OpenPlotter3 – Marine NAVTEX – Telecommunications, Navigation & Electronics

Introduction

In the previous post I looked at using OpenPlotter3 for receiving HF Weather Fax (Ref.1). In this post we will use OpenPlotter3 to receive Marine MF NAVTEX (Ref.2). NAVTEX transmissions contain valuable marine information such as navigation warnings, weather forecasts, ice information and MSI specific messages. The world is divided into different regions and different stations transmit in certain time slots. Ref.3 gives a list of stations and their transmission schedules. Ref.4 shows a map of NAVTEX transmitters by area.

NAVTEX Reception

Fig1. OpenPlotter3 Marine HF NAVTEX Equipment Setup

Figure 1 shows the reception setup. This is identical to my previous post (Ref.1), I am assuming that we have OpenPlotter3 setup with Gqrx, Fldigi and Pulse Audio and we have a decent HF antenna. My location in Toronto is not adjacent to a coast, but unfortunately there is an AM/IBOC station sitting on 530KHz with the OFDM Bart-Head sitting right on top of the 518KHz frequency as shown in Figure 2. I was about to give up, but then I remembered a new innovation called KiwiSDR which is a network of web based HF receivers some of which are open to the public. So I can receive remotely from a location close to a NAVTEX transmitter to demonstrate the principle.

KiwiSDR Reception

Fig.3 NAVTEX Transmitters Navarea4 West Atlantic

Fig.4 KiwiSDR Receiver Locations in Canada

Fig.5 KiwiSDR Receiver Screen Showing NAVTEX Frequency 518KHz

Figure 3 shows a table of NAVTEX transmitter locations (Ref.3) and Figure 4 shows a map of KiwiSDR receivers (Ref.5). Stations transmit for 10 minutes every 4 hours in a staggered fashion. I was able to receive Sydney & Portsmouth successfully using stations between Cape Cod and Nova Scotia. When you enter the kiwidr, select the medium wave band, usb mode and set the frequency to 517KHz as in Figure 5. Enter your call sign or vessel id. Transmissions start right on time. I record the audio using Audacity and playback into FLDIGI.

Experiments with my RTL-SDR_V3 and V2/Upconverter indicate that the you have to be careful with the gain setting in the MW band. Watch the SNR as you adjust the gain, a lower gain may be preferable. My V3 cuts off at 500KHz, so 490KHz may not be accessible depending on your unit.

NAVTEX Decodes

Fig.6 Fldigi Decode Sydney NAVTEX May 14th

Fig.7 Sydney NAVTEX Transmission

Fig.8 Fldigi Decode Portsmouth NAVTEX May 14th

Fig.9 Portsmouth NAVTEX Transmission

SD Card Backup

Once you setup OpenPlotter3 for the various functions you need, it makes sense to make a backup just in case something goes wrong. The RPi4 has a very useful utility called SD Card Copier. You can obtain an inexpensive usb drive with a slot for your sd card to use for this purpose.

Fig.11 YouTube Video OpenPlotter3 – Marine NAVTEX

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References

#1. – “OpenPlotter3 – Marine HF Weather Fax”
https://jeremyclark.ca/wp/telecom/openplotter3-marine-hf-weather-fax/

#2. – “NAVTEX”
https://en.wikipedia.org/wiki/NAVTEX

#3. – “List of NAVTEX Stations”
https://en.wikipedia.org/wiki/List_of_Navtex_stations

#4. – “Map of NAVTEX Stations”
https://community.wmo.int/en/navtex-stations

#5. – “List of KiwiSDR Public Receiver Stations”
http://rx.linkfanel.net/