VHF Channel Assignments
Seven channels account for the vast majority of USCG exam questions on VHF radio. Channels 16 and 70 are the two most tested — understand their rules cold before exam day.
ALL vessels — mandatory watch
Never use for routine traffic. MAYDAY, PAN-PAN, SECURITE, and initial calls only. Watch required by law.
Vessels communicating with USCG
Primary USCG working channel in the US. Most USCG broadcasts and SECURITE traffic goes here after Channel 16 announcement.
Recreational vessels (alternative to Ch. 16 for calling)
FCC designated alternative calling channel for recreational vessels. Reduces traffic on Ch. 16. Bridge-to-bridge on inland waters in some areas.
Recreational vessels — working channel after Ch. 16 contact
Popular recreational working channel. Not for commercial traffic. Available for boat-to-boat after initial contact on Ch. 16 or Ch. 9.
Recreational vessels — secondary working channel
Used when Ch. 68 is busy. Non-commercial only. Do not use for ship-to-ship commercial traffic.
Automated DSC controllers (not voice)
VOICE IS STRICTLY PROHIBITED on Ch. 70. This channel is reserved exclusively for DSC digital distress alerts and routine DSC calls. Never speak on this channel.
Vessel-to-vessel recreational traffic
Simplex only. Popular offshore and blue-water working channel. One of the few channels recommended for ship-to-ship non-commercial use.
Simplex vs. Duplex
Most VHF marine channels are simplex — transmit and receive on the same frequency. Only one party can speak at a time. Release the PTT button when done speaking (say "over").
Some channels (WX channels, coast station working channels) are semi-duplex or duplex — the coast station transmits on one frequency while vessels receive on another. This is why some channels appear with split frequencies in the ITU channel plan.
Transmitter Power and Range
A standard marine VHF radio transmits at 25 watts on high power and 1 watt on low power. VHF is line-of-sight — range depends primarily on antenna height above the water.
Range formula: miles = 1.42 x (square root of antenna height in feet). A 9-foot antenna on a small powerboat gives roughly 4-5 nm range to a vessel at water level. A 60-foot mast on a sailboat extends range to 11 nm. A USCG tower at 100 feet can communicate out to about 14 nm.
DSC — Digital Selective Calling
DSC is the digital alerting layer of modern marine radio. It does not replace voice communications — it is the automated digital alert that wakes up radios and transmits your identity and position before you transmit voice. Understand the four call priorities and the complete distress sequence.
Highest — supersedes all other communications
Procedure: Press and hold DISTRESS button 3-5 seconds. Radio auto-transmits MMSI, position, time, nature of distress on Ch. 70. Then voice MAYDAY on Ch. 16.
Grave and imminent danger to life or vessel. Sinking, fire aboard, person overboard, medical emergency.
Second highest — urgent message concerning safety
Procedure: Use DSC to send urgency call on Ch. 70 selecting urgency category, then voice PAN-PAN three times on Ch. 16, announce working channel, switch and repeat.
Medical advice needed, vessel in difficulty but not sinking, person in water (non-life-threatening), urgent navigation hazard.
Third priority — navigational or meteorological safety information
Procedure: DSC safety call on Ch. 70 (optional for SECURITE), then voice SECURITE three times on Ch. 16, announce working channel, broadcast message on working channel.
Navigational hazard, large debris, traffic separation warning, important meteorological observation, vessel test firing into shipping lane.
Lowest — normal operational communications
Procedure: Call on Ch. 16 or Ch. 9, identify both vessels, agree on working channel, switch and conduct business. Keep calls brief.
Vessel-to-vessel coordination, marina arrangements, bridge openings, crew communications, commercial operations.
MMSI — Maritime Mobile Service Identity
Your MMSI is a unique 9-digit number that identifies your vessel on the DSC system — the maritime equivalent of a phone number. When your DSC radio sends a distress alert, it automatically broadcasts your MMSI. Coast Guard and other vessels use your MMSI to look up your vessel name, type, hailing port, and emergency contact in the USCG's database.
US Vessel MMSI Format
Starts with 338 (US country code). Format: 338XXXXXX. Nine digits total. Ship stations begin with 3; coast stations with 0; search and rescue with 00; group calls with 0.
How to Register
Recreational vessels: free registration through BoatUS or Sea Tow. Commercial vessels: register via FCC Ship Station License application (Form 605). MMSI is programmed into the radio — do not change it without updating the registration.
