NZ MVOC (VHF Radio) key terms

The spoken distress signal used on VHF radio to announce that a vessel or person is in grave and imminent danger and needs immediate help. It has absolute priority over all other radio traffic.

The spoken urgency signal used when a vessel or person is in trouble but not in immediate danger of loss of life. It concerns the safety of a vessel or person and requires urgent but not distress-level assistance.

The spoken safety signal that announces a message about navigation safety or important warnings, such as a drifting hazard, a broken-down vessel, or a weather warning. It is repeated three times before the message.

The initial transmission that announces a distress situation, sent either as a DSC alert on channel 70 or as a voice Mayday call on channel 16. It starts the rescue response.

A digital distress message sent on VHF channel 70 by a DSC-equipped radio, normally by pressing and holding the distress button. It automatically includes the vessel's MMSI and, if connected, its position.

A transmission made by a station other than the one in distress to relay a Mayday call on behalf of a vessel that cannot transmit itself, or whose distress call was not acknowledged. The relaying station identifies itself clearly.

The reply to a distress call, confirming that the message was received. A coast station or rescue coordination centre normally acknowledges; other vessels should wait briefly before acknowledging if no shore station responds.

The phrase spoken by a station controlling distress traffic to impose radio silence so that only stations involved in the rescue may transmit. It is repeated as necessary during the emergency.

The spoken phrase, from the French 'silence fini', used to cancel silence and announce that normal radio working may resume after distress traffic has ended.

The spoken instruction, from the French 'prudence', used to ask stations to restrict or limit their transmissions because of an emergency, without imposing full radio silence. It is used during urgency or safety traffic.

The VHF marine calling and distress channel (156.800 MHz). It is used for initial contact, distress, urgency and safety calls, then stations move to a working channel to continue the conversation.

The VHF channel reserved solely for Digital Selective Calling (DSC). Voice transmissions are not permitted on it, because digital signals would interfere with DSC alerts and routine calling.

A VHF channel used for the main conversation after initial contact on channel 16. Common choices include channels 6, 8, 72 and 77; the choice depends on the area and the purpose.

A VHF channel where both stations transmit and receive on the same frequency, so only one station can talk at a time. Most ship-to-ship channels are simplex.

A VHF channel that uses separate transmit and receive frequencies, allowing both stations to speak at once. These are mainly used for public correspondence and some port operations, not for ship-to-ship working.

Maritime Mobile Service Identity, a nine-digit number that uniquely identifies a vessel or coast station in digital radio systems. It is programmed into DSC and AIS equipment and is used in distress alerts.

Automatic Identification System, a transponder system that broadcasts a vessel's identity, position, course and speed to other vessels and shore stations. It helps with collision avoidance and search and rescue.

A radio that can both transmit and receive on the marine VHF band. Marine VHF sets are normally limited to about 25 watts maximum power and are designed for short-range, line-of-sight communication.

The aerial that radiates and receives radio signals. Its height and mounting strongly affect VHF range, because VHF communication is essentially line of sight over the horizon.

The principle that VHF radio waves travel in nearly straight lines and are not reflected by the ionosphere. Range therefore depends mainly on antenna height and the curvature of the earth.

Digital Selective Calling, a digital calling system built into modern VHF radios. It sends preformatted alerts and calls on channel 70 and is used for distress, urgency, safety and routine calling.

The shore-based centre that coordinates search and rescue operations. In New Zealand, the Rescue Coordination Centre New Zealand (RCCNZ) coordinates maritime SAR and responds to distress alerts.

The New Zealand government agency responsible for maritime safety, including the regulation of marine radio and the licensing of operators. It also oversees search and rescue policy and boating safety education.

Marine VHF Operator Certificate, the New Zealand operator qualification for marine VHF radio. It is issued under the maritime radio framework and demonstrates competence in distress, urgency, safety and routine procedures.

The authority to operate a marine VHF radio. In New Zealand, a valid Marine VHF Operator Certificate is generally required to operate a VHF radio on a vessel, and the certificate is personal to the holder.

A unique identifier assigned to a radio station, used to identify the station during transmissions. Vessels may also use their name, and New Zealand vessels may use an allocated call sign.

The practice of stating the identity of the calling and called stations at the start of a transmission, and again at the end if the exchange is lengthy. It lets other stations know who is transmitting.

A short test transmission to confirm that a radio is transmitting and receiving properly. It is requested on a working channel, not channel 16, and the reply reports signal strength and readability.

A measure of how clearly a voice transmission is heard, usually reported on a scale from 1 (unreadable) to 5 (perfectly readable). It is part of a standard radio check report.

A measure of how strong a received transmission is, usually reported on a scale from 1 (faint) to 5 (very strong). Together with readability it describes reception quality in a radio check.

The standard set of words used to spell out letters clearly over the radio, such as Alpha, Bravo, Charlie. It reduces errors when passing names, call signs and positions.

Standard phrases such as Over, Out, Roger, Say again and I spell, used to manage a radio exchange clearly. They keep transmissions short and unambiguous, especially in emergencies.

Two separate procedural terms. 'Over' means the speaker has finished and expects a reply; 'Out' means the exchange is finished and no reply is expected. They should not be combined as a single phrase.

The part of a distress call that states the vessel's position, using latitude and longitude or a clear bearing and distance from a known landmark, so rescuers can locate it quickly.

A brief statement in a distress or urgency call describing what has gone wrong, such as sinking, fire, flooding, grounding or medical emergency. It tells rescuers what resources are needed.

Regular radio broadcasts of weather, navigation warnings and safety information for mariners. In New Zealand they are broadcast on VHF and other services, often introduced by the Securite signal.

A forecast or alert about hazardous weather such as gale or storm force winds, issued for mariners. Warnings are broadcast as safety messages and should prompt operators to consider shelter or route changes.

Emergency Position Indicating Radio Beacon, a device that transmits a distress signal to satellites to alert rescue authorities. It complements VHF radio and is registered to the vessel and its MMSI.

A portable, battery-powered VHF radio, typically limited to about 5 watts. It is useful as a backup and in a liferaft, but its range is much less than a fixed set with a masthead antenna.