SPACECRAFT POINTING MODE AND DESCRIPTIONS

Version 2.1
Written by Maud Barthelemy
2/10/2006
Modified by Maud Barthelemy
30/01/2007


This document describes the values for the SPACECRAFT_POINTING_MODE keyword. 

First, the definition of the SPACECRAFT_POINTING_MODE keyword is:
"The spacecraft_pointing_mode element provides the pointing mode of the      
spacecraft.  The definition of the modes and the standard values are given   
via the SPACECRAFT_POINTING_MODE_DESC element, which shall always accompany  
this keyword."    

Reference: VEX-ESF-IF-5005, Issue 2.2, 2005-05-20

1) NADIR

This pointing mode is used for science observations normally around          
pericentre. The Z axis of the sc points towards the Venus centre. The X axis 
of the s/c is perpendicular to the ground track.

2) ACROSSTRACK

The ACROSSTRACK pointing mode is a derivative of the NADIR pointing mode. It 
is computed by applying a rotation around the s/c Y-axis. The rotational     
angle shall be computed such that the projection of the s/c Z axis onto the  
Venus surface is separated by a constant predefined angle from the           
sub-satellite point as seen from the Venus centre. The angle is measured in  
degrees and shall be specified in the data product label.

3) ALONGTRACK

The ALONGTRACK pointing mode is a derivative of the NADIR pointing mode. It  
is computed by applying a rotation around the s/c X-axis. The rotational     
angle shall be computed such that the projection of the s/c Z axis onto the  
Venus surface is separated by a constant predefined angle from the           
sub-satellite point as seen from the Venus centre. The angle is measured in  
degrees and shall be specified in the data product label.

4) EARTH

HGA boresight axis pointing to the Earth (either HGA 1 or 2),                
    - Y-axis of the S/C (SA axis) is either closest to the ecliptic plane    
    normal or perpendicular to the plane defined by the earth and sun        
    directions. 
    - In addition, in case of the Y axis ecliptic option, the direction of   
    the Y axis (north/south) can be selected.

5) INERT

Inertial attitude is used to point a payload towards a fix direction. The    
direction is given in right ascension and declination. The rotational degree 
of freedom around this direction can be used to optimise power/thermal       
conditions.

6) NADIR_POW 

Nadir with yaw steering with s/c Z-axis as the LOS:
The s/c Z-axis is pointing towards the Venus centre. The Y-axis is           
perpendicular to the Sun. The position of X axis is selected such that no sun
illumination of the -X face occurs.

7) SPECULAR

Nadir with yaw steering with HGA axis as the LOS:
The HGA is pointing towards the Venus centre. The Y-axis is perpendicular to 
the Sun. The position of X axis is selected such that no sun illumination of 
the -X face occurs.

8) CUSTOM

The SPEC mode pointing is a user custom pointing. It aimed at the            
implementation of user specific pointing requirements which cannot be        
accommodated via a specific interface.

9) MOSAIC

The aim of mosaic pointing is to cover the full Venus disc from apocenter by 
performing a raster motion.
For each of the raster points the attitude is computed based on the nadir    
pointing with yaw steering for power optimisation and applying one rotation  
along the S/C Y axis and one rotation along the S/C X axis (in this order).