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General Communication Rules

A rather limited number of radio communication channels have to be shared by all mariners. In order to guarantee the optimum use of these communication resources, some general communication rules have been adopted:

  • One of the primary rules in using radio communications, is not to interrupt any ongoing communication on the selected channel. Before transmitting, it should always be checked whether the selected channel is free and not in use.

  • Transmission should first be tried with low HF-power levels first. The distance covered by radio waves is roughly proportional to the cube root of the transmitted power (to double the covered distance, the power has to be increased by a factor of 8!). Marine VHF radio usually has a low power mode (1 Watt) and a high power mode (25 Watt). The low power mode will typically cover a range of 10 nautical miles, whereas the high power mode will cover typically 30 nautical miles.
    The mechanical construction of the antenna (height!) and the electrical matching of the transmitter to the antenna (no reflection) are more important factors for the performance of the radio than HF-power.

  • Since only a limited number of - shared - channels is available for marine communication, messages should be short (less than 60 seconds), clear (avoids re-transmission) and valuable (information exchange). It usually is a good idea to write down a summary of the information that should be communicated, in advance.

  • Routine communication must always be conducted on a free "working" channel. So as soon as the contact with the the desired station has been established on the calling channel (VHF channel 16 or MF 2182MHz) the further communication will have to be immediately shifted to a channel, which is available for the type of communication planned. It should be checked whether this "working" channel is free, before contacting the other station.

Other rules involve spelling and clarity of communicated information. For this purpose some extra rules have been adopted, which are summarized below.


When in external communication spelling is required, only the following - international - spelling scheme should be used:

Letter Code Letter Code
  A Alfa   N November
  B Bravo   O Oscar
  C Charlie   P Papa
  D Delta   Q Quebec
  E Echo   R Romeo
  F Foxtrot   S Sierra
  G Golf   T Tango
  H Hotel   U Uniform
  I India   V Victor
  J Juliet   W Whisky
  K Kilo   X X-ray
  L Lima   Y Yankee
  M Mike   Z Zulu
Number Code
  0 Nadazero
  1 Unaone
  2 Bissotwo
  3 Terrathree
  4 Kartefour
  5 Pantafive
  6 Soxisix
  7 Setteseven
  8 Oktoeight
  9 Novenine
  . Decimal

IMO Standard Marine Communication Phrases (SMCP)

IMO's Standard Marine Communication Phrases (SMCP) were adopted by the 22nd Assembly in November 2001 as resolution A.918(22) IMO Standard Marine Communication Phrases. The resolution adopts the Standard Marine Communication Phrases (SMCP) and recommends a wide circulation to all prospective users and all maritime education authorities.

The SMCP was developed for use by seafarers, following agreement that a common language - namely English - should be established for navigational purposes where language difficulties arise and the IMO SMCP has been developed as a more comprehensive standardized safety language, taking into account changing conditions in modern seafaring and covering all major safety-related verbal communication.

The IMO SMCP includes phrases which have been developed to cover the most important safety-related fields of verbal shore-to-ship (and vice-versa), ship-to-ship and on-board communications. The aim is to get round the problem of language barriers at sea and avoid misunderstandings which can cause accidents.

The IMO SMCP builds on a basic knowledge of English and has been drafted in a simplified version of maritime English. It includes phrases for use in routine situations such as berthing as well as standard phrases and responses for use in emergency situations.

The following are some recommendations from the above mentioned SMCPs.

  • If any part of the message are considered sufficiently important to need safeguarding, the keyword "repeat" will be used followed by the corresponding part of the message.

  • When a mistake is made in a message, the phrase "mistake" followed by "correction" and the corrected part of the message will be used.

  • Responses to questions should be clear, indicating the nature of the answer at the beginning of the response:

    • "positive, ..." or "yes, ..." for an affirmative response,
    • "negative, ..." or "no, ..." for a negative response,
    • "stand by for ... minutes" when the requested information is not immediately available,
    • "no information" when the requested information cannot be obtained.
    • "over" means that you expect a reply.
    • "out" means you are finished and do not expect a reply. It is contradictory to say "over and out."
  • Numbers should generally be spoken in separate digits. Three-digit numbers such as courses and bearings are spoken as 3 separate digits: e.g. "one-five-zero degrees" for 150°.
    Two-digit numbers may be spoken directly, but will be repeated as separate numbers: e.g. "fifteen point six, repeat, one-five-decimal-six" for 15.6.

  • When a position is communicated, Latitude and Longitude will be expressed in degrees and minutes (and decimals of a minute if necessary). When the position is related to a mark, the bearing shall be in the 360° notation from true north and shall be that of the position FROM the mark: e.g. "My position bearing one-three-seven degrees from xxx lighthouse, distance two-decimal-four miles".

  • Time should be expressed in the 24-hour notation indicating whether UTC, zone-time or local time is being used.

Calling Procedure for Routine Communication

Since the introduction of the Digital Selective Calling, a radio station (or a group of stations) can be called selectively. The technique is comparable to a dial phone call in the telephone network. To establish a direct connection to a specific radio stations case, the Maritime Mobile Service Identity (MMSI, comparable to the "phone number") of that station must be known. In this case, the station can be called directly through the DSC controller and if the called station replies, the DSC controllers will switch the involved transceivers automatically to the selected working channel (working channel is selected by the station being called). After this connection has been established, voice communication can start immediately on the working channel.

Also calls to "all stations" or calls to "all ships" can be established this way. However, the former distress and calling channels (VHF channel 16 and HF 2182 kHz) may still be used for setting up communication with radio stations that are not equipped with DSC capability.

The Routine Calling Procedure used is internationally standardized and needs to be strictly adhered to in order to preserve the calling channels as the distress, call and reply channel. All communication on the calling channels should be kept to a minimum. Especially on the VHF channel 16 routine calling should be tried first in low-power mode (1 Watt) in order not to interfere with other communication traffic.

It is the responsibility of the calling station to select an appropriate working channel to shift communication as soon as contact with the desired station has been established. The only exception to this rule is when dealing with a coast station, who have preferred working channels and which will be indicated during the calling procedure.

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