In limb mode the instrument looks through the atmosphere forwards in the orbital direction of flight and at a tangent to the Earth's surface.

Limb measurements record the solar light scattered by the atmosphere and are performed during the dayside transit of each orbit. The line of sight follows a tangential path through the atmosphere, detecting solar radiation that is scattered and transmitted from the scattering point into SCIAMACHY's field of view.

In this mode, both elevation and azimuth mirrors are used. The instrument scans segments of atmosphere 960 km across and 3 km high at the tangent point, starting at the Earth's horizon and stepping upwards to the maximum altitude of 100 km. An across track scan lasts 1.5 s, giving a swath width of about 960 km at the tangent point. During the scan the tangent altitude is kept fixed by correcting for the curvature of the Earth's surface. At the end of a horizontal scan the line-of-sight is stepped upwards, and the next horizontal scan is performed in the reverse direction. In all, tangent altitudes from 0 to 100 km are measured, with a vertical resolution of about 3 km.

Fig 1.3.2.1.2.1: Limb mode scanning geometry of the SCIAMACHY instrument.

Image courtesy of :