Validation of the ALADIN Airborne Demonstrator A2D by end-to-end simulations and measurements

An airborne prototype of ALADIN (Atmospheric Laser Doppler lidar Instrument) was developed to analyse the measurement concept, to validate and optimize the data processing algorithms, and to examine atmospheric backscatter and internal signals at the receivers. The ALADIN Airborne Demonstrator A2D is designed to perform wind measurements in a downward viewing geometry, similar to how ALADIN will operate in space. To validate the A2D, an end-to-end simulator was developed at DLR (Deutsches Zentrum für Luft und Raumfahrt, or German Aerospace Centre), representing the properties of the A2D and various atmospheric models, and including the signal processing modules. This offers the possibility to study wind measurements for different atmospheric and instrumental parameters, and to analyse the performance of the A2D from ground and aircraft. The results of the simulations are used furthermore to develop and optimise the signal processing algorithms, knowing the properties of the modelled signal. Various algorithms were developed, analysed, and validated and the most suitable are used to determine the atmospheric wind speed. The ALADIN receiver consists of two spectrometers: one to detect aerosol (Mie) backscatter and one to detect molecular (Rayleigh) backscatter, and the Doppler shift is determined from these measurements. The Rayleigh receiver is a radiometric detector, whereby the Doppler shift is determined from a change in intensity. It employs the principle of the double-edge method in a new implementation of the Fabry-Perot interferometer, called the sequential technique. The Mie receiver consists of a Fizeau interferometer, which was not used for atmospheric wind measurements before to our knowledge. The Doppler shift is determined from the spatial location of the Mie signal at the detector by employing the fringe imaging technique. First measurements of atmospheric backscatter with the A2D were performed at DLR from ground and aircraft, and it was the first time that a direct detection Doppler wind lidar had been deployed on an aircraft. Signals between the aircraft and ground, along with backscatter from clouds, and signals of the Earth’s surface, were detected by the instrument, showing the capability to identify the ground and cloud return at the receivers. Atmospheric wind was not derived during these first functional tests, but the wind speed measurement accuracy was assessed by the Doppler shifted signal of the surface of a building