Rayleigh wind retrieval for the ALADIN airborne demonstrator of the Aeolus mission using simulated response calibration.

Aeolus, launched on August 22^nd in 2018, is the first ever satellite to directly observe wind information from the surface up to 30 km on a global scale. An airborne prototype called ALADIN Airborne Demonstrator (A2D) was developed at the German Aerospace Centre (DLR) for validating the Aeolus measurement principle based on realistic atmospheric signals. To obtain accurate wind retrievals, the A2D uses a measured Rayleigh response calibration (MRRC) to calibrate its Rayleigh channel signals. However, the atmospheric and instrumental variability currently limit the reliability and repeatability of this MRRC. Thus, a procedure for a simulated Rayleigh response calibration (SRRC) is developed and presented in this paper to resolve these limitations of the A2D Rayleigh channel MRRC. The transmission functions of the A2D Rayleigh channel interferometer, consisting of the double-edge Fabry-Perot interferometers (FPIs), are firstly characterised and optimized based on measurements performed during different airborne and ground-based campaigns. The optimized FPI transmission function is then combined with the molecular Rayleigh backscatter spectrum to derive an accurate A2D SRRC which can finally be implemented in the A2D wind retrieval. Using dropsonde data as a reference, a statistical analysis based on data from a flight campaign in 2016 reveals a bias and a standard deviation of line-of-sight (LOS) wind speeds derived from an SRRC of only 0.05 m s^-1 and 2.52 m s^-1, respectively. Compared to the result derived from a MRRC with a bias of 0.23 m s^-1 and a standard deviation of 2.20 m s^-1, the accuracy improved while the precision is considered to be at the same level. Furthermore, it is shown that SRRC allows the simulation of receiver responses over the whole altitude range from the aircraft down to sea level, thus overcoming limitations due to continuous ground elevation during the performance of airborne instrument response calibrations. [ABSTRACT FROM AUTHOR]