Name: CALCULATE_LSHELL Purpose: Calculates the l shell value given spacecraft position data in GSM coordinates Inputs: Spacecraft position vector, an array of [time, x, y, z] where time is double precision, and x, y, z, are in GSM coordinates and have units in RE Outputs: The l shell value, or -1L on failure Usage: lshell = calculate_lshell(pos_gsm_re) See Also: tkm2re.pro, thm_crib_calculate_lshell.pro History: Notes: This routine requires IDL geopack routines, returns -1L if not installed $LastChangedBy: egrimes $ $LastChangedDate: 2014-03-17 08:22:00 -0700 (Mon, 17 Mar 2014) $ $LastChangedRevision: 14543 $ $URL: svn+ssh://thmsvn@ambrosia.ssl.berkeley.edu/repos/spdsoft/trunk/external/IDL_GEOPACK/calculate_lshell.pro $