Navigation Electronics & GPS
for the Captain's License Exam

1. GPS Fundamentals & Trilateration

The Global Positioning System (GPS) is a constellation of at least 24 satellites operated by the US Department of Defense orbiting approximately 20,200 km (12,550 miles) above Earth. Each satellite transmits continuous time and position signals. Your receiver uses these signals to calculate its position through a process called trilateration.

How Trilateration Works

Each GPS satellite continuously broadcasts the time the signal was sent, along with its precise orbital position (ephemeris data). Your receiver knows the speed of light (299,792 km/s), so it calculates the distance to the satellite by measuring how long the signal took to arrive. This calculated distance is called a pseudorange.

With one satellite, you're somewhere on a sphere of that radius around the satellite. With two satellites, you're on the intersection of two spheres — a circle. With three satellites, you're at one of two possible points where three spheres intersect. The fourth satellite eliminates the ambiguity and — critically — corrects for the receiver's clock error (since receiver clocks are not as accurate as the atomic clocks in the satellites).

The exam distinguishes trilateration (uses distance measurements) from triangulation (uses angle measurements). GPS uses trilateration. Traditional radio direction finding uses triangulation.

Key GPS Terms

2. GPS Accuracy: SA Removal, WAAS, and DGPS

Standard GPS has a theoretical accuracy of around 10-15 meters CEP (Circular Error Probable, meaning 50% of positions are within that radius). Before May 2, 2000, the US military degraded civilian GPS signals intentionally using Selective Availability (SA) — limiting accuracy to about 100 meters. SA was turned off permanently in 2000, dramatically improving civilian GPS accuracy. Two augmentation systems further improve accuracy for mariners.

GPS Accuracy Comparison

HDOP and PDOP Explained

Even with augmentation, GPS accuracy varies based on satellite geometry. HDOP (Horizontal Dilution of Precision) quantifies this geometry. Imagine satellites spread evenly around the sky at different elevations: that's optimal geometry with low HDOP. If satellites cluster in one part of the sky, the spheres of their pseudoranges overlap at shallow angles, making the intersection imprecise — HDOP rises.

GPS Error Sources

3. Chart Datums vs. GPS Datum

One of the most tested — and most practically dangerous — topics in electronic navigation is the relationship between the datum your GPS uses and the datum on which your chart was created. A position that looks like it's in the center of a channel can plot 200 meters off on a mismatched chart.

What Is a Datum?

The Earth is not a perfect sphere — it bulges at the equator and is slightly flattened at the poles. A geodetic datum is a mathematical model that defines the size and shape of the Earth and the origin point of the coordinate system. Different datums position the coordinate grid slightly differently on the Earth's surface.

GPS uses WGS-84, which is a global datum optimized for satellite navigation. Older charts used local datums (like NAD-27 for North America) that were optimized for surveying accuracy in a specific region. The coordinate shift between datums can be significant enough to ground a vessel navigating in shoal water.

4. Chartplotter Operation

A chartplotter integrates a GPS receiver with an electronic chart display, showing your vessel's real-time position on the chart. Modern multifunction displays (MFDs) combine chartplotting with radar overlay, AIS display, sonar, and weather data on a single screen. Understanding the core functions is essential for both the exam and real-world operation.

Core Chartplotter Functions

Planning a Route on a Chartplotter

Entering a route before departure — not while underway — is best practice. The exam frequently tests the proper sequence for verifying a chartplotter route:

5. COG vs. Heading, SOG vs. Speed Through Water

Understanding the difference between GPS-derived motion data and compass or log data is fundamental to electronic navigation. These pairs appear on virtually every exam. The key insight: GPS measures motion relative to the ground; compass and log measure relative to the water.

