The purpose of the mission is to provide global measurements of aerosols and clouds required to obtain a better understanding of the aerosols and clouds role in the climate system, and to improve our abilities to predict long-term climate change and seasonal-to-interannual climate variability.
CALIPSO is a three-year earth science mission. The Satellite will catch-up with the A-Train, it will orbit the earth at an altitude of 705 km, and a nominal inclination of approximately 98.2 degrees. Other Satellites composing the A-Train are shown on the left.

The CALIPSO instrument suite will measure vertical distributions of aerosols and clouds in the atmosphere, as well as the optical and physical properties of aerosols and clouds, which influence the Earth's radiation budget.

From passive measuring techniques to active ones

Conventional space-based cloud and aerosol observation techniques measure the visible or infra-red radiation that is reflected by their targets. They are called "passive" techniques, as opposed to "active" ones where the instrument itself is the source of the radiation used to sample the target.
However, because they are so thin and are often found in multi-layered structures, cirrus clouds cannot normally be captured by such passive imaging techniques. Due to the way they work, the instruments of this type are not able to distinguish, with sufficient vertical resolution, the different cloud layers or the overlapping of aerosols and clouds, which results in significant distortion of cloud classification and limits our ability to study the interactions between aerosols and clouds.
In order to penetrate the secrets of the atmosphere, two instruments are currently offering us new opportunities: the lidar and the millimetric radar, or "cloud radar". These have both already been tested during ground-based or airborne measurement campaigns. In addition, the lidar has already been used in space, both on the Space Shuttle, for the Lite experiment which took place in 1994 and on board the Glas satellite, which was launched in 2003 by NASA, in order to take altimetric measurements of ice cover. With regard to the Cloudsat cloud radar, it will be the first time that such an instrument, a thousand times more sensitive than a classic meteorological radar, has flown in Space.
During the course of each orbit, both instruments (with a footprint of 90 metres for the Calipso lidar and 700 m for the Cloudsat radar) will sample the column of atmosphere located vertically underneath the satellite. Ground sampling will be performed every 330 m for the lidar and every 1.1 km for the radar. The vertical resolutions of the two instruments are 30 m for the lidar and 500 m for the radar. Measurements of the vertical dimension of the atmosphere will thus complement the horizontal dimension, which can only be done by wide field imagers.

Did you say lidar?

A lidar, (which stands for Light detection and ranging), works on the same principle as a radar, except that microwaves are replaced by light waves emitted by a laser (ultraviolet, visible or infra-red). As it travels through the atmosphere, the short (a few tens of nanoseconds), but highly intense pulse emitted by the Calipso laser is scattered by the gas molecules, aerosols and cloud particles.
The properties of the atmosphere – physical variables and concentration of elements – are deduced from the intensity and the spectral characteristics of the light reflected back in the direction of the emitting source.
This light is captured by a telescope before being analysed. Because the laser pulses are very short, it is possible to analyse the distance of the "echos" and to "reconstitute" the vertical structure of the atmosphere. The lidar can be used by day or night, although daytime measurements are distorted by natural light detected at the same time.
The signal thus measured by the satellite can be expressed as an equation, which groups the atmosphere's backscattering, depolarisation and attenuation properties. Initial processing is performed on this signal to extract essential information about the different strata of the atmosphere: altitude at the base and the top of the layers, classification into thin clouds, aerosols, liquid water clouds or ice clouds. Other algorithms then characterise the optical properties of the particles detected: pure backscattering profile and optical thickness of aerosols and clouds. Then this data has to be complemented by data from other sensors, passive radars or radiometers, in order to remove any uncertainty from the lidar equation.

The green ray Unlike sunlight, the "green ray" emitted by the Caliop lidar on Calipso is completely polarised in one direction. Atmospheric scattering modifies this incident polarisation and so the measurement of this depolarisation provides a wealth of information about the nature of the particles, especially their geometry. Since the analysis of ground-based measurements has shown that depolarisation 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.

 

Lidar or radar While the lidar and the radar are similar in theory, their detection capabilities are quite different, due to the fact that 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 the radar operates at 94 GHz, or 3.2 mm. The lidar can thus detect the tops of clouds and the base of those clouds through which it can travel very precisely, because 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 the lidar, which could also miss any layers which are 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. The radar signal is 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 clouds of liquid water. It can also detect precipitation. Thus the radar can 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, the lidar is more effective when studying thin clouds and aerosols, whereas the radar performs 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.