1.5.3 Occultation obliquity
The occultation obliquity is defined as the angle between the motion of the line of sight (with respect to the atmosphere) and the direction of the Earth's centre. The altitude chosen to calculate the obliquity is fixed to 35 km for any occultation. For a tangent occultation, this altitude may be never reached, and in that case, the obliquity is arbitrarily fixed to 90°. For a purely vertical occultation, the obliquity is equal to 0°. The obliquity varies in the same direction as the azimuth direction of the line-of-sight. This is illustrated in Figure 1.7 for an example set of 5000 occultations. For occultations taking place in the orbital plane (azimuth value close to 0), the obliquity is close to 0 (close to vertical occultations). Occultations measured with a field-of-view outside the orbital plane (larger azimuth value) are oblique.
Figure 1.7: Occultation obliquity (°) versus instrument azimuth direction (°) (measurements made between 08/03/2004 and 20/03/2004).