Position Auto-Embed
Connect the VHF radio to a GPS or chartplotter via NMEA 0183 or NMEA 2000. The radio automatically embeds your GPS position into every DSC distress call. Without GPS input, the radio sends a distress alert but without position — the USCG knows you are in trouble but not where.
DSC Position Update and Urgency Calls
Modern DSC radios allow you to send a routine position update — a digital transmission of your current GPS position — to another vessel or coast station. This is useful for routine ship reporting. Urgency calls (PAN-PAN) via DSC follow the same process as distress but select the urgency priority category and do not require the sustained button press.
After any DSC distress alert is sent, the radio automatically reverts to monitoring Ch. 16 for the acknowledgment from the USCG or another vessel. Do not change channels until you receive an acknowledgment or have given the voice MAYDAY.
GMDSS — Global Maritime Distress and Safety System
GMDSS is the international framework that defines how vessels communicate in distress based on where they are operating. The Sea Area classification tells you exactly what equipment is required. SOLAS vessels must comply fully; OUPV commercial operators need to know the framework and requirements for the areas where they operate.
Within range of at least one VHF DSC coast station
Approximately 20-30 nm from shore
Coastal waters of US, most Caribbean, inside the 20 nm coastal zone
Required Equipment
- ›VHF radio with DSC (Ch. 70) capability
- ›EPIRB (406 MHz Category I or II)
- ›SART or AIS-SART
- ›Two-way VHF radiotelephone (survival craft)
Watch Requirements
Continuous watch on VHF Ch. 16 and DSC Ch. 70
Beyond A1, within range of MF DSC coast station
Approximately 150-200 nm from shore
Offshore Gulf of Mexico, nearshore Atlantic and Pacific passages
Required Equipment
- ›All A1 equipment
- ›MF radio (2182 kHz) with DSC capability
- ›Automatic MF watch on 2187.5 kHz DSC
Watch Requirements
Continuous watch on MF 2187.5 kHz DSC and VHF Ch. 16
Between 70 N and 70 S, beyond A1 and A2, within Inmarsat coverage
Global between 70-degree parallels
Offshore ocean passages, Pacific crossing, Atlantic crossing south of 70 N
Required Equipment
- ›All A1 and A2 equipment
- ›Inmarsat-C or VSAT terminal, OR MF/HF radio with DSC
- ›NAVTEX receiver
- ›Enhanced Group Call (EGC) receiver
Watch Requirements
Inmarsat-C or HF DSC watch; VHF Ch. 16 in range
Polar regions beyond Inmarsat coverage (above 70 N or below 70 S)
Arctic and Antarctic waters
Arctic transits, Northwest Passage, Southern Ocean below 70 S
Required Equipment
- ›All A1, A2, and A3 equipment
- ›HF radio with DSC (Inmarsat unavailable)
- ›Iridium satellite phone or Iridium EPIRB
Watch Requirements
HF DSC watch on appropriate frequency; Iridium terminal
EPIRB and SART in GMDSS
A 406 MHz EPIRB (Emergency Position Indicating Radio Beacon) is required on GMDSS vessels in all sea areas. Category I EPIRBs float free and activate automatically on submersion. Category II EPIRBs require manual deployment. All GMDSS-compliant EPIRBs transmit your vessel's identity via a programmed 15-hex ID code to COSPAS-SARSAT satellites, which relay the alert to the USCG within minutes.
A SART (Search and Rescue Transponder) or AIS-SART must be carried in each survival craft for vessels in A2 and beyond. When activated by a radar pulse, the SART creates a line of 12 dots on searching vessel radars, pointing toward the survivor's location. AIS-SARTs transmit a GPS position via the AIS system instead.
MF/HF SSB Radio — Offshore Communications
Single Sideband (SSB) radio in the Medium Frequency (MF) and High Frequency (HF) bands provides voice communications beyond VHF range — from 50 miles to thousands of miles offshore depending on frequency and propagation conditions. Understanding SSB operation, frequency selection, and propagation is essential for GMDSS compliance and offshore passage planning.
HF Propagation — How It Works
Skywave Propagation
HF radio waves bounce off the ionosphere — a layer of charged particles 50-600 km above Earth's surface. This "skywave" propagation allows HF signals to travel thousands of miles beyond the horizon. The ionosphere is energized by solar radiation, so propagation conditions vary dramatically with time of day, season, and the 11-year solar cycle.
