Expected signal strength of this Bluetooth device for a receiver (phone / other Bluetooth device) placed at 1 meter, used for distance estimation.
Value is in negative dB (e.g. a value of s1=70 is considered to be -70dB)
Calibrated during device prototyping, and adjusted by each device for any changes in it’s transmit power with respect to the transmit power at calibration time (e.g. +2dB transmit power during advertising ⇒ s1=68, if 0dB transmit power at calibration time had yielded an RSSI of -70dB).