Development and Test of a Automatic Gain Controller for a Lidar-Detector Based on Airborne Measurements of Cirrus Clouds

Kurzfassung

In commercial aviation, clear-air turbulence (CAT) represents a major risk of damage to the aircraft or injury to the passengers. Gust load alleviation (GLA) is one strategy to mitigate loads caused by CAT and has been under development for decades. More recent techniques involve the use of Doppler Wind Lidar (DWL) to measure wind fields ahead of the plane and provide information for a feed forward GLA controller. This thesis deals with the receiver subsystem of such a DWL, developed by the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt, DLR). During cruise flight, the measurement range may be occasionally contaminated by cirrus clouds, which due to stronger back-scatter will lead to over-saturation of the detector. The goal of this work is to design, implement and test a automatic gain controller (AGC) for the lidar detector that will keep the return signal level within a specified range around a setpoint, regardless of varying external conditions. In order to set up a representative validation experiment, a study of back-scattering dynamics using data from airborn lidar observartions, and theoretical information is performed. This study yields characteristic variations both in magnitude and frequency of back-scattering on cirrus clouds. The AGC is based on a proportional-integrating (PI) controller architecture and is able to keep the detector output within 10 % of a specified setpoint at ca. 150 % input signal variability.