Practical Application: Current Effects

Consider a vessel steering 090 degrees magnetic (heading) at 8 knots STW. A 2-knot current is setting from 000 degrees (flowing southward). The current pushes the vessel to starboard. GPS shows:

6. Cross-Track Error (XTE)

Cross-Track Error is the perpendicular distance between your vessel's current GPS position and the planned course line drawn between active waypoints. It tells you how far off-track you have drifted — not how far you are from the next waypoint, but how far sideways you have moved from the straight-line route.

XTE is especially critical when navigating in channels with shoals on both sides. Many chartplotters allow you to set an XTE alarm that sounds when you drift beyond a set distance from the course line. Set XTE alarms conservatively — give yourself time to react before the hazard zone.

7. AIS: Class A/B, MMSI, CPA/TCPA

The Automatic Identification System (AIS) is a VHF transponder system that continuously broadcasts vessel identification, position, course, speed, and status data. AIS operates on two dedicated VHF channels: 161.975 MHz (AIS 1) and 162.025 MHz (AIS 2), using TDMA (Time Division Multiple Access) technology that allows thousands of vessels to share the channels without interference.

AIS Data Fields

Class A vs. Class B Comparison

CPA and TCPA

CPA (Closest Point of Approach) is the minimum distance between two vessels if both maintain their current course and speed. TCPA (Time to Closest Point of Approach) is how many minutes until that minimum distance occurs. AIS-equipped chartplotters calculate CPA and TCPA automatically for every AIS target and can display alarms when a target is predicted to close within a set CPA threshold.

MMSI

The Maritime Mobile Service Identity (MMSI) is a unique 9-digit number assigned to a vessel or station. It is the primary identifier in AIS and DSC (Digital Selective Calling) systems. US recreational vessels obtain an MMSI through the FCC or through organizations like BoatUS or Sea Tow. MMSI numbers are: 9 digits for ships, beginning with a 3-digit Maritime Identification Digit (MID) indicating the vessel's country. Coast stations use a 7-digit MMSI beginning with 00. SAR (Search and Rescue) aircraft use MMSIs beginning with 111. A vessel cannot make or receive an individual DSC call without a programmed MMSI.

8. Radar Operation

Marine radar transmits pulses of microwave energy and detects their reflections from targets. The time between transmission and return determines range; the antenna bearing when the return is received determines direction. Radar is indispensable in restricted visibility and for detecting targets that may not carry AIS. Understanding the controls and their interactions is heavily tested on the exam.

Radar Controls

Radar Measurement Tools

Radar in Restricted Visibility

Rule 19 of COLREGS governs conduct in restricted visibility. Vessels must: proceed at a safe speed adapted to conditions; have their radar operational and properly adjusted; plot or otherwise assess the risk of collision for every radar contact; take avoiding action in ample time, avoiding crossing ahead of a vessel forward of the beam.

9. Radar Plotting

Radar plotting — also called the RAPS method (Relative and Actual Plot System) or manual radar plotting — is the systematic method for determining a target's course, speed, CPA, and TCPA from a series of radar observations plotted on a maneuvering board or directly on the radar display. ARPA/MARPA automates this process, but the exam requires understanding the manual method.

Manual Radar Plot Steps

ARPA vs. MARPA

10. Depth Sounders, Transducers & Speed Logs

A depth sounder (echo sounder) transmits an acoustic pulse from a transducer, measures the time for the pulse to return from the bottom, and displays depth. Depth = (sound velocity x travel time) / 2. The speed of sound in seawater is approximately 4,800 feet per second (1,500 m/s) — the sounder uses this value (or an adjustable calibration) to convert travel time to depth.

Transducer Types

Sounder Calibration and Offset

Most depth sounders can be configured to display depth in three ways: depth below the transducer (raw measurement), depth below the keel (subtract the keel-to-transducer distance), or depth below the waterline (add the transducer depth below the waterline). For practical navigation, depth below the keel is most useful — it tells you how much water you have under your deepest appendage.

