Computer Science – Sound
Scientific paper
Dec 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufmin21a1197a&link_type=abstract
American Geophysical Union, Fall Meeting 2006, abstract #IN21A-1197
Computer Science
Sound
0315 Biosphere/Atmosphere Interactions (0426, 1610), 0394 Instruments And Techniques, 0426 Biosphere/Atmosphere Interactions (0315), 0452 Instruments And Techniques, 1694 Instruments And Techniques
Scientific paper
We report progress in developing a laser technique for the remote measurement of the tropospheric CO2 concentrations from orbit. Our initial goal is to demonstrate a lidar technique and instrument technology that will permit measurements of the CO2 column abundance in the lower troposphere from aircraft. Our final goal is to develop a practical space instrument and mission approach for active CO2 measurements at the 1 ppmv level. This would allow continuous measurements of CO2 mixing ratio, both day and night, over land and ocean surfaces, under realistic atmospheric scattering conditions. Measuring the CO2 mixing ratio in the troposphere from space is quite challenging. High signal-to-noise ratios and measurement stabilities are needed for accurate mixing ratio estimates. Our laser sounder approach has some fundamental advantages over passive sensors which use sunlight. It always uses a common nadir/zenith measurement path and the narrow laser divergence angles produce small laser footprints. The laser source allows it to measure in sunlight and darkness over different surfaces giving full global coverage. It can measure continuously over the ocean, to cloud tops and through broken clouds. The lasers are pulsed and potential measurement errors from aerosol scattering can be greatly reduced by using time gating in the receiver. Our approach uses a dual channel laser altimeter/spectrometer, which continuously measures at nadir from a near polar circular orbit. It uses several tunable fiber lasers for simultaneous measurement of the absorption from CO2 and O2, and aerosol backscatter in the same path. It directs the narrow co-aligned laser beams from the instrument's lasers toward nadir, and measures the energy of the laser echoes reflected from land and water surfaces During the measurement its lasers are tuned on- and off- a selected CO2 line near 1572 nm and a selected O2 line near 768 nm in the Oxygen A band at kHz rates. The receiver uses a 1-m diameter telescope and photon counting detectors and measures the energies of the laser echoes from the surface along with scattering from any clouds and aerosols in the path. The extinction and densities for the CO2 and O2 gases are estimated from the ratio of the on and off line signals via the differential optical absorption technique. The lasers are pulsed, which allows the receiver to gate out signals which were scattered by aerosols in the path. The lasers are rapidly tunable and have spectral widths much narrower than the gas absorption lines. Our technique tunes the on-line wavelengths tuned to the sides of the gas absorption line. This exploits the atmospheric pressure broadening of the gas lines to weight the measurement sensitivity to the atmospheric column below 5 km, where CO2 variations caused by surface sources and sinks are largest. Simultaneous measurements of O2 column are made using an identical approach with an O2 line. We have calculated several characteristics of the technique, and have demonstrated key elements of the laser, detector and receiver approaches in the laboratory. We have measured O2 in an absorption cell and CO2 over a 206 m horizontal path using breadboards. We will describe these and provide more details on our approach in the paper.
Abshire James B.
Collatz James G.
Jian Pu
Kawa Randolph S.
Krainak Michael A.
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