Title: The ASCENDS CarbonHawk Experiment Simulator (ACES)
Presenting Author: Michael Obland
Organization: NASA LaRC

Abstract:
The ASCENDS CarbonHawk Experiment Simulator (ACES) is a NASA Langley Research Center project funded by NASA's Earth Science Technology Office (ESTO) Instrument Incubator Program (IIP) that seeks to advance technologies critical to measuring atmospheric column carbon dioxide (CO2) mixing ratios in support of the NASA Active Sensing of CO2 Emissions over Nights, Days, and Seasons (ASCENDS) mission. The technology advancements targeted include: (1) increasing the power-aperture product to approach ASCENDS mission requirements by implementing multi-aperture telescopes and laser transmitters, (2) incorporating high-efficiency, high-power Erbium-Doped Fiber Amplifiers (EDFAs), (3) development of a high-bandwidth, low-noise HgCdTe detector and transimpedence amplifier (TIA) capable of long-duration autonomous operation on the Global Hawk aircraft, and (4) advanced algorithms for cloud and aerosol discrimination. The ACES instrument architecture is being developed for operation on a high-altitude aircraft and will be directly scalable to meet the ASCENDS mission requirements. The above technologies are viewed as critical towards developing an airborne simulator and spaceborne instrument with lower platform consumption of size, mass, and power, and with improved performance. ACES transmits five laser beams: three from commercial EDFAs operating near 1.57 microns, and two from the Exelis oxygen (O2) Raman fiber laser amplifier system operating near 1.26 microns. The three EDFAs are capable of transmitting up to 10 watts average optical output power each and are seeded by compact, low noise, stable, narrow-linewidth laser sources stabilized with respect to a CO2 absorption line using a multi-pass gas absorption cell. The Integrated-Path Differential Absorption (IPDA) lidar approach is used at both wavelengths to independently measure the CO2 and O2 column number densities and retrieve the CO2 mixing ratio. The ACES receiver uses three fiber-coupled 17.8 cm diameter athermal telescopes. The transmitter assembly consists of five fiber-coupled laser collimators and an associated Risley prism pair for each laser to align the outgoing lasers to the telescope field of view. The backscattered return signals collected by the three telescopes are combined in a fiber bundle and sent to a single low noise detector. The detector/TIA effort has improved the existing detector subsystem by increasing its bandwidth to 4.7 MHz from 500 kHz and increasing the duration of autonomous, service-free operation periods from 4 hours to >24 hours. The new detector subsystem enables higher laser modulation rates to be explored, which provides greater flexibility for implementing thin-cloud discrimination algorithms as well as improving range resolution and error reduction. The cloud/aerosol discrimination algorithm development by Langley and Exelis features a new suite of algorithms for the avoidance of bias errors in the return signal induced by the presence of intervening thin clouds. Multiple laser modulation schemes are being tested in an effort to significantly mitigate the effects of thin clouds on the retrieved CO2 column amounts. Full instrument development concluded in the spring of 2014. ACES completed six test flights on the Langley Hu-25 aircraft in July, 2014, and recorded data at multiple altitudes over land and ocean surfaces with and without intervening clouds. Preliminary data from these test flights will be presented.