pro magfld, jd, p, b
;
; Given a double-precision Julian date and fraction (UT) and a geocentric
; position vector (direction, distance) in the Earth-centered inertial
; (ECI) frame, this procedure returns the Earth's magnetic field vector
; (direction, magnitude) in the ECI frame.
;
; B. Knapp, 1998-11-19, 1999-06-25, 2001-04-20 (WMM2000), 
;           2006-03-27 (WMM2005)
;
; The present implementation uses the "World Magnetic Model 2005" (WMM2005)
; which includes a secular variation which is valid from 2005 to 2010.
;
; The output argument  STATUS  is returned non-zero if an error
; occurred.
;
; Print usage?
  if n_params() lt 3 or n_elements( p ) ne 4 then begin
     print, " "
     print, " MagFld is a procedure which given a UT date/time (as a Julian"
     print, " Day Number and fraction) and a geocentric position vector in"
     print, " the Earth-centered inertial (ECI) frame, returns the Earth's"
     print, " magnetic field (main field) vector in the same ECI frame.  The"
     print, " input position vector p must be a 4-vector with the first 3"
     print, " elements being a unit vector giving the direction, and the 4th"
     print, " element the distance from the center of the Earth in km.  The"
     print, " output vector b is also a 4-vector, with the first 3 elements"
     print, " a unit vector giving the field direction, and the 4th element"
     print, " the field magnitude in nTesla.  The input Julian Day Number"
     print, " and fraction must be sufficiently precise to specify the UT"
     print, " time-of-day, since the Earth's magnetic field depends on the"
     print, " geographic longitude.  The World Magnetic Model 2005 is used,"
     print, " which specifies a secular variation valid from 2005 to 2010."
     print, " "
     print, " Usage:"
     print, "     magfld, jd, p, b"
     print, " "
     return
  endif
;
; Use 32-bit or 64-bit shareable object library?
  mbits = ((!version.release ge 5.4) ? !version.memory_bits : 32)
  dll = getenv('bgkroot')+string(mbits,"('/lib/astron',i2,'.so')")
;
; Cast inputs and define outputs for the external call
  jd_loc = double( jd )
  p_loc = double( p )
  b = dblarr( 4 )
  result = call_external( dll, 'magfld_arg_', jd_loc, p_loc, b )
;
  return
  end