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Presented is a noncontact model and radiometric sensor developed to facilitate core body temperature extraction. The system has been designed as a close-proximity sensor to detect thermal emissions radiated from deep inside the human body. The radiometer uses a cavity-backed slot antenna (CBSA) designed to account for performance degradation which occurs in the near field of the human body. Tissue-simulating materials with electrical properties similar to the human body have been identified, and layered configurations of these phantoms are used to mimic the electromagnetic characteristics of a human stomach volume: hence, defines the core model. Positioned approximately 7 mm from the core model, the sensor tracks the change in brightness temperature as the subsurface physical temperature is varied. A mathematical noncontact model (NCM) is subsequently used to correlate the observed brightness temperature to the subsurface temperature, accounting for artifacts induced by the sensor's remote positioning from the specimen. The accuracy of the NCM is validated through an analysis of the measurement data extracted from the test bed before and after applying the model. The results illustrate that the measurement is highly sensitive to the antenna impedance match and atmospheric temperature. The measurement is less sensitive to the physical temperature of the instrument and emissivity of the specimen. The results of this study demonstrate that radiometric sensors are capable of close proximity, subsurface extraction of biological data given that certain parameters are closely monitored.