1. VHF Radio Operation and DSC
Why VHF Is the Primary Marine Communication Tool
VHF (Very High Frequency) marine radio operates between 156–174 MHz and provides reliable line-of-sight communication typically ranging from 5 to 20 nautical miles between vessels and up to 60 miles to shore stations. Every vessel equipped with VHF is required to monitor Channel 16 — the international distress, safety, and calling channel — whenever the radio is on and not actively communicating on a working channel.
Key VHF Channels to Know for the Exam
| Channel | Primary Use |
|---|---|
| 16 | International distress, safety, and calling — mandatory watch |
| 6 | Inter-ship safety communications |
| 9 | Recreational calling channel (alternative to 16) |
| 13 | Bridge-to-bridge navigation (1 watt only) |
| 22A | U.S. Coast Guard working channel |
| 70 | Digital Selective Calling (DSC) — no voice transmissions |
| WX1–WX3 | NOAA weather broadcasts (receive only) |
Digital Selective Calling (DSC)
DSC is a digital protocol built into Class D and Class E VHF radios that allows automated distress alerting and vessel-to-vessel calling. The distress button, when held for five seconds, transmits the vessel's MMSI number, nature of distress, and — if connected to GPS — position and time, on Channel 70. Receiving stations acknowledge the alert automatically, reducing response time dramatically.
MMSI Numbers
A Maritime Mobile Service Identity (MMSI) is a unique 9-digit number assigned to a radio station or vessel. U.S. vessels can obtain an MMSI from the FCC (with a Ship Station License) or from Sea Tow or BoatUS for recreational vessels. The MMSI must be programmed into the DSC radio before use. Vessel MMSIs begin with a 3-digit Maritime Identification Digit (MID) representing the country (USA = 338).
MAYDAY Calling Procedure
A MAYDAY is declared only when there is immediate danger to life or vessel. Transmit on Channel 16 (and simultaneously trigger DSC if equipped):
- MAYDAY MAYDAY MAYDAY
- THIS IS (vessel name spoken three times, then MMSI)
- MAYDAY (vessel name once)
- My position is (lat/long or bearing and distance from known point)
- Nature of distress (sinking, fire, medical emergency, etc.)
- Number of persons on board
- Any other useful information (vessel description, life raft deployed, etc.)
- OVER — await response; if none, repeat on Channel 16 and try 2182 kHz MF
Power Settings and FCC Licensing
Standard VHF radios transmit at 25 watts (high) or 1 watt (low). Channel 13 (bridge-to-bridge) must be used at 1 watt only. In U.S. waters, recreational vessels are generally exempt from FCC Ship Station Licensing for VHF, but any voyage to a foreign port requires a license. Commercial vessels always require an FCC Ship Station License. Operators of commercial vessels require a Restricted Radiotelephone Operator Permit (RR) at minimum; GMDSS-equipped vessels require a GMDSS Radio Operator's License.
2. AIS — Automatic Identification System
AIS uses VHF radio frequencies to broadcast vessel identity, position, course, speed, and other data to nearby vessels and shore stations. It was introduced to supplement radar for collision avoidance and improve maritime traffic management. AIS data is also received by satellites (S-AIS), extending global tracking coverage.
Class A vs. Class B Comparison
| Feature | Class A | Class B |
|---|---|---|
| Vessel Type | SOLAS vessels, passenger ships, large tankers | Recreational and smaller commercial vessels |
| Mandatory? | Yes — SOLAS requirement | Voluntary in most jurisdictions (required for some commercial) |
| Transmit Power | 12.5 W | 2 W |
| Update Rate (underway) | 2–10 seconds depending on speed/turn | 30 seconds |
| Static Data | Full IMO data set | Reduced data set (no voyage/ETA fields) |
| Channels Used | VHF 87B (161.975 MHz) and 88B (162.025 MHz) | Same VHF 87B and 88B |
AIS Data Fields
AIS broadcasts two types of data. Static data is programmed once and includes MMSI, vessel name, call sign, IMO number, vessel type, and dimensions. Dynamic data is generated automatically and includes position (lat/long), UTC time, COG (course over ground), SOG (speed over ground), heading, rate of turn, and navigational status (underway, anchored, not under command, etc.). Class A also broadcasts voyage-related data: destination, ETA, draught, and hazardous cargo type.
AIS Limitations
- !AIS does not replace radar — many vessels (especially small craft) are not equipped with AIS.
- !AIS data can be spoofed, delayed, or turned off. A vessel may be present without an AIS signal.
- !AIS channels can become congested in busy ports, causing delayed updates.
- !Position accuracy depends on the transmitting vessel's own GPS quality.
