They are reports from aircraft that are Mode S equipped, which generally provides identity / callsign / squawk / altitude in response to secondary radar interrogations, but doesn’t spontaneously emit the position or speed information that’s contained in the ADS-B extended squitter messages.
They’re the “positionless” flights on your receiver’s local map table.
My angle of interest is the tech / pushing the limit - in fact the whole point of my doing this was to:
a)Play with an RPi
b)Play with Linux
c)access the “mysterious signals in the ether”
d)make something cool with a bit of wire and a £20 board
So far, so good!
Thanks to all at FA and the great community for the support and providing the service etc really supportive and no “anti-noob” elitist vibe which you can get in some places.