NASA Goddard Space Flight Center has developed an integrated-path, differential absorption (IPDA) lidar approach to measure atmospheric column-averaged CO 2 mole fraction (XCO 2 ). This pulsed laser approach uses a step-locked laser diode source, a fiber laser amplifier, and a high-sensitivity detector. The approach allows measurements of CO 2 absorption and time-resolved laser backscatter profiles. These enable accurate estimates of XCO 2 and range to the Earth's surface and cloud tops under a wide variety of conditions, including darkness, at low sun angles, through broken cloud fields, and the presence of aerosols. The CO 2 Sounder lidar team has demonstrated airborne measurements during several campaigns over the past decade.