Speed Log and Temperature Sensor

Many transducers are combination units that also include a paddlewheel or impeller speed log (measuring STW) and a water temperature sensor. The paddlewheel must be kept clean — growth fouling slows the wheel and causes STW to read artificially low. Temperature sensors are used for fishfinding (thermoclines), weather monitoring, and calibrating doppler logs.

11. Electronic Chart Systems: ECDIS, ECS, RNC, ENC

The exam distinguishes carefully between different types of electronic chart systems and the two chart formats — raster and vector. Understanding which system legally replaces paper charts, and which chart format supports active navigation alarms, is critical for both the exam and for compliance aboard SOLAS-regulated vessels.

RNC vs. ENC: The Key Difference

The exam frequently tests the fundamental difference between raster and vector charts. An RNC is a photograph of a paper chart — the depths, shoals, and symbols are pixels, not data. There is nothing to query, and the system cannot automatically determine whether your intended track crosses a shoal.

An ENC stores every chart feature as a discrete object with attributes: a depth sounding is a data point with a value; a shoal is a polygon with a depth attribute; a wreck has a type, clearance depth, and hazard status. ECDIS can automatically check your planned route against all these objects and alarm if you're routing into shallow water or across a prohibited area.

12. Autopilots

An autopilot uses sensors (compass, GPS, wind instrument) and an actuator (hydraulic pump, belt drive, or tiller arm) to control the rudder and maintain a set course without continuous human input at the helm. Autopilots significantly reduce helmsman fatigue on long passages but require careful monitoring and immediate disengage capability.

Autopilot Limitations and Watch Requirements

Engaging an autopilot does not reduce the watch requirements under COLREGS. Rule 5 requires a proper lookout by sight and hearing at all times. An autopilot holding course cannot see a lobster pot float, debris, a vessel without AIS, or a breaking wave. At minimum, a person must be positioned to maintain a proper lookout while the autopilot is engaged — never leave the helm position unattended in trafficked or congested waters.

Autopilots are particularly dangerous near other vessels because they hold course regardless of developing collision situations. The helmsman must still monitor CPA/TCPA, assess risk, and disengage the autopilot to make any COLREGS-required avoiding action.

13. VHF DSC (Digital Selective Calling)

VHF radios with DSC (Digital Selective Calling) capability transmit and receive digital distress alerts on Channel 70. All VHF radios sold in the US since 1999 are required to have DSC. A DSC distress call transmits your MMSI, nature of distress, and GPS position automatically to the Coast Guard and nearby vessels — dramatically improving response time compared to a voice-only Mayday.

DSC Distress Procedure

14. Loran-C (Historical)

Loran-C (Long Range Navigation) was a ground-based hyperbolic radio navigation system that operated on 100 kHz. It used time differences between signals from a master station and secondary stations to position vessels within approximately 0.25 nautical miles during optimal conditions — acceptable for coastal navigation but far less accurate than GPS.

The US Loran-C system was shut down in 2010 after GPS became the primary navigation standard. The exam may include historical questions about how Loran-C worked or why it was shut down. Some countries and maritime authorities have explored eLoran (enhanced Loran) as a GPS backup system, but no eLoran network is currently operational in US waters.

15. Celestial Navigation as a Backup

GPS is the dominant navigation system for mariners, but the prudent navigator maintains celestial navigation skills as a genuine backup. A solar CME, GPS jamming, equipment failure, or power loss can leave an offshore vessel without electronic positioning. Celestial navigation, using only a sextant, nautical almanac, sight reduction tables, and an accurate timepiece, can determine position to within 1-2 nautical miles anywhere on Earth.

Sun Line (LOP) for a Running Fix

The most practical celestial technique for offshore passages without GPS:

16. Battery & Power Management

Electronic navigation systems are entirely dependent on battery power. A vessel that loses 12V DC power loses GPS, chartplotter, radar, VHF, AIS, and depth sounder simultaneously. Understanding current draw and proper battery management is part of the seamanship requirements for the captain's license.

Best Practices for Marine Electrical Systems

17. Exam FAQ — Navigation Electronics