This description file describes the convention used to describe the Venus    
Express spacecraft orientation, especially in nadir pointing mode.           

Version 1.1
Written by Maud Barthelemy
21/07/2006
Modified by Maud Barthelemy
30/01/2007

The SPACECRAFT_ORIENTATION element defines the orientation of Venus Express  
in respect to its spacecraft reference frame. The orientation is given as a  
3-tuple (x,y,z).
When the spacecraft is in nadir pointing mode, the +Z axis points towards the
centre of Venus and the +X axis is perpendicular to the ground. Unlike Mars  
Express, the speed vector of the spacecraft can be tilted from the +Y axis   
direction at an angle a. The 3-tuple (x, y, z) defines the coordinates of the
normalised spacecraft speed vector. As a consequence, we have:
    x = sin a
    y = cos a
    


                          Y ^
                            |
                            |    ^
                            |   /  V 
                            |a / 
                            | /  
                            |/_________> X
                      
          View of the spacecraft speed vector in the (X,Y) plane