Title: GODDARD PROGRAM FOR MEASUREMENT OF CARBON DIOXIDE USING A BROADBAND LIDAR  
Primary Author: Heaps, William
Organization: NASA GSFC
Co-Author(s): William S. Heaps, NASA Goddard Space Flight Center; Elena Georgieva, University of Maryland Baltimore County; Wen Huang, Science Systems and Applications, Inc

Abstract:
In order to better understand the budget of carbon dioxide in the Earthís atmosphere it is necessary to develop a global high precision understanding of the carbon dioxide column. In order to uncover the ëmissing sinkî that is responsible for the large discrepancies in the budget as we presently understand it calculation has indicated that measurement accuracy on the order of 1 ppm is necessary. Because typical column average CO2 has now reached 380 ppm this represents a precision on the order of .25% for these column measurements. No species has ever been measured from space at such a precision. In recognition of the importance of understanding the CO2 budget in order to evaluate its impact on global warming the National Research Council in its decadal survey report to NASA recommended planning for a laser based total CO2 mapping mission in the near future.

The extreme measurement accuracy requirements on this mission places very strong requirements on the laser system used for the measurement. This work presents an overview of the characteristics necessary in a laser system used to make this measurement. Consideration is given to the temperature dependence, pressure broadening, and pressure shift of the CO2 lines themselves and how these impact the laser system characteristics

We have been examining the possibility of making precise measurements of atmospheric carbon dioxide using a broad band source of radiation. This means that many of the difficulties in wavelength control can be treated in the detector portion of the system rather than the laser source. It also greatly reduces the number of individual lasers required to make a measurement. Simplifications such as these are extremely desirable for systems designed to operate from space.

Our current system employs an OPA pumped by a small ND:YAG laser (50 mJ at 15 Hz). We have achieved a signal to transmit that is 14 mJ in a 4 nanosecond wide pulse with a spectral width about 3 nm wide centered at ~1571 nm. CO2 has several strong spectral lines in this region. Our receiver is sensitive in this region. It consists of an 8 inch telescope coupled to our two channel detector system. One channel contains only a broad band filter while the second channel employs a Fabry-Perot to strongly select for regions of strong CO2 absorption. Both channels use avalanche photodiode detectors. We have not tested the complete system yet but expect to have early test results by the time of the conference.