National Use Field of National User Protocol

The national user protocol is a special coding format having certain data fields, indicated as "national use", which are defined and controlled by the national administration of the particular country which is identified by the country code field.

The national user protocol may be either a short or a long format message, as indicated by the format flag (bit 25). The correct BCH code(s) must be encoded in bits 86-106, and in bits 133-144 if a long message is transmitted.

Once the beacon has been activated, the content of bits 1 to 106 must remain fixed, however, bits 107 onwards are permitted to be changed periodically. In the case of the long format message, if bits 107 onwards are changed, then the 12-bit BCH code must also be recomputed. Changes to the content of the beacon message shall not occur more frequently than once every 20 minutes.

It should be noted that the content of distress alert messages encoded with the national user protocol cannot be interpreted by Cospas-Sarsat MCCs, and will be forwarded to SAR services only as hexadecimal data.