Physics
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufm.a41c0118s&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #A41C-0118
Physics
[0394] Atmospheric Composition And Structure / Instruments And Techniques, [1640] Global Change / Remote Sensing, [3360] Atmospheric Processes / Remote Sensing
Scientific paper
We are developing a differential absorption lidar (DIAL) for measuring the CO2 column concentrations from space for the ASCENDS mission. Our technique uses two pulsed laser transmitters to simultaneously measure the total column absorption by CO2 in 1570 nm band and O2 extinction in the Oxygen A-band by periodically stepping the laser wavelength at predetermined wavelengths across the absorption lines. The reflected laser signals from the surface and clouds are collected by the receiver telescope and detected by a set of single photon counting detectors. We used pulsed lasers and time resolved photon detection to distinguish the surface echoes from cloud and aerosol backscattering and to measure the column height. . The total column absorption at a given wavelength is determined from the ratio of the received laser pulse energy to the transmitted energy. The column gas concentrations and the spectral line shape are determined from curve fitting of the column absorptions as a function of the wavelength. We have built an airborne lidar to demonstrate the CO2 column measurement technique from the NASA Lear-25 aircraft. The airborne lidar scans the laser wavelength across the CO2 absorption line in 20 steps. The line scan rate is 450 Hz, the laser pulse energy is 25 uJ, and laser pulse widths are 1 usec. The backscatter photons are collected by a 20 cm telescope and detected by a near infrared photomultiplier tube. The detected photons are binned according to their arrival times with the use of a multichannel scaler. Several airborne measurements were conducted during October and December 2008, and August 2009 with many hours of CO2 column measurement data at the 1571.4, 1572.02 and 1572.33 nm CO2 absorption lines. The measurements were made over a variety of land and water surfaces and some through thin clouds. We also made several improvements to the instrument for the later flights. Measurements from early flights showed the receiver signal and noise levels were consistent with predictions And Cloud echoes could reliably be identified and separated from the surface echoes. Here we report a detailed assessment of the receiver signal to noise ratio from the airborne measurements , including those from recent flights. We will show samples of the received signal and noise from different altitudes and over types of ground surfaces. We will also determine the surface height variability and range measurement precisions from the time offlight of the laser pulses.
Abshire James B.
Allan Graham R.
Hasselbrack W. E.
Riris Haris
Sun Xiaosong
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