Ground Wave
MF signals (particularly around 2 MHz) also propagate as ground waves — following the curvature of the Earth along the sea surface. Ground wave range is typically 100-400 nm and is most reliable at night when atmospheric noise is lower. This is why 2182 kHz is the international MF distress frequency — consistent ground wave coverage within the GMDSS A2 area.
Frequency vs. Range vs. Time of Day
- 4 MHz: Best at night, short to medium range (300-1,000 nm). Low D-layer absorption at night allows skywave.
- 8 MHz: Good day and night, medium range (500-3,000 nm). Most versatile offshore band for vessel communications.
- 12-16 MHz: Best during daylight, long range (1,000-5,000 nm). High solar activity improves these bands significantly.
- Dead Zone: There is often a skip zone between ground wave range and where skywave first returns — no signal in this gap. Change frequency to close the skip zone.
| Frequency | Band | Purpose |
|---|---|---|
| 2182 kHz | MF | International Distress and Calling — MF equivalent of VHF Ch. 16 |
| 2187.5 kHz | MF | MF DSC Distress and Safety |
| 4125 kHz | HF (4 MHz) | HF Distress and Safety (voice) |
| 6215 kHz | HF (6 MHz) | HF Distress and Safety (voice) |
| 8291 kHz | HF (8 MHz) | HF Distress and Safety (voice) |
| 12290 kHz | HF (12 MHz) | HF Distress and Safety (voice) |
| 4209.5 kHz | HF | NAVTEX (HF supplement) |
| 14300 kHz | HF (20m Ham) | Maritime Mobile Net (Ham — not licensed commercial) |
Winlink and Pactor — Offshore Email via HF
What is Winlink
Winlink is a worldwide radio email system operated by the Amateur Radio Safety Foundation (ARSF). It allows vessels with a ham radio license (or a dedicated marine SSB license) to send and receive email via HF radio when far offshore and out of satellite phone coverage or when satellite costs are a concern.
Pactor Modem
Pactor is a high-performance HF digital mode used by Winlink for email transfer over SSB radio. A dedicated Pactor modem (SCS brand is most common) connects between the SSB radio and a laptop. Pactor III and IV offer the fastest throughput — up to several kilobits per second under good propagation conditions.
Frequency Selection
Winlink operates on multiple HF frequencies in the 4, 6, 8, 12, 16, and 18 MHz bands. The correct frequency depends on time of day and distance to the nearest Winlink HF gateway station. Winlink Propagation Tool (VOACAP-based) helps select the best frequency. Lower frequencies (4-8 MHz) work better at night and shorter ranges; higher frequencies (12-18 MHz) work better during the day at longer ranges.
Licensing Requirements
To use Winlink on the ham radio frequencies, you need at minimum an FCC Amateur Radio Technician license, but HF access requires a General or Extra class license. Alternatively, maritime SSB stations can use Winlink on marine SSB frequencies under the Ship Station License without a ham license, connecting to Winlink RMS stations on marine frequencies.
Practical Offshore Use
Many offshore sailors use Winlink for weather GRIB file downloads (compressed forecast files), position reporting to family, and receiving text-only email. A typical GRIB file for 5-day offshore weather takes 1-3 minutes to download on Pactor III under good conditions. SSB/Winlink is an excellent backup when satellite phone airtime is expensive or equipment fails.
Satellite Communications — Offshore Options Compared
Four satellite systems dominate offshore marine communications. Each has a distinct coverage area, capability level, and cost profile. Understand the difference between Inmarsat-C (GMDSS text-only) and the higher-bandwidth systems for commercial offshore operations.
Inmarsat-C
Coverage
Global between 76 N and 76 S (geostationary satellites)
Data Rate
600 bps — text messaging only, no voice
Best For
GMDSS compliance for A3 areas, distress alerting, ship reporting, weather fax
Limitations
No voice capability. Very slow data. No real-time communication. Poor above 76 degrees latitude.
Cost: Low per-message cost. Terminal cost approx. 1,000-3,000 USD. GMDSS standard.
Iridium
Coverage
Truly global — includes poles (66 low-Earth-orbit satellites)
Data Rate
2.4 kbps voice and data; Iridium Certus up to 700 kbps
Best For
Polar voyages, voice calls anywhere on Earth, GMDSS A4, redundant communications
Limitations
Higher per-minute cost than Inmarsat. Smaller antenna footprint requires good sky view. Base Iridium data is slow.
