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A novel and simple frequency-domain wireless interrogation method for surface acoustic wave (SAW) resonator sensors operating up to 850??C is presented. The method is based on directly measuring the scattering parameter S11 with a frequency-swept interrogating signal and extracting the group delay. Multiple resonators can be simultaneously addressed by frequency multiplexing the resonator responses. A new high-temperature SAW resonator capable of operating up to 1000??C has been used as a temperature sensor. The design uses the newly developed high-temperature Pt/Rh/ZrO2 thin-film electrodes and open-circuit gratings on langasite (LGS, La3Ga5SiO14). Reported is the first successful operation of a wireless SAW resonator at temperatures up to 850??C. The SAW resonator was exposed multiple times to 850??C, providing consistent performance.