Airborne Doppler Wind Lidar Technology Demonstration for Aeolus - from Pre-launch Campaigns to Mission Performance Validation

Kurzfassung

The Atmospheric LAser Doppler Instrument (ALADIN) on-board Aeolus was the first European lidar and the first direct-detection Doppler wind lidar (DWL) in space. To support the processor development and operational preparation of the ESA mission, for the first time an airborne technology demonstrator was implemented in parallel. The ALADIN Airborne Demonstrator (A2D) was developed, featuring the complex receiver optics including the spectrometers of the ALADIN pre-development model. A2D also consists of a frequency-stabilised ultra-violet laser, a Cassegrain telescope and the same dual-channel optical and electronical receiver design to measure wind speeds along the instrument’s line-of-sight by analysing particulate (Mie) and molecular (Rayleigh) backscatter signals. The A2D has been deployed by DLR for studies in the optical laboratory as well as during various ground and airborne field campaigns since 2005. A scanning, high-accuracy coherent DWL operating at 2-µm wavelength acted as a reference by providing wind vector profiles for the A2D and Aeolus validation. The combination of both DWLs enabled to explore the ALADIN-specific wind measurement technology under various atmospheric conditions. The two instruments were operated in parallel on the DLR Falcon research aircraft in a downward looking configuration similar to Aeolus, but offered higher horizontal resolution as well as the possibility to influence and monitor all relevant system parameters. Resulting from these pre-launch activities, the operational experience and algorithm development for Aeolus grew to a level, which paved the way for the fast commissioning and quick success of the mission after its launch in August 2018. With ESA support, DLR conducted four airborne validation campaigns during the first three years of the Aeolus mission, performing coordinated underflights along the Aeolus track in different geographical regions and operational states of the mission. After three campaigns in Central Europe and the North Atlantic region around Iceland in 2018 and 2019, DLR contributed to the Joint Aeolus Tropical Atlantic Campaign (JATAC) around Cape Verde in September 2021. Thanks to the high degree of commonality with the satellite instrument in terms of design and measurement principle, the collocated A2D and 2-µm wind observations acquired during these campaigns provide valuable information for the optimization of the Aeolus wind retrieval and related quality control algorithms. The mitigation of deficiencies that were identified in the Aeolus products as well as algorithm improvements developed for the A2D wind retrieval contribute to the enhancement of the Aeolus data quality and will continue to be relevant for testing re-processed Aeolus products. Meanwhile, the A2D is being further developed towards a 2nd - generation Aeolus-type instrument with a 100 Hz UV laser. The versatile instrument set-up allows for considering design modifications like the implementation of a cross-polar channel in linear or circular polarization configuration and the test of different spectrometer or detector concepts. The paper summarizes the mission-relevant results from pre-launch campaigns obtained by comparative wind observations of Aeolus and the DLR airborne DWLs. It is highlighted that a further successful mission support for a potential Aeolus-2 candidate can be founded on this heritage and an accordingly modified 2nd generation A2D.