Signal-to-Noise Ratios of the ASCENDS CarbonHawk Experiment Simulator (ACES) for Atmospheric CO2 Measurements

Abstract:

The ASCENDS CarbonHawk Experiment Simulator (ACES) system has been developed at NASA Langley Research Center to advance technologies in support of the NASA Active Sensing of CO₂ Emissions over Nights, Days, and Seasons (ASCENDS) mission and to demonstrate them initially from a high-altitude airborne platform. With a multiple fiber-amplifier-based Swept-Frequency Intensity-Modulated Continuous-Wave (SF-IM-CW) high-power laser transmitter and a multiple-aperture receiver, the ACES system provides simultaneous measurements of the differential optical depth at the 1571-nm CO₂ absorption line and the associated range between the transmitter and target on an airborne platform. The precise measurement of the CO₂ differential optical depth and the range, determined by signal amplitudes and phases in the ACES returns, together with the temperature, pressure, and water vapor information at the same location, make it possible to retrieve the column-averaged CO₂ dry air mixing ratio (XCO₂). The Signal-to-Noise Ratios (SNRs) of both return-signal amplitudes and phases of three simultaneously-received SF-IM-CW signals in the ACES system directly affect the measurement precision of the differential optical depths and the ranges between the ACES system and the Earth’s surface or the tops of intermediate cloud layers. In this paper, we present results from numerical simulations and experimental measurements of ACES SNRs based on the laboratory-determined system parameters and flight experiments over ocean and land areas. These results will be used in the design of an IM-CW CO₂ Integrated Path Differential Absorption (IPDA) lidar system for ASCENDS mission.