An Intensity-Modulated CW Laser Absorption Spectrometer Approach for Column CO2 Measurements for the ASCENDS Mission

The 2007 NRC Decadal Survey of Earth Science and Applications from Space recommended Active Sensing of CO2 Emissions over Nights, Days, and Seasons (ASCENDS) as a mid-term, Tier II, mission. As part of the development of a capability for the NASA ASCENDS mission, NASA Langley Research Center (LaRC) and ITT EXELIS are collaborating to develop and demonstrate an approach for measuring column CO2 from space. A Multi-Functional Fiber Laser Lidar (MFLL) system has been used to demonstrate high precision column CO2 measurements from aircraft platforms. The MFLL operates using an intensity modulated CW laser absorption spectrometer (LAS) to make simultaneous CO2 and O2 column measurements in the 1.57-micron and 1.26-micron regions, respectively, to retrieve dry column-averaged CO2 mixing ratios.

MFLL has been operated onboard the NASA UC-12 and DC-8 aircraft over the past several years to mature the measurement technique and demonstrate high-precision column CO2 measurements over several land and ocean surfaces under a variety of atmospheric conditions. The MFLL is configured to transmit continuously on- and off-line wavelengths using common Erbium Doped Fiber Amplifiers (EDFAs) and transmit optics. On- and off-line laser signals are modulated independently and lock-in detection is used to retrieve lidar signals. The transmitted on- and off-line laser energies are monitored with high precision in the reference output channels. The Integrated Path Differential Absorption (IPDA) approach is used to determine column CO2 and O2 number density amounts. In situ CO2, pressure, and temperature measurements are used in the retrieval algorithms and to evaluate the accuracy of the remote MFLL measurements. The MFLL measurements have demonstrated insensitivity to variations in surface reflectivity and compare well with column optical depths derived from in situ CO2 measurements. Encoded modulation schemes have been used to minimize the influence of thin clouds on column CO2 measurements and to determine the range to the surface. We present examples of these measurements from the past flight campaigns and plans for the upcoming field experiment in February-March 2013.

Ground-based range testing with the MFLL was conducted at LaRC during July-August 2012. We present a summary of results from the MFLL ground tests using various targets at a distance of 860 m. During these tests, measurements were made using well characterized targets, controlled background conditions, and a variety of intermediate ranges, partially reflecting and light-attenuating targets that are used as simulants for thin clouds. A variety of modulation schemes were tested to discriminate against cloud interferences and for evaluating precise ranging capability. Profiles of in situ CO2, relative humidity (RH), pressure, and temperature along the range were made to validate the CO2 column measurements. Models have been developed to predict the precision and accuracy of measurements with the MFLL. The ground-based measurements are valuable to validate these models. Results from the ground-based tests and modeling will be used to interpret measurements from aircraft by the MFLL, and for projecting the capability to space. Results from this field testing and modeling will be discussed in this presentation.