Differential Radiometers Using Fabry–Perot Interferometric Technique for Remote Sensing of Greenhouse Gases

A new type of remote-sensing radiometer based upon the Fabry-Perot (FP) interferometric technique has been developed at NASA´s Goddard Space Flight Center and tested from both ground and aircraft platforms. The sensor uses direct or reflected sunlight and has channels for measuring the column concentration of carbon dioxide at 1570 nm, oxygen lines sensitive to pressure and temperature at 762 and 768 nm, and water vapor (940 nm). A solid FP etalon is used as a tunable narrow bandpass filter to restrict the measurement to the gas of interest´s absorption bands. By adjusting the temperature of the etalon, which changes the index of refraction of its material, the transmission fringes can be brought into nearly exact correspondence with the absorption lines of the particular species. With this alignment between absorption lines and fringes, changes in the amount of a species in the atmosphere strongly affect the amount of light transmitted by the etalon and can be related to gas concentration. The technique is applicable to different chemical species. We have performed simulations and instrument design studies for CH₄, ¹³CO₂ isotope, and CO detection.