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Sometimes in FT-8 mode I receive calls like this one:

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It happens 2-3 times a week.

The problems are: 1) I never could look up such call signs 2) I tried to reply, but never got answer 3) I don't know what /R means.

Are these real calls or maybe just a noise that WSJT-X somehow managed to decode? So far I didn't log such events as QSO's. Maybe I should?

UPD: As Marcus pointed out, the checksum collision in FT-8 is very unlikely. Also I managed to figure out that "/R" means a repeater https://en.wikipedia.org/wiki/Amateur_radio_call_signs I believe it means that the signal was sent automatically by the device, not that the operators works through the repeater. Still I wonder - should I add repeaters to the logbook?

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    $\begingroup$ "noise that WSJT-X somehow managed to decode" <-- that's extremely unlikely; FT-8 does a 12 bit CRC on 75 bit data; the likelihood of a sequence of bit flips breaking both the data and the checksum in a way that looks "valid" again is extemely slim – especially since FT-8 encapsulates these 87 bits in a rate $\frac12$ low-density parity check code. I haven't done the math, but my guess would be that it's more likely that you'll be struck by lightning on your way to pick up your lottery jackpot while randomly happening to be the Queen of England. $\endgroup$ – Marcus Müller Mar 3 at 12:45
  • $\begingroup$ Thank you, Marcus. I didn't know how long the checksum in FT-8 is. In this case I must agree that the collision is very unlikely. Still I never heard of /R suffix. What could it mean? $\endgroup$ – Aleksander Alekseev - R2AUK Mar 3 at 12:48
  • $\begingroup$ OK, I manage to figure out that "/R" means a repeater en.wikipedia.org/wiki/Amateur_radio_call_signs I believe it means that the signal was sent automatically by the device, not that the operators works through the repeater. Still I wonder whether I should add repeaters to the logbook. $\endgroup$ – Aleksander Alekseev - R2AUK Mar 3 at 12:59
  • $\begingroup$ @MarcusMüller The caveat, of course, is that the checksum is subject to the same errors as the message body. $\endgroup$ – Glenn W9IQ Mar 3 at 14:51
  • $\begingroup$ @MarcusMüller I've definitely seen false decodes, stuff like superficially-Chinese callsigns that don't exist in any database, with grids in the middle of the ocean. The strength of the FT8 code has to be weighed against the sheer number of decodes the software attempts, and its determination to sift out the weakest possible signals. $\endgroup$ – hobbs - KC2G Mar 6 at 3:44
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FT8 decoding can use a technique called a priori (AP) whereby it uses naturally accumulating information for the purposes of increasing apparent sensitivity by about 4 dB. There is an increased chance of false decodes when AP is enabled since AP is essentially sophisticated guess work. The technique looks at its guessed result and compares it to the parity information and displays the guess if there is a specified level of confidence of it being correct. There is no assurance that it is correct. The term often used for these false AP decodes is exotica - referring to a rare, decoded call sign that doesn't exist.

While it is entirely possible that the sender is a pirate station, the fact that the V0 prefix is not allocated by the ITU is a good indicator that you are experiencing exotica. The /R is likely part of the exotica decode - not an indication of a repeater. Another good indication of an exotica decode is a decode of an unlikely grid square.

Because the decoded call sign does not exist, when you respond, the other station does not recognize the exotica call as their call sign so no QSO takes place.

If you wish to avoid exotica, at the expense of some loss of apparent sensitivity, simply disable the AP feature under the decode menu.

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