An airborne gas-filter correlation analyzer was designed and assembled to monitor atmospheric trace gases. Flight tests indicated that a methane version of this nondispersive infrared instrument performed properly while measuring CH(4) concentration profiles as small as 1 ppm from altitudes of 0.15 up to 1.2 km. A unique multiple chopping scheme was developed to simplify the gas-filter technique and to provide an easy mechanical nulling procedure. The single-beam rotating-cell optical system integrated with a multipass sampling cell and the signal processing circuit are described in detail.