5.2 B₁ – Transmitter Identification Character

  5.2.1 The transmitter identification character is a single letter which is allocated to each transmitter. It is used to identify the broadcasts which are to be accepted by the receiver and those to be rejected, and also the time slot for the transmission.

  5.2.2 In order to avoid erroneous reception and interference of transmissions from two stations having the same transmitter identification character, it is necessary to ensure that such stations have a large geographical separation. Allocation of transmitter identification characters by alphabetical sequence to adjacent sites can also cause problems; hence, consecutive transmitter identification characters are not normally allocated to adjacent stations. Experience has shown that this removes the risk of a station which over-runs its time slot masking the phasing signal of an adjacent station which is about to begin its transmission.

  5.2.2 NAVTEX transmissions have a designed maximum range of about 400 nautical miles. The minimum distance between two transmitters with the same transmitter identification identifier must, therefore, be sufficient to ensure that a receiver cannot be within range of both at the same time.

  5.2.3 Close co-ordination between transmitting stations in adjacent NAVAREAs/METAREAs is necessary to achieve this separation. For this reason, national administrations shall request the advice of the IMO NAVTEX Co-ordinating Panel at an early stage in the planning of a new NAVTEX service. The Panel will allocate B₁ transmitter identification characters in such a way as to minimize the risk of interference occurring.

  5.2.5 Table 2 shows the transmitter identification characters and their associated transmission start times used by the IMO NAVTEX Co-ordinating Panel to evaluate and allocate transmitter identification characters A to X, regardless of the geographical position of the station anywhere in the world. Each transmitter identification character is allocated a maximum transmission time of 10 minutes every 4 hours. Because the NAVTEX system always utilizes a single frequency, it is fundamental to its successful operation that the following time slots are strictly adhered to, and that broadcasts do not overrun their allotted 10 minutes.

  5.2.6 In some regions, it has become necessary to accommodate a large number of stations. In extreme cases, it has even been necessary to reuse some transmitter identification characters for a second time within a region. Where this occurs every effort is made to ensure stations with the same character are as far apart as possible to reduce the risk of mutual interference.