A microwave radiometer (26) includes an antenna-lens assembly (30) for receiving, and a waveguide designed to provide a common path (48) for propagating, atmospheric signals. Circuitry (35) is responsive to the atmospheric signals for generating signals representing the water vapor and liquid content in and the temperature of the atmosphere. For calibration a blackbody assembly (41) is mounted in the near field of the antenna lens assembly. The circuitry responds to the blackbody signals to represent them as first reference signals. A noise diode assembly (49) adds a microwave signal to the waveguide when a blackbody signal is processed. The circuitry responses to blackbody assembly are used to provide data that is used to obtain signals in response to the atmospheric signals.
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