In the first ASCENDS flight test campaign, which was conducted using the NASA DC-8 during 6-18 July 2010, the MFLL system obtained surface reflectances and CO2 column measurements from altitudes of 2.5 to 13 km over the Central Valley of California; the desert of southeastern California/Nevada; the Pacific Ocean off of the Baja Peninsula; Railroad Valley, Nevada; and the DOE ARM CF in Lamont, Oklahoma. In these flight tests MFLL used IM frequencies near 50 kHz, and from an average altitude of 7 km over land, MFLL demonstrated CO2 column measurements with a signal to noise ratio (SNR) >500 (equivalent to CO2 precision <0.7 ppmv) for a 1-s average and a SNR >1200 (CO2 <0.3 ppmv) for 10-s average. Over all altitudes the comparison of remote MFLL and in situ CO2 measurements reflected a CO2 difference <0.9 ppmv with a standard deviation <3.2 ppmv. Note that the same empirically-determined altitude-dependent MFLL calibration was applied objectively to all science flight data. The major change to the MFLL system in 2011 was the implementation of several different IM modes, which could be quickly changed in flight, to directly compare the precision and accuracy of MFLL CO2 measurements in each mode. The different IM modes that were evaluated included "fixed" IM frequencies near 50, 200, and 500 kHz; frequencies changed in short time steps (Stepped); continuously swept frequencies (Swept); and a pseudo noise (PN) code. The Stepped, Swept, and PN modes were generated to evaluate the ability of these IM modes to desensitize MFLL CO2 column measurements to intervening optically thin aerosols/clouds.
MFLL was flown on the NASA Langley UC-12 aircraft in May 2011 to evaluate the newly implemented IM modes and their impact on CO2 measurement precision and accuracy, and to determine which IM mode provided the greatest thin cloud rejection (TCR) for the CO2 column measurements. Within the current hardware limitations of the MFLL system, the "fixed" 50-kHz results produced similar SNR values to those found previously. The SNR decreased as expected with increasing IM frequency with the SNR (500 kHz) equal to 31% of SNR (50 kHz). The absolute accuracy of the 50-kHz CO2 measurement showed a previously observed altitude-dependent trend that was greatly reduced at 200 kHz. Laboratory experiments have duplicated this effect which results mainly from IM frequency cross talk between LAS wavelengths in the erbium-doped fiber amplifier (EDFA) and which is reduced when operating at higher IM frequencies. Performance of the Stepped, Swept, and PN modes were evaluated in close time proximity to each other, and these results will be discussed in this paper.
MFLL was modified to add the O2 measurement capability in June 2011, and a series of flight tests were conducted on the NASA DC-8 from 25 July to 12 August 2011 over similar local land and ocean targets as in 2010 and with additional long-range flights over the corn fields of Iowa, forests in northern Wisconsin, and snow and ice fields of southwestern Canada. Comparisons of MFLL CO2 and O2 column measurements with in situ measurements from this airborne campaign will also be presented.