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