The NOAA Fish Lidar,31–33 developed in the Earth System Research Laboratory over a number of years, was installed in the back of a four-seat Cessna-177 aircraft with the rear seats removed (Fig. 2). Two flights were made each day of the deployment at a flight altitude of about 300 m and a speed of about . The lidar transmitted 100 mJ of green (532 nm) light in a 12-ns pulse at a rate of 10 pulses per second. It was pointed 12 deg off nadir to minimize the specular reflection from the sea surface. The laser beam divergence was set so that the diameter of the laser spot on the surface was 5 m, which is large enough that the power density at the surface is safe for humans and marine mammals.34 The scattered light from the water column was collected by two telescopes whose fields of view were matched to the laser beam divergence. Each telescope was equipped [as shown in Fig. 2(a)] with a polarizing filter, a field-of-view aperture, and an interference filter to reduce background light. The light collected by each telescope was detected by a photomultiplier tube, logarithmically amplified to increase the dynamic range, and digitized at a rate of samples per second. Between the morning flight and the afternoon flight on September 16, we changed the polarization characteristics of the lidar. In the morning, the transmitter was linearly polarized, and the two telescopes were filtered to receive light co-polarized with the transmitter and cross-polarized to it. In the afternoon, the transmitter was changed to right-hand circular polarization, and the receiver telescopes were filtered to receive co- and cross-circularly polarized light.