iPhone/iPad/iPod app to decode NAVTEX marine transmissions
First observed two days ago, there seems to be a new (to us HF listeners, anyway) network of HF ditter CW transmissions. The purpose of this network, as well as who is operating it, is unknown. It is possible they are for propagation monitoring. Based on observations of listeners and propagation characteristics, it would appear that at least some of the transmissions are coming from North America, possibly the Central US.
The transmissions do not occur at the same time on all frequencies. It appears that each transmitter steps through the frequencies. The following image shows the received signal on three of the frequencies (click on the image to view it as a larger size):
As you can see, thea transmission on each frequency begins right after the transmission on the previous frequency ends. This data was obtained by running a netSDR receiver in 500 kHz wide I/Q capture mode. The resulting recording file was then demodulated at each frequency of interest.
You can also see the second (weaker) dit on the 11150 tranmsission, that occurs shortly before the stronger main dit. (It is less obvious before the second dit, but you can see it, if you squint)
Each pulse (dit) is 130 milliseconds long, and they repeat every 6 seconds.
How to find these transmissions:
I find that using an SDR is the easiest way, as you can observe a large portion of the spectrum at once. I use a 500 kHz wide view, and step through HF, looking for the periodic dits. But you can certainly use any radio. Note that the frequencies are all multiples of 25 kHz. They also sometimes occur in groups of three relatively associated frequencies. There are likely additional frequencies that have not yet been discovered.
If you’re hearing any of these transmissions, or have discovered possible additional frequencies, please let us know with a comment!
Transmissions on the following frequencies have been observed (all in kHz):