Laser Sounder for the ASCENDS Mission - Space Instrument Study
James
B
Abshire, NASA-Goddard, james.b.abshire@nasa.gov
(Presenting)
Xiaoli
Sun, NASA-Goddard, xiaoli.sun@gsfc.nasa.gov
Mark
Stephen, NASA-Goddard, mark.stephen@gsfc.nasa.gov
Jeffrey
Chen, NASA-Goddard, jeffrey.chen@gsfc.nasa.gov
Emily
Wilson, NASA-Goddard, emily.wilson@gsfc.nasa.gov
Luis
Ramos-Izquierdo, NASA-Goddard, luis.a.ramos-izquierdo@nasa.gov
Haris
Riris, NASA-Goddard, haris.riris@gsfc.nasa.gov
Our group is developing the laser sounder approach to measure tropospheric CO2 concentrations from space for the ASCENDS Mission. This work is ongoing and has been supported by the NASA ESTO ACT and IIP programs.
The laser sounder approach uses the 1570-nm CO2 band and a pulsed dual channel laser absorption spectrometer. This uses differential lidar absorption measurement in an altimeter mode, and continuously measures at nadir from a near-polar circular orbit. It uses several tunable fiber laser transmitters allowing simultaneous measurement of the absorption from a CO2 absorption line in the 1570 nm band, O2 extinction in the oxygen A-band, as well as surface height and aerosol backscatter in the same measurement path. It directs the narrow co-aligned laser beams toward nadir, and measures the energy of the pulsed laser echoes reflected from land and water surfaces. During the measurement, the lasers are tuned across a selected CO2 line and a region between two O2 lines near 765 nm. The lasers have spectral widths much narrower than the gas absorption lines and are wavelength tuned at kHz rates. The receiver uses a telescope and photon counting detectors, and measures the background light and energies of the laser echoes from the surface, along with scattering from any clouds and aerosols in the path. The gas extinction and column densities for the CO2 and O2 gases are estimated from the ratios of the on and off line signals. We use pulsed laser signals and time gating to isolate the laser echo signals from the surface, and to reject photons scattered from thin clouds and aerosols in the path, which can otherwise bias retrievals.
We have recently completed a second design study for the space instrument. For the study, we selected a nominal sun-synchronous orbit with an altitude of 500 km and equator crossing time of 1:30 pm, and a receiver telescope with 1.5 m diameter. An overview of the study results will be shown.
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