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Unified carrier TT&C system integrates tracking, ranging, speed measurement, telemetry, remote control, communication, data transmission and other functions in one. The noise power spectral density is one of the downlink channel basic parameters of the telemetry and tracking and command (TT&C) system equipment, which reflects the receiving ability of the channel for weak signal. Regulating the input noise power spectral density of baseband to a reasonable range is the prerequisite for completing aerospace TT&C task. The basic composition and design requirements of downlink channel of unified carrier TT&C system were introduced, and the transmission characteristic of noise power spectral density transferring in downlink channel was analyzed, then the fundamental reference and principle for demarcating noise power spectral density in different voltage levels and different command systems were proposed. Combining with a certain type of S-band unified carrier TT&C system practical engineering, the principle and specific calibration method of noise power spectral density were analyzed. In the method, according to different application environments and TT&C systems, noise power spectral density was calibrated reasonably, based on fully considering the allocation criteria of gain and accurate calculating the level of signal noise at all levels, in order to ensure the reliability and stability of the system. The method could provide the reference and basis for testing the indicators of the device and setting the aerospace TT&C mission parameters.