This file summarises the convention used to describe the direction of flight  
of the Venus Express spacecraft, especially in the NADIR pointing mode (see   
VEX_POINTING_MODE_DESC.TXT).                                                  
                                                                              
Version 1.1                                                                   
Written by Maud Barthelemy                                                    
21/07/2006                                                                    
Modified by Maud Barthelemy                                                   
30/01/2007                                                                    
Modified after ASPERA Peer review by Maud Barthelemy                          
23/02/2009                                                                    
                                                                              
The SPACECRAFT_ORIENTATION element defines the Venus Express direction of     
flight with respect to its spacecraft reference frame. The direction of flight
is given as a 3-tuple (x,y,z). The paragraph below describes how to derive the
(x,y,z) values.                                                               
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 track. 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