New concept for modulation of a continuous wave (CW) lidar system that allows accurate retrieval of column integrated CO2 in the presence of clouds for the ASCENDS mission

The Active Sensing of CO2 Emissions over Nights, Days, and Seasons (ASCENDS) mission is a lidar sensing project proposed by the decadal survey. The requirement is to accurately map sources, sinks and concentrations of global CO2 levels across the globe. The lidar system relies on the ratio between light scattered from the ground at two slightly different wavelengths around the 1.57um CO2 absorption line. One wavelength is tuned to the peak of this absorption and is referred to as `online'. The other is positioned slightly away from this spectral peak and is referred to as `'offline'. Using the ratio of light scattered from the ground at these two wavelengths it is possible to accurately derive the column integrated CO2 within the atmosphere. ITT have developed a continuous wave (CW) solid state laser concept that would be ideal for the ASCENDS mission. Its advantage over a pulsed laser system (e.g. CALIPSO) is an expected longer lifetime as it utilizes more reliable technology, such as that used in the telecommunications industry. However, a pulsed laser concept does have the advantage that the effects of cloud scattering can be more easily separated from the ground scattered signal. For a CW system, light scattered from clouds represents noise in the CO2 retrieval as it has passed through a lower path length than that scattered from the ground. This can lead to an underestimate of the column density. This paper details a new signal processing concept for a CW CO2 sensing lidar that enables an effective removal of the cloud scatter noise signal. It relies on the use of a modulated LIDAR pulse which allows accurate simulation of the cloud noise and hence its removal, leaving the ground scattered signal with accuracy 1% or better.