venerdì 10 marzo 2017
Digital " Ladders " captured and demodulated
Following the post at the links :
This is the video of the digital ladder that I called "ghost" .
It pops out in two parts of the spectrum .
In the second part ( at 10.7 MHz ) I have demodulated it in baseband with a wideband spectrum with components spaced 50 and 200 Hz .
Some second after the end of receiving this ladder you can see and listen the transit of a typical continuous sweeping ionosonde .
The BW of the modulated signal was evaluated graphically to be around 20kHz as in the following images and the modulation time to be around 0.6 s .
The full-resolution images make it possible to read the axis labels, so the vertical marks are approx 10 secs. The chirpsounder in the ghost10k image (above the irregular-step broadband trace) is therefore sweeping 500kHz in
10 secs, which makes it a 50kHz sounder.
Digisondes step at a uniform rate. The DPS4D steps 25kHz every 640msec. So the traces on the ghost10k image is not a Digisonde since the steps are
irregular in both the time and frequency axes. I don't see any traces in
these "ladder" images which could be a Digisonde. There is more information about the Digisondes on the UMLCAR and DIGISONDE websites.
My guess is that the irregular-step signals are path sounders. They emit a burst of MFSK on many channels in sequence to keep track of which frequencies are open for the path they are using. It's not surprising that they follow a sequence up the spectrum.