Calibration of a cw infrared Doppler lidar.

A moving scattering target used as a transfer standard allows absolute calibration of the response of a cw Doppler lidar to an atmospheric target. The lidar in this study operated at a 10.6-microm wavelength. Consideration of the distribution of radiant energy density near the focus of the lidar transceiver permits measurement of a backscatter coefficient from a distributed array of scatterers, such as atmospheric aerosols, based on the diffuse reflectance of the surface of the transfer standard. The minimum detectable signal for our system with a 5-sec averaging time corresponds to a backscatter coefficient of 2.4 x 10(-12) m (-1) sr (-1) +/- 2.5 dB, which is ~ 9 dB greater than the theoretical threshold. Calibration shows that the lidar response is 5+/-1 dB less than the ideal limit for signal powers well above the minimum detectable signal.