NAVTEX (Navigational Telex) is an international automated medium frequency direct-printing service for delivery of navigational and meteorological warnings and forecasts, as well as urgent marine safety information to ships. It was developed to provide a low-cost, simple, and automated means of receiving this information aboard ships at sea within approximately 370 km (200 nautical miles) off shore.
NAVTEX broadcasts are primarily made on the Medium frequencies of 518 kHz and 490 kHz. The international NAVTEX frequency is 518 kHz, and these broadcasts should always be in English. Regional transmission of NAVTEX uses 490 kHz specifically for broadcasts in local languages. It is not used in the U.S.
DroidNavtex is able to decode NAVTEX messages from your receiver through the phones/tablets microphone or through a connected interface. Special audio filters even decode very weak signals through the microphone and make DroidNavtex a very affordable alternative to high priced decoders.
An “Auto mode” finds the frequency and the sideband of the Navtex signal automatically. With “Auto mode” turned off the frequency and sideband can be adjusted manually.
A built-in database stores the received messages which can then be filtered, sorted and shared. Messages can be deleted individually, in groups or by age.
To save battery power 7 timers can be set to start DroidNavtex at a given time to decode messages. All timers can be set in local time or UTC.
For DroidNavtex to decode messages the phones/tablets microphone needs to be placed as close as possible to the receivers speaker.
Note: Most phones of tablets produce interference on the longwave and medium wave bands. This can disturb the quality of the received signal. A solution would be to connect a headphone or external speaker to the receiver and position it at least 2 feet away from the receiving antenna. The microphone of the phone or tablet should then be placed close to the speaker or headphone.
Decoding in “Auto tune”
In “Auto tune” mode DroidNavtex detects the frequency of the Navtex signal automatically. This works good when the signal of the Navtex station is medium to strong with normal interference.
Decoding without “Auto tune”
For weak Navtex signals and noisy conditions it is suggested to switch off the “auto tune” mode in the settings .
The frequency then need to be adjusted as well as the correct sideband. When a Navtex signal is present the 2 vertical red lines must be aliened with the 2 white lines on the waterfall by dragging them to the left or right as seen in the picture above. The receiver needs to be set to USB (upper side band). To decode the LSB (lower side band) reversed decoding can be set in the settings.The volume of the receiver should be adjusted to show as many green bars illuminated in the spectrum view.
The decoding screen can hold up to 1000 lines.
The screen can be erased by clicking on the “eraser” in the top left corner.
The Oscilloscope shows the bits of the Navtex stream after they passed the audio filters. A clean reception and decoding will show sharp corners. Weak signal will show more round corners.The Oscilloscope display can be turned on or off in the settings.
The “Spectrum” view can be also turned on or off in settings. If the “Auto mode” is turned off the “Spectrum” view can not be turned off since it is needed to adjust the frequency.
The history screen displays the past received messages.
The messages are arranged in table. The left column shows the
Display a message:
Deleting individual messages:
Share a message:
· all messages
Only the filtered messages will be deleted. All messages which are not displayed in the table will not be deleted and remain in the memory.
The timers will wake up the phone/tablet and start DroidNavtex and close it after the set duration.
Each individual timer can be set by pressing the icon.
NAVTEX Message Format
Start of message
ZCZC begins the message.
Transmitter identity (B1)
This character defines the transmitter identity and its associated coverage area.
Subject indicator character (B2)
The subject indicator character is used by the receiver to identify different classes of messages below. The indicator is also used to reject messages concerning certain optional subjects which are not required by the ship (e.g. LORAN C messages might be rejected in a ship which is not fitted with a LORAN C receiver).
NAVTEX broadcasts use following subject indicator characters:
Note: Receivers use the B2 character to identify messages which, because of their importance, can not be rejected (designated by a 1). The subject indicator characters B, F and G are normally not used in the United States since the National Weather Service normally includes meteorological warnings in forecast messages. Meteorological warnings are broadcast using the subject indicator character E. U.S. Coast Guard District Broadcast Notices to Mariners affecting ships outside the line of demarcation, and inside the line of demarcation in areas where deep draft vessels operate, use the subject indicator character A.
Serial number of message (B3, B4)
These two characters define the serial number of each B2 message type (class). Generally serial numbers start with the numbers ’01′, however in special circumstances, the numbers begin with ’00′. This forces the receiver to print the message.
Time of origin
The time of the transmission of the message is in UTC.
The full text of the message follows.
End of message
The end of the message is asserted when the characters "NNNN" are received.