A microwave ablation system comprises a microwave applicator including an antenna configured to deliver microwave energy to ablate tissue, and a microwave generator coupled to the microwave applicator via a transmission network and configured to generate a microwave signal and transmit the microwave signal to the antenna. A radiometer is configured to measure noise temperature from a signal propagating through the transmission network. A controller is configured to turn off the microwave signal generated by the microwave generator for a predetermined period and to control the radiometer to measure noise temperature during the predetermined period. In addition at least one thermocouple is configured to measure temperatures, and the controller identifies the noise temperature measured by the radiometer as a noise temperature of tissue based on the measured temperatures.
