Title: Advances in the CO2 Sounder Lidar for Measurements from Aircraft and in Scaling for Space
Presenting Author: Xiaoli Sun
Organization: NASA GSFC
Co-Author(s): James B. Abshire, Anand Ramanathan, Haris Riris, Graham R. Allan, William E. Hasselbrack, Jianping Mao, Mark Stephen

Abstract:
We have demonstrated an improved pulsed, multiple-wavelength IPDA lidar for measuring the atmospheric CO2 concentrations. The CO2 Sounder lidar measures the shape of the 1572.33 nm CO2 absorption line and the range to scattering surfaces including the ground and the tops of clouds. Airborne measurements have used both 30 and 15 wavelength samples. In recent ASCENDS airborne campaigns the lidar used a new high precision step-locked laser seed source, and sensitive HgCdTe APD detector. Analyses of 2014 airborne measurements show the retrievals of lidar range and XCO2 worked well and in several flights the agreement of the lidar retrievals with the in-situ measured XCO2 was better than 1ppm. Additional flights were made in 2016 with the lidar using a larger laser spot to reduce speckle noise and with a receiver with improved optical transmission. These changes improved the lidar sensitivity x3, and the retrievals showed 0.7 ppm precision over the desert in 1-second averaging time, and agreed with the in-situ measured column XCO2 to within 0.5 ppm. We have also updated our measurement model and retrieval approach for the airborne lidar. The CO2 absorption line shape is fit to the received energy at each wavelength to determine XCO2. The receiver model includes effects of solar background, photon detection shot noise, detector dark current, preamplifier noise and laser speckle. The retrieval employs a least-square fit to the CO2 line that is linear in optical depth. There are also terms to solve for and mitigate potential sources of measurement bias, including Doppler shift and dependence of lidar transmission versus wavelength. The random errors from the 2016 airborne measurements agree well (within a factor of 1.4) with the measurement model. Our team has also demonstrated other capabilities needed for a space lidar. One is a fiber laser power amplifier that will increase the average laser power to 20W. Ongoing work is improving the engineering of the key laser components, and is scheduled to take a laser and a detector module through space environmental testing to achieve TRL 6 in 2017. When the lidar performance model is evaluated for a space design using 20W laser power, a 1.5 m diameter telescope, and is at 400 km orbit altitude, the results show random errors of 0.4 ppm over desert surfaces in 1 second averaging time. This is 3 times better than the present ASCENDS mission requirement.