Cost: Terminal 500-1,500 USD. Per-minute voice rates apply. Certus terminals higher cost.
VSAT (Very Small Aperture Terminal)
Coverage
Global between approximately 75 N and 75 S via geostationary satellites
Data Rate
1-20+ Mbps depending on service tier — true broadband
Best For
Commercial vessels needing crew welfare internet, video conferencing, large data transfers
Limitations
Large dish antenna (30-120 cm). High equipment and service cost. Dish cannot track satellite effectively in heavy seas. No polar coverage.
Cost: Equipment 5,000-50,000 USD. Monthly plans 500-5,000+ USD.
Fleet Broadband (Inmarsat)
Coverage
Global between 76 N and 76 S via Inmarsat I-4 satellites
Data Rate
Up to 432 kbps (FB500) — voice and IP data simultaneously
Best For
Offshore commercial vessels needing voice plus moderate-speed internet; GMDSS primary
Limitations
No polar coverage. Higher terminal cost than Iridium. Stabilized antenna required for small vessels.
Cost: Terminal 3,000-15,000 USD. Service plans vary widely by usage.
GMDSS Compliance via Satellite
For GMDSS Sea Area A3, a vessel may meet its communication requirements using either Inmarsat-C (with enhanced group call receiver and NAVTEX) or MF/HF SSB radio with DSC. Inmarsat-C is the most common choice on commercial vessels for its simplicity and low equipment cost. However, Inmarsat-C is text-only and cannot be used for voice distress communications — for that, a separate VHF (A1) and MF (A2) radio are still required. Iridium satisfies A4 area requirements where Inmarsat coverage does not reach.
AIS — Automatic Identification System
AIS is the vessel tracking system that continuously broadcasts position, identity, course, and speed via VHF data channels. Understanding Class A versus Class B differences is among the most frequently tested topics in the communications section of the OUPV exam.
| Parameter | Class A | Class B |
|---|---|---|
| Class A Update Rate — Underway | Every 2 seconds at over 23 knots; every 6 seconds at 14-23 knots; every 10 seconds at under 14 knots | Every 30 seconds underway regardless of speed |
| Class A Update Rate — At Anchor | Every 3 minutes | Every 3 minutes |
| Transmit Power | 12.5 watts (minimum) — 2 watts reduced power mode allowed in port | 2 watts maximum |
| Static Data Transmitted | MMSI, IMO number, call sign, vessel name, vessel type, dimensions, EPIRB MMSI | MMSI, call sign, vessel name, vessel type, dimensions (no IMO, no EPIRB MMSI) |
| Dynamic Data Transmitted | Position (GPS), COG, SOG, heading, rate of turn, navigational status | Position, COG, SOG (no rate of turn, no navigational status) |
| Voyage Data | Draft, destination, ETA, number of persons aboard | None — voyage data not transmitted |
| Who Must Carry | SOLAS vessels: all ships 300 GT+ on international voyages; all passenger ships; all tankers | Optional for recreational vessels; required by some flag states for commercial vessels under SOLAS size |
| Priority on AIS Channels | Class A has transmission priority — Class B yields to Class A | Lower priority; may not transmit if Class A traffic saturates the channel |
AIS Uses and Benefits
- ›Collision avoidance — displays nearby vessel positions on chartplotter
- ›Traffic separation scheme monitoring by VTS authorities
- ›SAR operations — AIS-SART locates survivors after DSC alert
- ›Port entry reporting — vessel arrival data to port authority
- ›USCG vessel traffic monitoring in high-density areas
- ›Offshore emergency: nearest vessel identification for assistance
AIS Limitations — Critical for Safety
- ›Not all vessels carry AIS — fishing boats, small recreational craft often do not
- ›AIS can be turned off or spoofed — never replace radar watch with AIS alone
- ›GPS outages degrade AIS accuracy — positions may be stale or wrong
- ›Channel saturation in busy ports may drop Class B transmissions
- ›AIS shows intended track, not guaranteed future position
- ›AIS range limited to VHF line-of-sight — typically 20-30 nm vessel-to-vessel
FCC Radio Licenses — What You Need and Why
Two separate FCC authorizations are required for commercial vessel radio operation. The Ship Station License belongs to the vessel. The operator license belongs to you. Know both — exam questions frequently test the distinction.
