I’d been trying to find something else I could do with my Nooelec RTL-SDR, having virtually exhausted all that could be done with my limited antenna set-up, when I came across NAVTEX.
NAVTEX is a method of disseminating meteorological and MSI (maritime safety information) to vessels using the medium and high frequency wave bands.
The primary transmissions are at 490 and 518 kHz at 100 baud using FSK modulation with a frequency shift of 170 Hz, with a further HF transmission on 4,209.5 kHz using FEC mode.
As is very often the case, there is a wealth of information on the Interweb to describe NAVTEX and so I won’t re-invent the wheel once more by going into those details. Purpose built receivers, with screen and/or paper tape printout, have been designed and are available for NAVTEX reception.
One thing I found surprising was the number of navigational warning light and warning aid failures that were present around our shores, from failed buoys to unlit wind farms!
This is a very straightforward exercise and didn’t require anything special in the way of an antenna, or specialised applications.
Fldigi makes a good job of decoding NAVTEX and works fine with the Nooelec SMART 5 RTL-SDR and SDR# receiver programme.
Again; as I have covered all of these items in previous articles, I will not go over each in detail again here. I’ll just highlight any specific set-up I found to be of use to ensure reception and decoding of the signals.
The antenna used was my fan dipole. The longest wavelength section is cut for 40 m, so a long way from 578 m! As in my MSF/60 kHz foray though, it worked fine for this purpose.
- As you will need to ‘pipe’ the audio from SDR# to Fldigi, a virtual audio cable will be required. Again, see earlier articles where I have outlined how to do this.
- Choose Navtex/SitorB mode in Fldigi and I also made the waterfall size a little larger than default.
- For SDR#:
Make sure you are using ‘Direct sampling (Q branch) as the sampling mode in ‘RTL-SDR Controller’. - In ‘Radio’:
- Switch off the IF Noise Reduction filter, if you use it.
- Set mode to USB with standard bandwidth of 2.5 kHz
- Set the receive frequency to just under 519 kHz – I set 516.5 kHz – to bring the signal to near the centre of the default audio passband on Fldigi – more
detail follows.
A screen shot of SDR#, when set-up and receiving a NAVTEX signal, looks like this:
NOTE:
As a signal strength reference; the signal at 558 kHz is Panjab Radio, an Asian commercial station which has been broadcasting for 24 years, and was about 10 dB stronger on the spectrum scale. It broadcasts from Crystal Palace and runs 2.5 kW.
- For Fldigi:
- After setting the ‘Op mode’, and waterfall height in ‘FFT Processing’, I brought the red signal tracks cursor to 1500 Hz and made sure AFC was ON. You can see that the middle of the cursor doesn’t sit directly over 1500 Hz and I found that the decoding was more reliable if you let the AFC reposition the cursor.
- After setting the ‘Op mode’, and waterfall height in ‘FFT Processing’, I brought the red signal tracks cursor to 1500 Hz and made sure AFC was ON. You can see that the middle of the cursor doesn’t sit directly over 1500 Hz and I found that the decoding was more reliable if you let the AFC reposition the cursor.
And here’s what a typical decoded signal appears like:
The basics are:
- ZCZC is the start of a message
- EA26 translates to:
o E: Niton
o A: Navigational warning
o 26: Message number - NNNN is the end of a message
The most difficult part of the process was waiting for a suitable signal to appear! Stations use 4 hour slot times, but may only use a couple of those times during a 24 hour cycle to broadcast weather information.
Once I’d proven that the system was working OK, I just left it ‘on idle’ to see what would be recorded in the Fldigi window.
The strongest signals, in Malmesbury, Wiltshire, were from Niton and the best times to watch were the 12:40Z and 16:40Z slots.