| Description: The emission_angle element provides a measure of the relationship between the instrument viewing position and the surface normal vector at the intercept point. Specifically, it is the angle, measured at the intercept point, between the local surface normal vector and a vector in the direction of the outgoing photon to the observing instrument. In the context of planetary rings, the local surface normal vector is always on the side of the ring plane illuminated by the light source. Thus, the emission_angle varies from 0 degrees when the spacecraft is viewing the illuminated side of the rings face-on, to 90 degrees when viewing the rings edge-on (that is, the spacecraft is in the ring plane), to 180 degrees when the spacecraft is viewing the unlit side of the rings face-on. |