3. Radar Operation, Ranges, and ARPA
Marine radar operates by emitting pulses of microwave energy and measuring the time required for echoes to return from targets. The result is displayed as a plan position indicator (PPI) showing the vessel at the center with surrounding targets as bright returns. Most commercial marine radars operate in X-band (9 GHz, 3 cm wavelength) for high resolution or S-band (3 GHz, 10 cm wavelength) for better performance in rain and at longer range.
Range Selection and Scale
Common range scales are 0.25, 0.5, 0.75, 1.5, 3, 6, 12, 24, and 48 nautical miles. Short ranges (0.25–1.5 nm) are used in restricted waters for piloting. Mid-ranges (3–6 nm) are the primary collision-avoidance range in open water. Long ranges (12–48 nm) are used for coastal navigation and tracking distant weather. COLREGS Rule 5 (Proper Lookout) and Rule 7 (Risk of Collision) require systematic use of radar.
Radar Controls and Adjustments
Gain
Amplifies the received signal. Too low: weak returns. Too high: noise/clutter obscures targets.
Sea Clutter (STC)
Suppresses near-range returns from wave action. Adjust carefully — can mask small targets.
Rain Clutter (FTC)
Reduces precipitation returns that obscure targets. Use sparingly.
Tuning
Optimizes the receiver to the transmitter frequency. Proper tuning maximizes target brightness.
Brilliance/Contrast
Display brightness — adjust for ambient light conditions.
VRM (Variable Range Marker)
Concentric circle overlay to measure target range precisely.
EBL (Electronic Bearing Line)
Radial line from center to measure target bearing.
IR (Interference Rejection)
Filters interference from other nearby radars.
ARPA — Automatic Radar Plotting Aid
ARPA automatically acquires and tracks radar targets, computing each target's:
- CPA — Closest Point of Approach (minimum range the target will pass)
- TCPA — Time to Closest Point of Approach
- True course and speed of the target
- Relative course and speed (how the target moves on a relative motion display)
The officer sets a CPA/TCPA guard zone. When a tracked target is predicted to violate the guard zone, an audible and visual alarm sounds. ARPA dramatically reduces the manual plotting workload, but regulations and good seamanship require the officer to understand and verify computed data — ARPA does not replace judgment under COLREGS.
Radar Display Modes
Head-Up (Unstabilized): The vessel's heading always points to 12 o'clock. Targets drift as the vessel turns. Simplest to interpret for immediate collision avoidance but disorienting when maneuvering.
North-Up (Course/Heading Stabilized): True north is always at 12 o'clock. Consistent with chart orientation. Most useful for coastal navigation.
Course-Up: The vessel's set course — not current heading — is at 12 o'clock. The display stabilizes when on track.
Radar and Collision Avoidance
COLREGS Rule 7 states that every vessel shall use all available means — including radar if fitted — to determine risk of collision. A bearing to a vessel that does not change (constant bearing, decreasing range — CBDR) indicates risk of collision. COLREGS Rule 8 requires collision avoidance action to be taken in ample time, be large enough to be readily apparent, and not result in another close-quarters situation. Radar is also used to fix position using known charted landmarks as radar targets (radar pilotage).
4. GPS and Chartplotter Operation
How GPS Works
GPS (Global Positioning System) uses a constellation of at least 24 satellites in medium Earth orbit. A GPS receiver measures the time delay of signals from at least four satellites and uses trilateration to compute latitude, longitude, and altitude. Civilian GPS accuracy is typically within 3–5 meters horizontally with modern receivers. WAAS (Wide Area Augmentation System) further improves accuracy to approximately 1–3 meters in the contiguous U.S. by using ground reference stations to correct satellite timing errors.
Key GPS/Chartplotter Data Fields
COG
Course Over Ground — actual track made good through water, accounting for current and leeway
SOG
Speed Over Ground — speed relative to the Earth's surface (not through water)
CMG
Course Made Good — the net direction from departure point to present position
DTG
Distance to Go — distance remaining to the next waypoint
VMG
Velocity Made Good — speed made toward (or away from) a waypoint
XTE
Cross-Track Error — how far off the planned track the vessel is
ETA
Estimated Time of Arrival at next waypoint based on current SOG
HDG
Heading — direction the bow is pointing (may differ from COG due to current/leeway)
HDOP
Horizontal Dilution of Precision — satellite geometry quality (lower is better)
Chartplotter Operation
A chartplotter overlays GPS position on electronic charts and provides navigation functions including route planning (waypoints and legs), anchor watch alarms, and off-course alarms. Routes are saved as sequences of waypoints; the active leg from one waypoint to the next is the planned track. The chartplotter continuously calculates XTE, bearing and distance to the next waypoint, and ETA.
