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

Abstract

Abstract. Airborne lidar sensing ahead along the flight direction can serve for notification 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 detection) project. However, the difficulties encountered during the processing of DELICAT data indicated the need of a better understanding the observational errors due to geometrical factors. In this paper, we discuss possible error sources imminent to this technique, related to fluctuations of the flight parameters, which may lead to an incorrect estimation of the turbulence intensity. We analyze the variations of backscattered lidar signal caused by fluctuations of the most important ahead sensing flight parameter, the pitch angle. The fluctuation values considered in the paper correspond to the error limits of the compensational gyro-platform used in the civil aviation. Uncompensated pitch angle fluctuations 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 aerosol layer. We formulate the criteria that allow the recognition of signal variations caused by pitch angle fluctuations. Influence of these fluctuations is shown to be stronger for aerosol variations on smaller vertical scales. An example of DELICAT observations indicating a noticeable pitch angle fluctuation impact is presented.