I have exchanged a couple of emails with Joe K1JT about the QRPp experiment and the -24 dB limit for the FT8 decoder. The following are Joe's remarks.

*###########################################################################*

*By convention all SNRs reported by the software package WSJT-X are*

*measured in a standard reference noise bandwidth of 2500 Hz.*

*Detailed measurements of FT8 sensitivity in various simulated*

*propagation conditions are summarized in a graph posted on the WSJT web site here:*

*http://physics.princeton.edu/p…T8_Decode_Probability.png*

*This plot was made by generating 1000 simulated FT8 signals at each*

*specified SNR, subjecting them to simulated propagation by nine standard*

*ITU propagation models, and counting the number of resulting decodes for*

*each case.*

*Propagation to the geostationary satellite QO-100 ahould closely*

*approximate the AWGN (additive white Gaussian noise) path, for which the*

*leftmost curve in the plot shows a 50% probability of decoding success*

*at SNR = -20.8 dB and a very small (less than 1%) probability at SNR =*

*-23 dB. The probability of decoding an FT8 signal with true SNR less*

*than -22 dB is extremely small.*

*Why is it, then, that the decoder sometimes reports SNRs as low -24 dB?*

*And why do we never see reported SNR -25 dB or lower?*

*All physical measurements have associated uncertainties. In principle*

*we could estimate and report an uncertainty along with each measured*

*value -- for example, something like SNR = -18 ± 2 dB. If WSJT-X were*

*intended as a measurement tool, we would do it this way; but for its*

*intended purpose such reports would be cumbersome, distracting, and*

*irrelevant.*

*Reported SNRs lower than -22 dB are nearly always underestimates of the*

*true value. To avoid unwarranted focus of attention on very small*

*reported SNRs, we arbitrarily clamp reported FT8 SNRs at -24 dB.*

*###########################################################################*