Though lidars and radars are similar in theory, their detection capabilities are quite different as their wavelengths are very far apart on either side of the distribution range for atmospheric particles. The laser source emits visible radiation at 532 nm and near infra-red radiation at 1,064 nm, whereas radars operate at 94 GHz, or 3.2 mm. Lidars can detect cloud tops and bases through which they can travel very precisely; beyond an optical thickness of about 3, the signal is too attenuated to be measured. The base of a cloud might therefore no longer be detected by lidars, which could also miss potential layers underneath the cloud. Measurements taken in clear skies are used to detect aerosol layers, the atmospheric boundary layer, and the surface, whose echo is used to calibrate the instrument. At 94 GHz, microwaves penetrate ice clouds practically without any attenuation. Radar signals are sensitive to the size of the particles multiplied to the power of six: consequently, it will not see the aerosols and will be more sensitive to ice clouds than to liquid water clouds. It can also detect precipitation. Radars can therefore detect both the top and the base of the clouds, even when they are thick, as long as there is no precipitation.
As a result, lidars are more effective when studying thin clouds and aerosols, whereas radars work better with low clouds. The complementarity of these two techniques explains why Calipso and Cloudsat will be flying in formation—Cloudsat will be bound to follow Calipso at an interval of less than 15 seconds.
Lidar profiles On the left: lidar measurements taken at night at 532 nm over the Tropics by the Lite lidar on the Space Shuttle, in September 1994. On the right: simulation based on Lite measurements of what Calipso would observe for daytime measurements.
The green ray
Unlike sunlight, the "green ray" emitted by the Caliop lidar on Calipso is completely polarized in one direction. Atmospheric scattering modifies this incident polarization and so the measurement of this depolarization provides a wealth of information about the nature of the particles, especially their geometry. Since the analysis of ground-based measurements has shown that depolarization of ice clouds depends to a large extent on the shape and orientation of the crystals of which they are comprised, it is thus possible to classify the particles into four types according to their shape: spheres, platelets, hexagonal columns, or polycrystals.