From the “International Electrotechnical Commission” webstore:
“IEC 61788-16:2013 [describes] the standard measurement method of power-dependent surface resistance of superconductors at microwave frequencies by the sapphire resonator method. The measuring item is the power dependence of Rs at the resonant frequency. This method is applicable for a frequency in the range of 10 GHz, for an input microwave power lower than 37 dBm (5 W).”