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Receiving NAVTEX Maritime Safety Information Broadcasts With Your Macintosh


The International Maritime Organization has designated NAVTEX as the primary means for transmitting coastal urgent marine safety information to ships worldwide.

If you're on a boat or other vessel, have a Macintosh and a radio, and need to received NAVTEX transmissions, then MultiMode is the program for you.

NAVTEX is transmitted on three frequencies. 518 kHz is the primary frequency, some local language transmissions also take place on 490 and 424 kHz.

A schedule of NAVTEX transmissions is available at this URL

NAVTEX broadcasts are sent using the SITOR-B method, using a pair of audio tones. All you need is a marine or other receiver that can tune in 518 kHz, and a Macintosh computer (laptops work fine, as long as they have sound input available). Just connect the audio (speaker) output of the radio into the microphone input of your Mac. If your Mac doesn't have sound input, you can use a USB sound input device, like the Griffin iMic.

Then download and run a copy of MultiMode. Select SITOR mode in MultiMode, and click the B button to select SITOR-B (NAVTEXT) mode. Check the Parse NAVTEXT box if you want the four letter codes displayed as plain text.

If your radio has a RTTY mode, use that, as it will produce the best results. Tune in to 518 kHz. Set the Center Frequency in MultiMode to that of the audio tones coming out of the radio, usually around 2200 Hz, check your radio's manual. You may have to click on the INV button in MultiMode if the audio tones from your radio are reversed. But usually you don't for RTTY mode.

If your radio does not have RTTY mode, then you will need to use SSB mode. You can use either LSB or USB. If you use LSB mode, tune your radio to 520 kHz. If you use USB mode, tune your radio to 516 kHz. Set the Center Frequency in MultiMode to 2000 Hz. If you are using USB mode, make sure the INV button is highlighted, to select INVERT mode, if using LSB mode, make sure it is turned off.

The above picture shows the tuning indicator in MultiMode, when a NAVTEX broadcast has been properly tuned in.

MultiMode will synchronize to any detected NAVTEX transmission, and start printing the received text. That's it!

The word PHASE will display in BLUE when the phasing signal is being received. GOOD will appear in green when valid text is beign received, and BAD will appear in red when invalid text is being received. That could be due to poor reception quality, interference, an incorrect center frequency selected, INV being incorrectly selected, or no valid NAVTEX transmission being received.

MultiMode - Decodes weather fax and satellite images on your Macintosh, without the need for extra hardware.

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Chris Smolinski