Fabry-Perot Based Radiometers for Precise Measurement of Greenhouse Gases

Differential radiometers based upon the Fabry-Perot interferometer have been developed and demonstrated that exhibit very great sensitivity to changes in the atmospheric column of carbon dioxide, oxygen, and water vapor. These instruments employ a solid Fabry-Perot etalon that is tuned to the proper wavelength by changing the temperature. By choosing the thickness of the etalon its multiple pass bands can be made to align with regularly space absorption features of the molecule under investigation. Use of multiple absorption features improves the optical throughput of the instrument and improves the stability of the instrument response with respect to environmental changes. Efforts are underway at Goddard to extend this technique to the carbon 13 isotope of carbon dioxide and to methane. These instruments are intrinsically rugged and can be made rather small and inexpensively. They therefore hold promise for widespread use in ground based networks for calibration of satellite instruments such as OCO and GOSAT. Results will be presented for ground based and airborne operations for these systems. The effects of atmospheric scattering, pointing errors, pressure broadening and temperature effects will be discussed with regard to achieving precision better than .5% required for validation of carbon dioxide column measured from space. Designs permitting the extension of the technique to an even larger number of atmospheric species will be discussed along with theoretical analysis of potential system performance.