Multi-wavelength Integrated Path Differential Absorption CO2 Lidar for Space Application

A multi-wavelength integrated path differential (IPDA) CO₂lidar has been developed for measuring atmosphere column CO₂ concentration from space. The CO₂ lidar uses fiber lasers to measure the atmosphere transmission about the CO₂ absorption line at 1572.33 nm. The laser is frequency locked to the absorption line via a small CO₂ gas cell and a control loop, stepping across the entire absorption line in 16 wavelengths. The receiver consists of a telescope, a bandpass filters, and an HgCdTe avalanche photodiode array. The lidar uses low pulse energy and high pulse rate measurement approach with a laser pulse energy of about 2.4 mJ and a pulse rate of 7.5 kHz. The receiver measures the received laser pulse energy and time of flight at each wavelength. The measured data are then fit to a CO₂ line shape function based on the meteorological data and the atmospheric model. A least squares retrieval algorithm is used to simultaneously solves the total column CO₂ mixing ratio, Doppler shift, off-line atmosphere transmission, and water vapor abundance. An airborne CO₂ lidar has been developed to validate the measurement technique and to demonstrate the technology readiness level of the instrument. The airborne instrument has flown successfully in several airborne campaigns in 2014 and 2017 and demonstrated <1 part per million (ppm) measurement precision and accuracy with 1-second averaging. An instrument model has also been developed and continuously updated with laboratory data and airborne measurement results. The model includes effects of solar background, photon detection shot noise, detector dark current, preamplifier noise, laser speckle noise, and inherent errors from least squares fit. The calculated measurement performance agreed with the airborne measurement data. The space borne CO₂ lidar is designed by scaling the receiver telescope size and the laser transmitter power of the airborne instrument for the much higher altitude. Using the same instrument performance model, the space borne CO₂ lidar is expected to achieve <0.5 ppm measurement precision with 1-second average time and 1-meter diameter telescope from 400 km orbit.