On the benefit of airborne demonstrators for space borne lidar missions

Kurzfassung

Laser based remote sensing with lidar is an attractive technique to remotely detect a variety of atmospheric parameters from space. It is independent of sunlight, provides accurate ranging information, and high accuracy of the target parameters. In the past years, various mission proposals using different lidar techniques were proposed for Earth observation. Current European lidar missions are ADM-Aeolus and Earth-CARE in the framework of ESA’s Living Planet Programme, as well as the German-French climate mission MERLIN jointly prepared by DLR and CNES. ADM-Aeolus will provide global wind information by means of a direct detection Doppler lidar. Earth-CARE comprises a high spectral resolution lidar (HSRL) for cloud and aerosol studies. MERLIN is an integrated path differential absorption lidar (IPDA) dedicated to measure the atmospheric column of methane. Furthermore, lidar mission proposals were investigated within feasibility studies such as WALES (WAter vapour Lidar Experiment in Space) or A-SCOPE (Advanced Space Carbon and Climate Observation of Planet Earth) but were put on hold due to missing heritage or lacking technology readiness level. In order to circumvent these deficiencies and support existing missions several airborne lidar instruments have been developed at DLR: A2D (ALADIN airborne demonstrator) was devised to demonstrate the measurement technique of ADM-Aeolus, namely to separately analyse the Doppler shift by aerosols and molecules using a Mie and Rayleigh interferometer. During several pre-launch validation campaigns comparisons with a coherent wind lidar were performed and important experiences collected. Those resulted in more than hundred recommendations to the ADM-Aeolus mission related to operation, validation, and retrieval optimisation. The WALES airborne demonstrator is a multi-wavelength water vapour differential absorption lidar simultaneously featuring a high-spectral-resolution lidar (HSRL) receiver for aerosol and cloud investigations. Within several hundreds of flight hours it could be proven that using four different wavelengths in the 935-nm absorption band of H2O as proposed for WALES is adequate to retrieve accurate water vapour profiles from the boundary layer to the lower stratosphere. In combination with a radar deployed aboard the same airborne platform this system also constitutes an adequate demonstrator for Earth-CARE. Finally, an integrated path differential absorption lidar (IPDA) has recently been developed and successfully tested to simultaneously measure methane and carbon dioxide columns below the aircraft at wavelengths of 1645 nm and 1572 nm, respectively. In particular, the airborne geometry is important to mimic the satellite instrument which uses the backscattered radiation from targets such as the ground or clouds. Here, we would take up the cudgels for continuing support and development of airborne demonstrators to advance space borne lidar missions. Airborne demonstrators help to address technological challenges associated for example with laser development, frequency stabilisation, or detection schemes. Due to their downward looking geometry they are indispensable for retrieval algorithm development, improvement and testing. In addition, all these instruments constitute important validation capabilities not only for active but also for passive space borne remote sensors. In view of the progress made, the feasibility of space borne lidar missions dedicated to water vapour and carbon dioxide should be revisited.