Title: Recent Progress in developing the CO2 Sounder Lidar as a candidate for the ASCENDS Mission
Presenting Author: James Abshire
Organization: NASA GSFC

Abstract:
The lidar on NASAs planned ASCENDS mission will measure atmospheric CO2 and O2 column absorption and range, allowing calculation of the atmospheric CO2 mixing ratio needed for flux estimates. The ASCENDS mission is important because its lidar approach will permit, for the first time, global atmospheric CO2 concentration measurements, including those at night, at high latitudes, through hazy and thin cloud conditions and to cloud tops. None of these are possible using passive sensors. Since 2010, the CO2 Sounder team has made significant progress in developing of the CO2 Sounder approach and laser technology for the ASCENDS lidar. The team previously demonstrated the airborne lidar on airborne flights in 2010, 2011, 2013. The results show that airborne measurements made in 2011 and 2013 were accurate to ~1 ppm for airborne altitudes between 6 and 12 km. Analysis also highlighted a new capability to measure CO2 column absorption and range both to cloud tops and the ground. Differencing allows a direct estimate of the CO2 concentrations in the boundary layer. The team developed a new HgCdTe APD detector in collaboration with DRS RSTA. Testing shows it is radiation tolerant, has analog response, highly linear dynamic range and ~x10 time more sensitivity than the IR-PMT detector used previously. In 2014 the CO2 Sounder team improved the airborne lidar by incorporating a new step-locked laser seed source and the HgCdTe APD detector. The team participated in the 2014 ASCENDS flights during August and September on the NASA DC-8. Flights were made over the California Central Valley, the redwood forests along the California coast, the desert near Edwards Air Force Base, and over growing agriculture in Iowa. Two flights were made under the track of OCO-2 satellite. The results show improvements from the locked seed laser and the detector. For scaling the laser to space, the project has demonstrated high power single frequency fiber laser preamplifier that emits > 320 uJ/pulse at 1572 nm. The fiber amplifier produced sufficient pulse energy to allow using 8 stages in parallel for a space transmitter. An alternative approach has been developed that uses a single planar waveguide gain element in a 4-pass configuration. A breadboard of this has been assembled at Raytheon and is being tested. A summary of all these results will be presented.