Low-cost gas correlation detection of methane

Two instruments have been designed for airborne remote sensing of landfill methane emissions using the infrared absorption of reflected sunlight. A gas correlation filter wheel and a length modulated cell have been produced to discriminate between methane and other interfering species and the performance of the two systems discussed. The two systems have been interfaced with an Indium Gallium Arsenide (InGaAs) 2D detector array and an Indium Antimonide (InSb) point detector. The InGaAs array detector response rate was found to be too slow so experiments were done using the InSb detector. The gas correlation filter wheel has been shown to detect levels of methane equivalent to 200ppmv with a 30m pathlength at the 3.3 mu m methane band with the InSb point detector. It has been predicted that it should be possible to detect levels equivalent to 20ppmv over a 30m pathlength at the 1.65 mu m band with the gas correlation filter wheel and a fast response InGaAs detector. The length modulated device was found to have far less sensitivity in comparison to the filter wheel system, but could have enhanced performance with improved design.