Ground return recognition at airborne cw Doppler lidar measurements

An airborne cw Doppler lidar measures the Doppler shift of aerosols in the focal volume which are moving with the wind, but also stronger reflections from the ground below the aircraft can be registered. Now one problem is to distinguish between the ground return and the aerosol signal. One possibility is to use the intensity of the back scattered signal, but the ground return may sometimes be very weak, for example at a higher flight level or over sea. Another way to deal with this problem is to use the statistical properties of the back scattered light also known as speckle effect. The back scattered light of different types of targets Like hard target, aerosol, and cloud seem to have different correlation times. At a pulsed Doppler lidar this statistical approach has to be modified due to the short correlation time of the laser pulse itself, but a cross correlation between emitted and received pulse may be a solution. This paper will present an analysis of the correlation time at the practical example of a cw Doppler lidar measurement obtained with the system ADOLAR. The measured wind field will also be discussed.