Ship Station License
Exam FocusIssued by: FCC — Held by: The vessel (not the operator) — Duration: 10 years, renewable
When Required
Required for vessels operating on US navigable waters that are equipped with a VHF, MF, HF, or satellite radio. Also required for vessels on international voyages.
How to Obtain
FCC Form 605 filed online at fcc.gov. Fee required. License posted or available aboard.
The Ship Station License authorizes the vessel's radio equipment to operate on specific frequencies. Calling sign assigned by FCC.
Restricted Radiotelephone Operator Permit (RROP)
Exam FocusIssued by: FCC — Held by: The individual operator — Duration: Lifetime — does not expire
When Required
Required for any person operating a ship radio station on domestic voyages involving VHF, MF, or HF voice communications. Required for commercial vessel operators.
How to Obtain
Pass FCC Element 1 examination at an authorized test site. No studying required — basic rules test. Fee paid to test site.
The RROP is the minimum operator license for commercial VHF radio use in US domestic waters. Recreational operators on domestic voyages technically do not need an RROP, but commercial operators do.
Marine Radio Operator Permit (MROP)
Issued by: FCC — Held by: The individual operator — Duration: Lifetime — does not expire
When Required
Required for operators on vessels on international voyages. Covers operation of all ship radio equipment including HF, GMDSS equipment, and EPIRB testing.
How to Obtain
Pass FCC Element 1 exam. MROP covers all RROP privileges plus international operations.
If your vessel operates internationally (including Canada, Mexico, Bahamas), every operator needs at minimum an MROP.
GMDSS Radio Operator License
Issued by: FCC — Held by: The individual operator — Duration: Lifetime — does not expire
When Required
Required for operators designated as the GMDSS-responsible officer on SOLAS vessels. Tests specific GMDSS equipment operation and procedures.
How to Obtain
Pass FCC Element 7 exam — comprehensive GMDSS written exam. More advanced than MROP.
Required for officers on SOLAS-class vessels. OUPV operators typically need the MROP, not the full GMDSS license, unless serving on SOLAS vessels.
Communications Log Requirements — 46 CFR
Federal regulations require commercial vessels to maintain a radio communications log. The log is the legal record of all safety communications and radio equipment status. Knowing what must be recorded — and for how long — is tested on the OUPV exam.
Watch establishment
Record the date and time the radio watch was established at the start of each operating period. Record operator name and license number.
Watch secured
Record the time the radio watch is terminated at the end of each operating period.
Distress communications
Record ALL distress signals heard or transmitted. Include: time, frequency/channel, identity of vessel in distress, nature of distress, position if given, and any response made.
Urgency communications (PAN-PAN)
Record all PAN-PAN calls heard or transmitted with same detail as distress entries.
Safety communications (SECURITE)
Record SECURITE broadcasts relevant to the vessel's voyage, especially those issued by your vessel.
Equipment malfunction
Record any equipment malfunction, the probable cause, the corrective action taken, and the time repaired or put back in service.
EPIRB and SART tests
Record monthly self-test of EPIRB and SART equipment, including the test result and battery/hydrostatic release expiration dates.
Position at time of distress communication
If your vessel participates in or witnesses a distress, record the vessel position at that time.
Retention Period and Inspection Rights
The radio communications log must be retained for a minimum of two years from the date of the last entry. USCG officers have the right to inspect the log at any time without a warrant. In the event of a maritime casualty, the log may be subpoenaed as evidence. Electronic logs are acceptable if they can be printed on demand. Failure to maintain the log, or falsifying log entries, is a federal offense.
SECURITE Calls and Navigation Warnings
SECURITE (pronounced say-CURE-ee-TAY) is the international safety signal for navigation and meteorological warnings. Knowing when and how to issue a SECURITE call — and when NOT to use MAYDAY or PAN-PAN — is a common exam scenario.
Step-by-Step SECURITE Procedure
- 1
On Channel 16, transmit: SECURITE, SECURITE, SECURITE — All stations, this is [vessel name and call sign].
- 2
Announce: Navigational warning will be broadcast on Channel 22A. Out.
- 3
Switch to Channel 22A (or the announced working channel).
- 4
Transmit: SECURITE, SECURITE, SECURITE — All stations, this is [vessel name]. [Message content: nature of hazard, position in LAT/LON or bearing/distance, time of observation, any additional detail].
