 X
  X component
of vehicle position (units of length) 
 Y
  Y component
of vehicle position (units of length) 
 Z
  Z component
of vehicle position (units of length) 
 XD
 
 X component
of vehicle velocity (units of length per time) first derivative
of X position

 YD
 
 Y component
of vehicle velocity (units of length per time) first derivative
of Y position

 ZD
 
Z component of
vehicle velocity (units of length per time) first derivative
of Z position
