An airborne microwave refractometer system was tested to determine its noise spectrum and the distribution of this noise within the system. Samples of atmospheric turbulence spectra are shown from about 180 to 1800 m above ground [about 1800 to 3300 m above msl (mean sea level)]. Total noise in a 0.03–15 cps passband is found to be about 0.04 ppm rms and to be approximately ``white.''