Use of Fourier Transforms and Auto-correlation to Identify Column Integrated CO2 Mixing Ratio from a Continuous Wave Lidar System for the ASCENDS Mission

Accurate understanding of carbon balance in the environment is critical to projections of the future evolution of the Earth’s climate. Uncertainties in the modeling of carbon sources and sinks remain large due to the limited set of observations from the current network of in-situ and surface measurements. Global, space borne measurements of atmospheric CO2 can reduce these uncertainties. As a result, the NRC Decadal Survey (DS) of Earth Science and Applications from Space identified Active Sensing of CO2 Emissions over Nights, Days, and Seasons (ASCENDS) as an important mid-term (Tier II) mission. The active space remote measurement of the column CO2 mixing ratio (XCO2) that is called for by the NRC in the ASCENDS mission requires the simultaneous measurement of the CO2 number density column and the O2 number density column to derive the average XCO2 column. The NRC recommendation calls for XCO2 to be measured to a precision of less than 2 ppm and must be made without bias from aerosols, dust, or clouds. ITT Exelis and NASA Langley have developed a continuous wave (CW) fiber laser concept that would be ideal for the ASCENDS mission. Its advantage over a pulsed laser system (e.g. CALIPSO) is an expected longer lifetime as it utilizes more reliable technology, such as that used in the telecommunications industry. For a CW system, light scattered from clouds represents noise in the CO2 retrieval as it has passed through a lower path length than that scattered from the ground. This can lead to an underestimate of the column density. This presentation shows how the use of auto-correlation and Fourier transforms are used to remove the cloud contamination. Some initial results from the Summer 2011 campaign are also presented.