last known position, time, course, speed
Begin dead reckoning immediately
Reduce speed in restricted or shallow waters
Post extra lookout; increase radar watch
Pull paper chart for area — determine safe course options
Take visual bearings on landmarks or aids to navigation
Monitor depth sounder against charted soundings
Consider VHF position report to Coast Guard if uncertain
Backup Navigation Methods
Visual Bearing Fix
Take compass bearings to two or more known fixed objects (lighthouses, towers, charted landmarks). Plot bearing lines on the chart — your position is at or near the intersection.
Depth Sounding
Match depth sounder readings to charted soundings along your DR track. Useful in areas with distinctive bottom topography.
Radar Fix
Use radar range and bearing to charted land features, buoys, or structures to establish position without GPS.
Paper Chart + Compass
The original backup. A paper chart, hand-bearing compass, and parallel rulers allow full navigation. Required to be aboard USCG-inspected vessels.
GPS Troubleshooting Checklist
Exam Strategy
COLREGS always govern
No matter how many electronics you have, COLREGS Rules 2–38 apply. GPS and AIS do not create or remove right-of-way. Exam questions will probe this — 'an AIS alarm sounds; you are the stand-on vessel — what do you do?' Answer: maintain course and speed unless collision is imminent (Rule 17).
COG ≠ Heading
In any question involving current or leeway, COG and HDG will differ. COG is what GPS shows; HDG is what your compass shows. To make good a desired course in a cross-current, steer into the current (HDG upstream of COG).
GPS failure = DR + paper chart
The exam will ask what you do when GPS fails. Know the sequence: last fix → DR → paper chart → visual bearings → depth soundings. A backup GPS device and paper charts are required equipment on inspected vessels.
Frequently Asked Questions
What is the difference between COG and heading on a GPS chartplotter?
COG (Course Over Ground) is the actual direction your vessel is moving relative to the earth, as measured by GPS. Heading (HDG) is the direction your bow is pointing, measured by your compass. The difference between them is caused by leeway (wind pushing the boat sideways) and current (water movement). In a 2-knot crosscurrent with no leeway, your COG may differ from your heading by several degrees.
What does Cross Track Error (XTE) mean on a chartplotter?
Cross Track Error (XTE) is the perpendicular distance between your vessel's current position and your intended course line. If XTE is 0.1 nm, you are one-tenth of a nautical mile off your plotted route. A positive XTE typically means you're to the right of the course; negative means you're to the left. Minimizing XTE keeps you on the planned route.
What is the difference between SOG and STW?
SOG (Speed Over Ground) is your actual speed relative to the earth's surface, measured by GPS. STW (Speed Through Water) is your speed relative to the water, measured by a paddlewheel or impeller log. They differ because of current: if you're making 8 knots through the water (STW) but a 2-knot favorable current is behind you, your SOG is 10 knots. For fuel planning and ETA calculations, SOG is used. For hull speed and engine performance, STW is relevant.
What chart formats are used in electronic navigation?
NOAA produces two main electronic chart formats. RNC (Raster Nautical Charts) are digital scans of paper charts — essentially photographs of the paper chart. ENC (Electronic Navigational Charts) are vector charts where all features are stored as discrete objects that can be queried, turned on or off, and automatically used for collision and grounding alarms. Most modern chartplotters and ECDIS systems use ENCs because they support active navigation features. NOAA provides both formats free at charts.noaa.gov.
What is AIS and what does CPA/TCPA mean?
AIS (Automatic Identification System) is a transponder system that broadcasts a vessel's MMSI (Maritime Mobile Service Identity), name, position, COG, SOG, and destination on VHF frequencies 161.975 and 162.025 MHz. CPA (Closest Point of Approach) is the minimum distance your vessel will pass from a target if both vessels maintain their current courses and speeds. TCPA (Time to Closest Point of Approach) is how many minutes until CPA occurs. AIS provides CPA/TCPA data automatically, but COLREGS still govern — AIS data does not override your obligation to maintain a proper lookout and take appropriate collision avoidance action.
What is dead reckoning and when do you use it?
Dead reckoning (DR) is the process of estimating your current position based on a known past position, elapsed time, speed, and course. The formula is: DR Position = Last Known Position + (Course × Speed × Time). You use it when GPS fails, in areas of poor satellite reception, or to predict a future position. A DR position is less accurate than a GPS fix because errors accumulate over time with current, leeway, and compass errors. DR positions should be updated whenever you obtain a reliable fix.
Do COLREGS still apply when using GPS and AIS?
Yes — absolutely. GPS and AIS are navigational aids, not substitutes for the Rules of the Road. COLREGS (the International Regulations for Preventing Collisions at Sea) govern all vessels regardless of what electronics they carry. You are required to maintain a proper lookout by sight and hearing (Rule 5), proceed at a safe speed (Rule 6), and take appropriate action to avoid collision (Rules 16–17). AIS data showing a CPA of 0.1 nm does not automatically trigger a right-of-way decision — you must apply COLREGS to determine who gives way.
What should you do if your GPS fails underway?
If GPS fails, immediately: (1) Note your last known position and time. (2) Begin dead reckoning using your compass course and log/speedo readings. (3) Consult your paper chart to identify landmarks, aids to navigation, and depth soundings. (4) Use a hand-bearing compass to take bearings on two or more fixed objects and plot a position fix. (5) Reduce speed if in congested or shallow waters until your position is confirmed. A backup GPS, paper charts, and compass are required on USCG-inspected vessels for exactly this reason.
Related Study Guides
Marine Chart Reading
Chart types, buoy systems, compass rose, depth soundings, and chart symbols.
Tides & Currents
Rule of Twelfths, tidal current, set and drift, and current triangles.
Chart Plotting Exam Guide
Dead reckoning, TVMDC compass math, speed/time/distance problems.
COLREGS Study Guide
Rules of the Road — lights, shapes, sound signals, and right of way.
VHF Radio Guide
DSC, MMSI, channel usage, and distress procedures.
Navigation General Overview
Full Navigation General study guide: tides, weather, anchoring, celestial.
Practice GPS & Electronic Navigation Questions
COG vs HDG, AIS/radar, dead reckoning, GPS failure backup — practice with USCG-style questions and instant feedback. Free on NailTheTest.
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