ressed as seconds past 
               J2000 TDB, at which the position of the target body 
               relative to the observer is to be computed.  `et' 
               refers to time at the observer's location. 
 
   ref         is the name of the reference frame relative to which 
               the output position vector should be expressed. This 
               may be any frame supported by the SPICE system, 
               including built-in frames (documented in the Frames 
               Required