NAVDAT DIGITAL BROADBAND SYSTEM FOR HIGH-RATE DATA TRANSMISSION FROM COAST STATIONS TO SHIPS

The existing narrowband direct data systems NAVTEX and TELEX which send information from shore stations to the ships at a low rate of 50 bps and parcels duration of 20ms are outdated already and do not meet modern requirements of information exchange. It is impossible to raise the rate by shortening a parcel duration due to the multipath effect and deformation defects of a message shape when signal passing through the Earth atmosphere. Deformation defects are resulted in extension of a message in time and superposition on each other. The KENTA Company (France) has introduced digital wideband system NAVDAT for MSI and security-related information. The system central frequency is 500 kHz with band of 10 kHz. 228 subcarrier frequencies are used. It is shown the dependency of a modulation mode and data transfer rate.They have refused from frequency modulation to reduce width of the signal spectrum in favor of multiply amplitude-phase encoding. The noise immunity of the system is solved with convolutional encoding which decreases the required signal-to-noise ratio at the receiver input by 6 dB. NAVDAT stations are deployed off the coast of Europe and Asia. NAVDAT data transfer rate in the presence of real interference increases hundreds of times in comparison with the existing systems. It has been shown necessity to increase signal-to-noise ratio at a signal reception area. Updated version of NAVDAT system is able to provide better data transmission rate due to some technological decisions: multifrequency signal transmission; multifrequency modulation (QAM) of signal; redundant convolutional coding. Digital wideband data transmission through updated NAVDAT provides a possibility to replace narrowband data transmission via TELEX.

RADIOTELEX, NAVTEX, NAVDAT, narrowband communication systems, data rate, convolutional encoding, multiply amplitude-phase modulation, RADIOTELEX, NAVTEX, NAVDAT

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