For well over 12-months, I have had my radio tuned to the WSPR frequency of 28.1246 MHz on the 10m band and I feed the decoded signals up to the WSPRnet website.
While I can see the WSPR signals clearly on the waterfall display, I can also see the very slow morse QRSS signals as well just a few hundred Hz below.
The image above shows the QRSS signals I could see on the 2nd of November 2023 and the locations of the stations are shown in the map at the start of the post.
The screenshot above shows a good capture of AE0V in the USA at about a distance of 6000kms. Ned, AE0V reports using a solar powered transmitter with no battery storage running 100mW into a 1/4 lamba stainless whip about 8m above the ground.
The signals from the USA and Canada are easily explained as they are via F2 layer propagation. The signals I find unusual are the ones from the England which are in the region of 500 to 650kms.
The trace from the 0.2-watt signals of G0PKT and G0MBA are there nearly all the time when the band is open. It's not F2 propagation in the usual sense as it's too close and it's not Sporadic-E.
I believe that it's backscatter just like what the military use for their over the horizon radar systems (OTHR).
In this case, the 28 MHz band is open with F2 layer propagation and the signals from G0PKT & G0MBA are being reflected back towards my location from some distant point.
As an example of how consistent these signals are, I have decoded the WSPR signal of G0PKT about 1,000 times in the last 3-weeks. And that's a signal that's supposed to be in my 'skip zone' where it's supposed to be hard to reach.
There's nothing new about this, it's just that in this modern age of weak signal modes and waterfall displays, we can now see these very weak signals more clearly.
If you're using FT8 on the higher HF bands and you see lots of reports from stations that about 200-600kms away then F2 backscatter is probably the reason.