Realization of a standard radiometer for microwave brightness-temperature measurements traceable to fundamental noise standards

We describe and demonstrate a standard radiometer for making microwave brightness-temperature measurements that are traceable to fundamental noise standards. The standard radiometer is based on a National Institute of Standards and Technology (NIST) waveguide radiometer for 18-26.5 GHz, fitted with an antenna to measure radiated power. The fraction of the antenna pattern subtended by the radiating target is determined by anechoic-chamber measurements in which we vary the temperature of the target and measure the received power. Sample measurement results with uncertainties are presented. The typical standard uncertainty for a brightness temperature of around 340 K is about 1 K. The approach should be extendable to other waveguide bands where NIST has radiometers and standards.