GPS Limitations — Critical for Exam
- !GPS gives position and COG/SOG — it does not indicate currents, leeway, or set and drift directly.
- !GPS signals can be blocked, jammed, or spoofed. Always maintain traditional navigational skills.
- !Chartplotter charts can be out of date. Always use current, properly licensed charts.
- !GPS altitude data is less accurate than horizontal position — do not rely on it for depth clearance.
5. GMDSS, EPIRB, and NAVTEX
GMDSS Sea Areas
The Global Maritime Distress and Safety System (GMDSS) replaced the traditional watch on 2182 kHz and standardized distress communications worldwide. GMDSS divides ocean coverage into four sea areas, each with increasing equipment requirements.
| Area | Coverage | Minimum Equipment |
|---|---|---|
| A1 | Within VHF DSC range of at least one coast station (20–50 nm) | VHF DSC radio, EPIRB (406 MHz), SART or AIS-SART |
| A2 | Within MF DSC range but outside A1 (approx. 150–400 nm) | All A1 equipment + MF DSC radio (2187.5 kHz watch) |
| A3 | Within INMARSAT coverage but outside A1/A2 (approx. 70 N/S) | All A2 equipment + INMARSAT SES or HF DSC radio |
| A4 | Polar regions beyond INMARSAT coverage | All A3 equipment — INMARSAT not usable; HF DSC required |
EPIRB — Emergency Position Indicating Radio Beacon
An EPIRB transmits a distress signal to the COSPAS-SARSAT satellite constellation when activated. Category I EPIRBs automatically activate when submerged to approximately 1–4 meters and float free of a sinking vessel. Category II EPIRBs are manually activated only. Both types transmit on 406 MHz (satellite uplink) and 121.5 MHz (homing beacon for SAR aircraft and vessels). The 406 MHz signal encodes the unique 15-digit hex ID.
EPIRB Registration Requirements
U.S. EPIRBs must be registered with NOAA at beaconregistration.noaa.gov before use. Registration links the 15-digit hex ID to:
- Vessel name, type, and length
- Owner name, address, and phone numbers
- At least two emergency contacts
- Home marina or usual port
Re-registration is required when vessel ownership changes or contact information changes. An unregistered or incorrectly registered EPIRB can delay SAR response by hours.
SART — Search and Rescue Transponder
A SART (Search and Rescue Radar Transponder) is a survival craft device that responds to 9 GHz (X-band) radar pulses from searching vessels or aircraft. When interrogated, it transmits a series of 12 dots on the rescuer's radar display along a line of bearing, converging to a target. AIS-SARTs are a newer alternative that transmit AIS signals; they appear as AIS targets on chartplotters of searching vessels.
NAVTEX Receivers
NAVTEX (Navigational Telex) is an automated MF broadcast system for maritime safety information. The primary international frequency is 518 kHz (English only). National language broadcasts use 490 kHz and 4209.5 kHz. NAVTEX receivers are programmed with a station selection code and message type codes. The receiver automatically stores and displays new messages, suppressing duplicates. Message types include:
6. Depth Sounders, Transducers, and False Readings
A depth sounder (or echo sounder) measures water depth by transmitting an ultrasonic pulse from a transducer mounted on the hull and measuring the time for the echo to return from the bottom. Depth = (speed of sound in water x time) divided by 2. Speed of sound in seawater is approximately 4,800 feet per second (1,463 m/s), varying with temperature and salinity.
Transducer Types
Through-Hull
Installed through the hull with a fairing block. Best accuracy and beam angle options. Requires a hull fitting (potential leak point). Preferred for offshore and performance boats.
Transom Mount
Mounted on the outside of the transom above the waterline. Easy installation, no hull penetration. Can be affected by turbulence from the propeller or hull at speed.
In-Hull (Shoot-Through)
Mounted inside the hull, transmitting through the fiberglass. No hull penetration, no antifouling issues. Signal loss through hull — not effective on cored hulls or steel/aluminum.
Beam Angle and Coverage
Transducers transmit in a cone shape. Narrow beam transducers (8–12 degrees) provide more accurate depth directly below and better target definition for fish finding. Wide beam transducers (20–45 degrees) cover a larger area, useful in shallow water or when maneuvering. The displayed depth is the shallowest point within the cone — in areas with significant bottom relief, the reading may be shallower than the depth directly under the keel.