- 5
Conclude: This is [vessel name], out. (No wait for reply required.)
When to Issue a SECURITE Call
Traffic Separation Scheme Broadcasts
The USCG broadcasts navigational warnings and traffic separation information on a scheduled basis — typically at H+05 and H+35 (5 and 35 minutes past each hour) on Channel 16 with traffic on Channel 22A. Times and channels vary by region. Check the Coast Pilot and USCG Broadcast Notice to Mariners for your area.
USCG Exam Focus Areas — What Gets Tested
These are the concepts that appear most frequently in USCG captain's license exam communications questions. Read each carefully — many candidates lose points on topics they think they understand.
Channel 70 is VOICE-PROHIBITED — DSC digital only
The most common wrong answer on communications questions: using Channel 70 for voice. It is strictly reserved for DSC digital distress alerts and DSC routine digital calls. Never transmit voice on Ch. 70. This is both a federal regulation violation and defeats the purpose of the DSC alerting system.
DSC distress alert does NOT replace the voice MAYDAY
The DSC alert on Ch. 70 is the digital alert. After sending it, you MUST switch to Ch. 16 and transmit the voice MAYDAY call. The DSC alert wakes up radios and gives position; the voice call provides critical detail. Both steps are required. Exam questions often ask what you do AFTER pressing the distress button — answer: switch to Ch. 16 and give voice MAYDAY.
MMSI must be registered BEFORE a DSC distress is useful
Your 9-digit MMSI number must be registered with a maritime authority (BOATUS, SeaTow, or FCC) so that when a DSC distress arrives at the USCG, they can immediately pull up your vessel name, home port, and emergency contact. An unregistered MMSI produces an alert that the USCG cannot immediately identify. Registration is free through BoatUS and Sea Tow for recreational vessels.
GMDSS Sea Area determines required equipment — know all four
The exam will describe a voyage and ask what GMDSS equipment is required. A1 adds VHF DSC. A2 adds MF radio. A3 adds Inmarsat-C or HF. A4 adds HF with DSC or Iridium. Each area is cumulative — A3 equipment includes all A2 and A1 requirements. Memorize the areas by range: A1 is coastal VHF range, A2 is MF range, A3 is satellite range, A4 is polar.
Class B AIS does NOT transmit voyage data or rate of turn
This distinction appears on exam questions comparing AIS classes. Class A transmits draft, destination, ETA, rate of turn, and navigational status. Class B omits all voyage data and rate-of-turn. Class A updates every 2-10 seconds underway; Class B every 30 seconds. Class A transmits at 12.5 watts; Class B at 2 watts. Class B yields transmission priority to Class A on the AIS data channels.
Radio log entries for distress calls are mandatory and permanent
Under 46 CFR and FCC regulations, every distress, urgency, and safety communication must be logged with time, frequency, vessel identity, and content. The log must be retained for at least two years. Failure to log a distress call is a separate violation from the underlying emergency. The radio log is the legal record of all safety communications and may be subpoenaed in casualty investigations.
RROP is lifetime — Ship Station License is 10-year renewable
The Restricted Radiotelephone Operator Permit (RROP) is a lifetime personal license that never expires. The Ship Station License belongs to the vessel and must be renewed every 10 years. Both are required for commercial VHF radio operation. The RROP goes with the operator, not the vessel. A new boat requires a new Ship Station License; an operator moving to a new vessel uses the same RROP.
Practice Questions with Answers
10 USCG-style communications questions with complete explanations. Work through each before reading the answer. Exam questions on this topic are generally straightforward if you know the rules — the challenge is eliminating plausible distractors.
1. A vessel in distress wants to send a DSC alert. Which channel does the DSC controller use to send the digital distress signal?
Answer: C — Explanation
Channel 70 (156.525 MHz) is reserved exclusively for DSC digital distress alerts and routine DSC digital calls. Voice is prohibited on Channel 70. The digital distress alert is transmitted automatically on Ch. 70 when the DISTRESS button is pressed and held. After the DSC alert, the operator switches to Channel 16 for the voice MAYDAY.
