SNR readings, query

[davefes]

I notice that when the RSSI gets down to about -120dB that the SNR readings jumps from a low value (say 1-2dB) to a high value (ie 56dB).

I see in def _handle_interrupt() that RSSI is modified depending on SNR, but can't see anything that modifies the SNR value. According to section 4.1.1.4 in the RFM96W datasheet it looks like SNR should be reported as negative dBs below some value (ie -120dBm, maybe depends of modem_config).

How do I interpret the very high SNR readings at low signal levels?