Free Practice Questions for GASM GMDSS ROC

Explanation: VHF Channel 16 operates on 156.800 MHz and is the international distress, urgency, safety, and calling radiotelephony channel. Under SOLAS and ITU Radio Regulations, vessels equipped with GMDSS installations must maintain a continuous listening watch on Channel 16 when underway. Once communication is established on Channel 16, stations must immediately shift to an appropriate working channel.

Explanation: VHF Channel 70 (156.525 MHz) is designated internationally under the ITU Radio Regulations exclusively for Digital Selective Calling (DSC). Voice transmissions are strictly prohibited on Channel 70 to avoid causing interference to automated distress, urgency, safety, and routine signaling. GMDSS ship stations must maintain a continuous automated watch on Channel 70 using a dedicated watchkeeping receiver.

Explanation: In maritime VHF, a simplex channel transmits and receives on a single identical frequency, meaning operators cannot speak and listen simultaneously. A duplex (or semi-duplex) channel uses two separate frequencies—normally separated by 4.6 MHz in the international VHF band—allowing coastal stations and public telephone networks to support simultaneous two-way conversation. Direct ship-to-ship communication is impossible on duplex channels without coast station relay.

Explanation: The Dual Watch (DW) feature enables a mariner to monitor a designated working channel (such as a port control or inter-ship channel) while the receiver automatically samples VHF Channel 16 at high-frequency intervals (typically every few hundred milliseconds). If a transmission is detected on Channel 16, the radio locks onto Channel 16 so the operator does not miss safety or distress broadcasts. This satisfies continuous watch requirements while conducting operational communications.

Explanation: IMO performance standards and ITU Radio Regulations specify that maritime VHF transmitters must have a maximum effective radiated power of 25 Watts. Equipment must also provide a manual low-power setting that reduces power to 1 Watt or less. The 1 Watt setting is required when operating inside harbors, locks, marinas, or during close-range maneuvering to prevent overwhelming other stations on the same frequency.

Explanation: The squelch control sets a threshold level in the receiver's audio circuitry to mute the loud rushing 'hiss' caused by thermal and atmospheric background noise when no carrier signal is received. The operator adjusts the squelch until the background hiss just disappears; setting it too high decreases receiver sensitivity and blocks faint or distant distress calls.

Explanation: Due to atmospheric refraction curving VHF radio waves slightly around the curvature of the Earth (the 4/3 effective Earth radius model), the theoretical radio line-of-sight range in nautical miles is approximately 2.5 × (√h1 + √h2), where antenna heights are measured in meters. For example, a ship antenna at 9 m (√9 = 3) and a coast station at 100 m (√100 = 10) yield an estimated range of 2.5 × (3 + 10) = 32.5 nautical miles.

Explanation: VHF Channel 13 (156.650 MHz) is designated worldwide as the primary bridge-to-bridge navigational safety communication channel. Navigating officers use Channel 13 to exchange maneuvering intentions and coordinate passing arrangements directly to prevent collisions, particularly in narrow channels, congested waterways, or traffic separation schemes.

Explanation: VHF Channel 06 (156.300 MHz) is reserved under ITU Appendix 18 for communication between ship stations and between ship stations and aircraft engaged in coordinated search and rescue operations. It may also be used for inter-ship safety communications when Channel 16 is clear.

Explanation: A ship station MMSI consists of exactly 9 numerical digits in the format MIDxxxxxx. The first three digits are the Maritime Identification Digit (MID), which denotes the vessel's flag state administration (for example, 271 for Turkey). The remaining six digits uniquely identify the individual ship station.