2. You are operating in an area approximately 100 nautical miles offshore. Which GMDSS Sea Area are you most likely in?
Answer: B — Explanation
Sea Area A2 extends beyond VHF DSC range (A1, approximately 20-30 nm) out to MF DSC coast station range, generally 150-200 nm. At 100 nm offshore you are beyond VHF range of most coast stations but within MF radio range. Sea Area A3 would apply only on ocean passages beyond MF range, within Inmarsat satellite coverage. Sea Area A4 is polar regions above 70 N or below 70 S.
3. What is the primary purpose of AIS Channel 70?
Answer: B — Explanation
Channel 70 (156.525 MHz) is designated exclusively for DSC (Digital Selective Calling) digital transmissions — distress alerts, urgency calls, safety calls, and routine DSC digital contacts. AIS uses its own separate channels 87B (161.975 MHz) and 88B (162.025 MHz). NAVTEX is a separate MF system operating at 518 kHz. No voice is permitted on Channel 70.
4. A vessel master must hold which FCC license to legally operate the ship's VHF radio on a domestic commercial voyage?
Answer: C — Explanation
The RROP (Restricted Radiotelephone Operator Permit) is the individual operator license required to operate a ship radio station commercially on domestic voyages. The Ship Station License belongs to the vessel, not the operator. The GMDSS Radio Operator License is a higher-level license required for SOLAS vessel GMDSS officers. An amateur radio license does not authorize marine commercial radio operations.
5. NAVTEX broadcasts maritime safety information on which frequency?
Answer: C — Explanation
The international NAVTEX frequency is 518 kHz in the MF band. This frequency provides coverage out to approximately 200-400 nm from shore. A secondary frequency of 490 kHz is used in some regions for national-language broadcasts. 156.8 MHz is VHF Channel 16. 2182 kHz is the MF international distress frequency. 406 MHz is the EPIRB satellite distress frequency.
6. Which of the following data does an AIS Class B transponder transmit that a Class A does NOT transmit?
Answer: D — Explanation
Class B is strictly a subset of Class A data. Class B transmits: MMSI, vessel name, call sign, vessel type, dimensions, position (GPS), COG, and SOG. Class B does NOT transmit: rate of turn, navigational status, IMO number, draft, destination, ETA, or number of persons aboard. Class A transmits all of these. There is no data that Class B transmits that Class A does not also transmit.
7. A captain observes a large floating shipping container in a busy shipping lane at night. The container is not lit. What is the correct radio procedure?
Answer: C — Explanation
SECURITE is the correct signal for navigational hazards — this situation does not rise to a distress (MAYDAY) or urgent safety of life situation (PAN-PAN). The correct procedure: transmit SECURITE three times on Channel 16, announce traffic will follow on Channel 22A (or another working channel), switch to Ch. 22A, transmit SECURITE three more times, then broadcast the hazard location, description, and time. Other vessels in the area need this information to navigate safely.
8. Under 46 CFR, how long must the ship radio communications log be retained?
Answer: C — Explanation
Under 46 CFR and FCC regulations applicable to commercial vessels, the radio communications log must be retained for a minimum of two years. The log must record all distress, urgency, and safety communications; daily watch establishment and securing times; equipment malfunctions and repairs; and operator license numbers. The log may be in paper or electronic format but must be available for USCG inspection at any time.
9. What is the correct VHF channel and procedure for contacting the US Coast Guard when you need non-emergency assistance?
Answer: A — Explanation
All initial USCG contact is made on Channel 16, which the USCG continuously monitors. After establishing contact, the USCG will typically direct you to Channel 22A (the primary USCG working channel) to conduct your business and keep Channel 16 clear for distress traffic. Calling directly on Ch. 22A without a Channel 16 announcement may miss the USCG watch if they are on another channel. Channel 70 is for DSC digital only, never voice.
10. An SSB MF/HF radio aboard a vessel uses Single Sideband modulation. What advantage does SSB have over AM for marine communications?
Answer: B — Explanation
Single Sideband (SSB) modulation suppresses the carrier wave and one of the two sidebands present in a full AM transmission. A standard AM signal wastes approximately two-thirds of its power transmitting the carrier (which carries no information) and the redundant sideband. SSB puts all of the transmitter power into the single sideband containing the actual voice information. This allows an SSB transmitter to achieve significantly greater range than an AM transmitter of identical power output — a critical advantage for offshore HF communications.
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NailTheTest delivers USCG-style practice questions covering VHF channels, DSC procedures, GMDSS sea areas, AIS, FCC licenses, and everything else in the communications section of the captain's license exam. Immediate answer explanations included.
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