Measurements of atmospheric brightness temperature fluctuations and their implications on passive remote sensing

Passive remote sensing of airborne chemicals at infrared wavelengths may be limited by temporal fluctuations in atmospheric brightness temperatures deltaT(t). Brightness temperatures in two infrared spectral bands were simultaneously measured on clear and cloudy days along three lines of sights. For time windows t < 3-5 s, deltaT(t) remained constant at the sensor noise level and rapidly increased as t increased. The fluctuation time scale for the cloudy day was longer than for the clear day. The long correlation time for T(t) limits the utility of signal averaging in improving detection signal-to-noise ratio (SNR). The simultaneous outputs of the two spectral channels during the clear day exhibited no spectral coherence at t < 3 s and limited coherence at t > 30 s. Measurements during the cloudy day were largely coherent. Consequently, band-by-band subtraction may have limited benefits.