This article documents measurement of the calibration of an IC-7300 S-meter in SSB mode using a continuous sine wave at 1kHz tone frequency.
There has been a long standing convention that S-meters are calibrated for 50μV in 50Ω to be S9, and S-points laid out at 6dB per S-point. IARU Region 1 formalised this with Technical Recommendation R.1 which defines S9 for the HF bands to be a receiver input power of -73 dBm (equivalent to 50μV in 50Ω).
A test was conducted where a Standard Signal Generator was connected to the receiver and slowly increased from -125dBm in steps of 1dB and the point at which the S-meter display segments lit was noted.
Above is a chart of the error between the S meter indication and the value per IARU Region 1 Technical Recommendation R.1.
It can be seen that the error is smallish near S9, but is gross (>25dB) at the high and low end of the S-meter scale.
Though this receiver might use SDR techniques for demodulation and spectrum display, Icom appear to have taken no effort to fix the S-meter calibration inaccuracy of their earlier receivers.