Satellite Lidar for Global Warming Gas Measurement

Global climate change studies require a higher spatial and temporal density of measurements of greenhouse gases to achieve increased precision. Confident predictions based on these models require a better knowledge of CO2 and CH4 sources and sinks in order to increase our understanding of the global atmospheric carbon cycle. Space-based observations of CO2 mixing ratios are an efficient way to generate the required database. These orbital observations are needed to define the spatial gradients from which sources and sinks can be quantified and separated from the seasonal fluctuation component. A precision of 1-2 ppmv, which represents 0.5 % of the average ambient concentration of CO2, will be needed. Measurements are primarily needed in the lower and middle troposphere as that is where the gradients arising from sources and sinks will be the largest. To fulfill the need for space-based observations of CO2 mixing ratios, we are pursuing system designs for active satellite-based sensors. These studies suggest that a satellite-borne lidar sensor operating at wavelengths in the near-IR offers real potential for making these measurements in the near future. We review the concepts and technologies for a satellite-borne, all solid-state, differential absorption lidar (DIAL) transmitter for the measurement of column densities of CO2, CH4, and O2. Approaches to sensing CO2 at 1.6 micron and at 2 micron were investigated. A laser transmitter at 1.6 micron offers the additional advantage for remotely sensing CH4. We have also estimated the impact of cirrus clouds on the measurement precision. We have outlined an algorithm for retrieving vertical profile of mixing ratios from the returns at selected wavelength offsets. We have examined several aspects of the lidar system as a whole. We carried out an analysis of the sensor transmitter performance, developed system concepts including a system functional block diagram, and developed mission profile concepts, including type and altitude of orbit.