Articles | Volume 10, issue 10
Research article
19 Oct 2017
Research article |  | 19 Oct 2017

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

Alexander Sergeevich Gurvich and Victor Alexeevich Kulikov

Abstract. Airborne lidar forward sensing 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 strong variations in signal level, which were observed during the DELICAT measurements but not explained, sometimes indicated the need of a better understanding the observational errors due to geometrical factors. In this paper, we discuss possible error sources pertinent to this technique, related to fluctuations of the flight parameters, which may lead to strong signal variations caused by the random deviations of the sensing beam from the forward flight trajectory. We analyze the variations in backscattered lidar signal caused by fluctuations of the most important forward-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 civil aviation. The part of the pitch angle fluctuations not compensated 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 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.

Short summary
The experimental and numerical simulation results of lidar sensing ahead of the airplane's flight direction were analyzed. Pitch angle fluctuations, even when compensated for within the error limits of a typical beam-steering system, can produce significant fluctuations in the backscattered signal. This effect appears in the presence of aerosol clusters with vertical size of about a few hundred meters. A simple criterion to distinguish these fluctuations from aerosol evolution is suggested.