; docformat = 'rst'
;+
;This function takes a given Julian Date time and returns the quaternion
;to transform from ECI to ECEF coordinate frames.
;
; :Examples:
;   quat_trans = eci_to_ecef(time)
;-
;+
; :Params:
;   t : in, required, type=double
;     Julian Date time used for the transformation.
;     
; :Returns:
;   Returns the quaternion to transform from ECI to ECEF coordinates.
;-
function eci_to_ecef,t

  if ~real_numeric_type(t) then begin
    doc_library, 'eci_to_ecef'
    return, -1
  endif
  ;calculate the transformation from ECI to ECEF
  return,quatx(3,-gmst(t)*!const.dtor)
end