False Bottom Readings
Several conditions produce inaccurate depth sounder readings that are critical to recognize on the captain's exam:
- Thermocline / Density Layer: A sharp temperature or salinity gradient can reflect the sonar pulse, producing a false bottom reading shallower than the actual depth. Common in stratified water bodies.
- Kelp, Seagrass, or Dense Weeds: Vegetation can return a strong echo, making the sounder display the weed bed rather than the actual bottom.
- Air Bubbles / Cavitation: Bubbles under the transducer (from hull turbulence, aeration, or cavitation) absorb or scatter the pulse, causing erratic or zero readings.
- Multiple Returns (Second Echo): In shallow water, the pulse may bounce off the bottom and the surface multiple times, displaying double or triple the actual depth on some units.
- Schools of Fish: A dense school of fish can return a strong echo and be misread as a bottom or obstruction.
Transducer Depth Offset
The sounder measures depth from the transducer face, not from the waterline or the keel. Captains must know whether the unit is set to display depth below transducer, below keel, or below waterline, and configure it appropriately to avoid running aground based on a misinterpreted reading.
7. Compass Types, Deviation, and Variation
Three Types of Marine Compass
| Type | Principle | Pros | Cons |
|---|---|---|---|
| Magnetic (Wet) | Magnetized card suspended in dampening fluid aligns with Earth's magnetic field | No power required, simple, highly reliable | Affected by vessel's own magnetic fields (deviation), sluggish in steep chop |
| Fluxgate | Electronic sensor detects magnetic field direction; no moving bowl | Digital output, integrates with autopilot and chartplotter, no deviation card needed if corrected electronically | Requires power, can be affected by electrical interference |
| Gyrocompass | High-speed gyroscope seeks true north via Earth's rotation | Indicates true north (no variation), unaffected by magnetic fields, stable in rough seas | Requires continuous power, settling time after startup, expensive, may have speed/latitude error |
Variation
Variation (magnetic declination) is the angle between true north and magnetic north at any given location on Earth, caused by the non-coincidence of geographic and magnetic poles. Variation is shown on nautical charts as isogonic lines (lines of equal variation) and in compass roses with the annual rate of change. Variation is always described as East or West. In the eastern United States, variation is currently West (magnetic north is to the west of true north). In the Pacific Northwest, variation is East.
Correcting for Variation
To convert a True course to Magnetic: add Westerly variation, subtract Easterly variation.
Example: True course 090, Variation 15 W — Magnetic course = 090 + 15 = 105 degrees Magnetic.
Deviation
Deviation is the error in a magnetic compass reading caused by the vessel's own magnetic field — from the engine, electrical wiring, metal fittings, loudspeakers, and electronic equipment near the compass. Deviation changes with the vessel's heading and is determined by swinging the compass (comparing the compass reading against known bearings on each major heading) and recording the results on a deviation card. Deviation is labeled East if the compass card is deflected east of magnetic north, West if deflected west.
The Compass Correction Formula
T → V → M → D → C
True — Variation — Magnetic — Deviation — Compass
Memory aid: “True Virgins Make Dull Companions”
Applying corrections going right (True to Compass): Add West errors, Subtract East errors.
Applying corrections going left (Compass to True): Add East errors, Subtract West errors.
Alternative memory aid for direction: “Compass to True? Add East. Error West? Add West going right.”
Loran-C (Historical)
Loran-C (Long Range Navigation) was a ground-based hyperbolic radio navigation system operating in the 90–110 kHz band. It determined position by measuring the time difference (TD) of signals from two or more synchronized transmitters. Loran-C was the primary electronic navigation system before GPS became widely available. The U.S. terminated Loran-C service on February 8, 2010. The USCG exam may include historical questions about Loran-C TDs and the concept of hyperbolic navigation.
8. ECDIS, ECS, and Integration of Electronics with Navigation
ECDIS — Electronic Chart Display and Information System
ECDIS is a navigation information system that complies with IMO Resolution MSC.232(82) and IHO standards S-52 and S-57. A properly type-approved ECDIS using current official ENCs (Electronic Navigational Charts) may replace paper charts on SOLAS vessels. ECDIS integrates position from GPS, vessel track, route planning, and safety depth contours with automated grounding alarms when the vessel approaches charted hazards.
| Feature | ECDIS | ECS (Chartplotter) |
|---|---|---|
| Type Approval | Required — IMO/IHO certified | Not required |
| Chart Types | Official ENCs (vector S-57 format) | ENCs and raster charts (varies by manufacturer) |
| Legal Status | Can replace paper charts on SOLAS vessels | Cannot replace paper charts or ECDIS |
| Anti-Grounding Alarms | Mandatory — look-ahead contour checks required | Optional/varies |
| Carriage Requirements | SOLAS vessels above 10,000 GT (phased in) | No carriage requirement |
| Typical Users | Commercial ships, passenger vessels, tankers | Recreational, small commercial, fishing vessels |
Electronic Chart Types
Vector charts (ENCs) store geographic data as objects with attributes — a depth sounding is a data point that the chart system queries for safe depth alarms. Raster charts are essentially scanned photographs of paper charts — the system cannot interpret individual data elements. Only vector ENCs support the full safety functionality required by ECDIS. NOAA distributes free ENCs for U.S. waters. UKHO and other hydrographic offices distribute ENCs through the AVCS (Admiralty Vector Chart Service).
