Remote sounding by the limb scattering technique

What is the limb ? In the context of remote sounding, it is simply the part of the atmosphere that is illuminated by the Sun.

Instead of observing only sunsets and sunrises, the scattering technique uses the bright limb. Different sunrays will enter the atmosphere and will be scattered toward the instrument, being partially absorbed or scattered along their respective optical ways.

Therefore, it is possible to perform a measurement at any time when the satellite is on the dayside. limbgeometry.pngIf this is done from an almost polar orbit, one can achieve a very efficient coverage of the globe over a few days. In the figure, the black dots represent the sounded regions over one day by a solar occultation instrument. The open circles represent the sequence of measurements from a sounder using the limb scattering technique for which an equivalent vertical resolution can be obtained. However, it should be noticed that the mathematical processing of the observed light intensity is much more complicated because rays may be scattered more than once and because of other contributions (the presence of clouds for instance).

The limb scattering technique has been proven to allow for accurate ozone retrievals. Nevertheless, a major difficulty is caused by the fact that it is very difficult to determine from which altitude in the atmosphere the observed ray of light is coming. Indeed, the instrument is not observing in the direction of a light source of known astronomical coordinates.

So far, all limb scattering experiments have been using limb scans to determine, with respect to a radiative model, the correct tangent altitude of the line-of-sight.

All these considerations lead us to the ALTIUS concept.