NAVTEX is an acronym meaning Navigational Telex, and as its name shows, it is a kind of narrow band radio teletype system for sending (by frequency shift keying) text messages expressed in a 7-unit code. The difference is that a NAVTEX transmitter transmits nine control characters (header code) ahead of the main message, so that the receiver can identify the station, message type and serial number automatically.

For navigation purposes, the world is divided into 21 areas (called Navareas). Each Navtex station has an identification code, from “A” to “Z”. The frequency assigned to Navtex are 518 kHz and another (490 or 4209.5 kHz), and many stations exist in the same service coverage. If the stations were to transmit without any rule, the system would collapse due to mutual interference. To avoid this problem, the following rules apply. • The transmission schedule is determined so that two or more stations having a common service area may not overlap in time. • Each station transmits with minimum required power to cover its service area (200 to 400 nautical miles nominal).