Studies of a Space Lidar Instrument for the ASCENDS Mission

We have developed a lidar technique for measuring the tropospheric C02 concentrations as a candidate for NASA's planned ASCENDS mission. Our technique uses two pulsed laser transmitters allowing simultaneous measurement of a C02 absorption line in the 1570 nm band, 02 extinction in the Oxygen A-band and surface height and backscatter. The lidar measures the energy and time of flight of the laser echoes reflected from the atmosphere and surface. The lasers are stepped in wavelength across the C02 line and an 02 line region during the measurement. 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 aerosols in the path. The gas extinction and column densities for the C02 and 02 gases are estimated from the ratio of the on- and off- line signals via the DIAL technique. Pulsed laser signals and a time gated receiver are used to isolate the laser echo signals from the surface, to reject laser photons scattered in the atmosphere, and measure the surface height and scattering profile in the path. 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 400 km and equator crossing time of 1:30 pm, and a receiver telescope with 1.5 m diameter.