Impact of pitch angle fluctuations on airborne lidar forward sensing along the flight direction

摘要： Airborne lidar forward sensing along the ?ight direction can serve for noti?cation of clear air turbulence (CAT) and help to prevent injuries or fatal air accidents. The validation of this concept was presented in the framework of the DELICAT (DEmonstration of LIdar-based CAT detec- tion) project. However, the strong variations in signal level, which were observed during the DELICAT measurements but not explained, sometimes indicated the need of a bet- ter understanding the observational errors due to geometrical factors. In this paper, we discuss possible error sources perti- nent to this technique, related to ?uctuations of the ?ight pa- rameters, which may lead to strong signal variations caused by the random deviations of the sensing beam from the for- ward ?ight trajectory. We analyze the variations in backscat- tered lidar signal caused by ?uctuations of the most impor- tant forward-sensing ?ight parameter, the pitch angle. The ?uctuation values considered in the paper correspond to the error limits of the compensational gyro platform used in civil aviation. The part of the pitch angle ?uctuations not com- pensated for by the beam-steering device in the presence of aerosol concentration variations can lead to noticeable signal variations that can be mistakenly attributed to wind shear, turbulence, or fast evolution of the aerosol layer. We formu- late the criteria that allow the recognition of signal variations caused by pitch angle ?uctuations. In?uence of these ?uc- tuations is shown to be stronger for aerosol variations on smaller vertical scales. An example of DELICAT observa- tions indicating a noticeable pitch angle ?uctuation impact is presented.