Integration of Electronics with Traditional Navigation
The captain's exam emphasizes that electronic navigation is a tool, not a replacement for traditional skills. The required approach:
- 1.Cross-check position continuously. GPS/chartplotter position should be verified by depth sounder, visual bearings, and radar fixes whenever possible. No single system is infallible.
- 2.Maintain paper chart competency. Electronic systems can fail. USCG regulations still require paper charts (or ECDIS) on inspected vessels. All captains must be able to plot a position and lay a course on a paper chart.
- 3.Understand sensor inputs. A chartplotter displaying AIS targets, radar overlay, depth contours, and your GPS track is only as reliable as each underlying sensor. Know what feeds what.
- 4.Radar overlay on chartplotter. Many modern systems overlay live radar on the chart. This aids situational awareness but can mask chart detail — use split-screen display when navigating in confined waters.
- 5.NMEA 0183 and NMEA 2000. Marine electronics communicate via NMEA 0183 (serial, point-to-point, older standard) or NMEA 2000 (network, multiple devices share a single bus). Understanding the data bus prevents troubleshooting confusion when a GPS loses connection to the chartplotter or autopilot.
Frequently Asked Questions — Marine Electronics Exam
What is GMDSS and what sea areas does it cover?
GMDSS (Global Maritime Distress and Safety System) is an internationally agreed set of safety procedures, equipment, and communication protocols. It defines four sea areas: A1 covers VHF DSC range (20–50 nm from shore); A2 covers MF DSC range (approximately 150–400 nm); A3 covers INMARSAT satellite coverage (roughly 70 degrees N/S); and A4 covers polar regions beyond A3, requiring HF radio. Equipment requirements escalate with each sea area.
What is the difference between AIS Class A and Class B?
Class A AIS is mandatory for SOLAS vessels. It transmits at 12.5 W with update rates of 2–10 seconds underway and includes full voyage data. Class B is used by smaller commercial and recreational vessels, transmits at 2 W with 30-second update intervals, and provides a reduced data set. Both classes use VHF channels 87B and 88B and are identified by MMSI.
How do you register an EPIRB and what happens when it activates?
EPIRBs must be registered with NOAA at beaconregistration.noaa.gov. Registration links the EPIRB hex ID to vessel name, owner contact, vessel type, and emergency contacts. On activation, the EPIRB transmits on 406 MHz to COSPAS-SARSAT satellites and on 121.5 MHz for homing. The Coast Guard checks the registration database and coordinates rescue. An unregistered EPIRB causes delays and false alarm investigations.
What is the difference between compass deviation and variation?
Variation is the angular difference between true north and magnetic north caused by the Earth's magnetic field — shown on charts as isogonic lines. Deviation is the error caused by the vessel's own magnetic influences and varies with the vessel's heading. Both are corrected in sequence using the formula True-Variation-Magnetic-Deviation-Compass (True Virgins Make Dull Companions).
What is ARPA and how does it aid collision avoidance?
ARPA (Automatic Radar Plotting Aid) automatically acquires and tracks radar targets, computing CPA (Closest Point of Approach), TCPA (Time to CPA), true course, and speed. It enables monitoring of multiple contacts simultaneously with guard zone alarms. ARPA does not replace COLREGS obligations — the officer must assess all information and make independent judgments.
What is NAVTEX and what information does it broadcast?
NAVTEX is an automated MF broadcast system transmitting maritime safety information on 518 kHz (international English) and 490 kHz (national language). Receivers print or display categorized messages covering navigational warnings, weather, ice reports, SAR information, and more. Range is approximately 300 nm. Duplicate messages are suppressed automatically.
What is the difference between ECDIS and ECS?
ECDIS is a type-approved system meeting IMO and IHO standards that can legally replace paper charts on SOLAS vessels using official ENCs. ECS is a general term for chartplotter software that does not meet ECDIS standards. Recreational chartplotters are ECS devices and cannot substitute for required paper charts or a type-approved ECDIS.
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