Atmospheric refractive effects on coherent lidar performance at 10.6 microm

The aperture diameter over which a lidar transceiver collects backscattered signal that can be coherently detected depends on refractive index inhomogeneities in the atmosphere. We have studied the change in coherently detected signal with change in aperture for a cw (focusing) lidar operating at 10.6-microm wavelength over a horizontal path 2 m above the ground at target ranges between 0.6 km and 2.6 km. We conclude that for average daytime conditions at a range of 1 km, for example, a transceiver aperture diameter of 27 cm gives approximately 65% of the coherent signal expected from an aperture of twice that diameter.