This paper discusses the latest flight test results of an intensity-modulated (IM) continuous-wave (CW) laser absorption spectrometer (LAS) that simultaneously operates near 1.57 μm for remote CO₂ column measurements and near 1.26 μm for remote O₂ column measurements. The CO₂ and O₂ column measurements are then combined to determine the column CO₂ mixing ratio (XCO₂). This IM-CW LAS system is under development for a future space mission to determine the global distribution of regional-scale CO₂ sources and sinks, which is the objective of the NASA Active Sensing of CO₂ Emissions during Nights, Days, and Seasons (ASCENDS) mission. This prototype of the ASCENDS system, called the Multi-frequency Fiber Laser Lidar (MFLL), has been flight tested for CO₂ measurements in eleven airborne campaigns since May 2005. This paper compares the most recent MFLL remote CO₂ column measurements against airborne in situ CO₂ measurements obtained during 2010 and 2011 UC-12 and DC-8 flight tests, and MFLL remote O₂ column measurements are compared with in situ surface pressure measurements obtained during 2011 DC-8 flight tests.

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.