14 Mutual Interference between NAVTEX Stations

  14.1 The two principal causes of interference are:

• a) transmission overruns; and

• b) excessive power output.

  14.2 Although NAVTEX continues to be generally reliable and an effective medium for the promulgation of maritime safety information, the world-wide infrastructure continues to expand and the volume of information that each Administration disseminates through the international NAVTEX service continues to increase. There is a danger that in some geographical areas, without firm management, both the system and system users may become overloaded with information on the single frequency used. This is of particular importance when handling messages of VITAL priority.

  14.3 Many stations are filling their allotted 10-minute time slots and an increasing number are over-running. Instances of interference with neighbouring stations, as a result of over-running the time allocation, are also increasing. Where adjacent stations have transmitter identification characters which follow alphabetically (i.e. adjacent time slots), if the first station over runs, it may mask the phasing signal of the second station such that, to the user, it seems as if the second station is off the air. Safety-critical information from the second station, although broadcast, may not be received by the system users. Over-run is usually caused by one or more of the following which must be avoided at all costs, preferably by controlling the volume of data broadcast:

• .1 a significant increase in safety-critical activity such as cable laying. Navigational warnings promulgating such activity often include numerous waypoints which are listed by Latitude and Longitude;

• .2 meteorological information provided in a manner which is not concise and easily assimilated by the system user or for a much wider area than is covered by the NAVTEX station; and

• .3 additional information provided for non-SOLAS system users, e.g., longer-range weather forecasts for fishing and recreational vessels.

  14.4 As the GMDSS spreads to non-SOLAS mariners, their requirements for information are often different from the SOLAS ships and may be determined at a national level. SOLAS ships trading internationally usually pass through the area of coverage of a NAVTEX transmitter in a day; for them a 24-hour weather forecast usually suffices. However, fishing vessels and recreational vessels often remain in the same vicinity for several days and may require much longer range forecasts which take up more transmission time.

  14.5 In order to keep the quantity of information that is broadcast on 518 kHz to manageable levels and to reduce avoidable interference on this frequency, Administrations must:

• .1 monitor the volume of data broadcast and, together with adjacent Administrations, actively manage the system to ensure that interference caused by over-running allocated time slots is eliminated; and

• .2 transmit non-English language broadcasts for SOLAS vessels and broadcasts of information provided specifically for non-SOLAS vessels on 490 kHz or 4209·5 kHz as required. B₁ characters for these frequencies will be allocated by the IMO NAVTEX Co-ordinating Panel, on request.

  14.6 Excessive power output also causes interference between stations with the same B₁ transmitter identification character/time slot, but located in different regions. This has particularly been identified at night, as the number of operational NAVTEX stations increases. Occasionally, this can be caused by atmospheric conditions, but is generally caused by excessive power output from one of the stations. It is recommended that Administrations restrict the power output from their transmitters to that required to cover the designated NAVTEX service area, particularly at night, in order to avoid interference. As a general rule, transmitted power shall not exceed 1 kW by day and 300 watts by night.

  14.7 When interference is detected, particularly when it affects the service to system users, the matter shall be addressed immediately. When the interference is with adjacent stations, attempts shall be made to resolve the problem locally. Advice may also be sought from the NAVAREA Co-ordinator. If this is unsuccessful, the IMO NAVTEX Co-ordinating Panel shall be alerted to the problem and their advice sought. When the interference is from a station with the same B₁ character in a different area, the NAVTEX Co-ordinating Panel must be contacted and they will initiate any necessary investigation/action.