FUNCTION: MVN_EPH_STRUCT PURPOSE: Creates a MAVEN ephemeris structure template. The structure elements are as follows: time : spacecraft event time (seconds since 1970-01-01/00:00:00) ;;orbit : orbit numbers (TBD) x_ss : s/c location in ss coordinates (x) y_ss : s/c location in ss coordinates (y) z_ss : s/c location in ss coordinates (z) vx_ss : s/c velocity in ss coordinates (x) vy_ss : s/c velocity in ss coordinates (y) vz_ss : s/c velocity in ss coordinates (z) x_pc : s/c location in pc coordinates (x) y_pc : s/c location in pc coordinates (y) z_pc : s/c location in pc coordinates (z) Elon : east longitude of s/c [radians] lat : latitude of s/c [radians] alt : s/c altitude above Mars surface [km] sza : solar zenith angle of s/c [radians] ;;lst : local solar time of s/c (0 = midnight) [radians] (TBD) ;;sun : sunlight flag (0 = s/c in Mars' optical shadow 1 = s/c illuminated by sun) (TBD) The coordinate systems are: SS = Mars-centered, sun-state coordinates: (MSO) X -> Sun Y -> opposite to Mars' orbital motion Z -> X x Y PC = Mars-centered, body-fixed coordinates: (IAU_MARS) X -> 0 deg longitude, 0 deg latitude Z -> +90 deg latitude (Mars' north pole) Y -> Z x X (= +90 deg east longitude) USAGE: mvn_eph = mvn_eph_struct(npts) INPUTS: npts: Number of ephemeris points. KEYWORDS: INIT: Initialize the structure with the specified float scalar. Default = NaN. CREATED BY: Takuya Hara on 2014-10-06 LAST MODIFICATION: $LastChangedBy: hara $ $LastChangedDate: 2014-11-26 11:42:32 -0800 (Wed, 26 Nov 2014) $ $LastChangedRevision: 16310 $ $URL: svn+ssh://thmsvn@ambrosia.ssl.berkeley.edu/repos/spdsoft/trunk/projects/maven/eph/mvn_eph_struct.pro $ MODIFICATION LOG: (YYYY-MM-DD) 2014-10-06: Initial version. This function is based on the MGS routine written by David L. Mitchell. The original routine name is 'mgs_eph_struct' (created on 1999-08-16, and the latest version is 1.4 on 2009-03-06). 2014-10-21: Added "vss" keyword. It can restore the spacecraft velocity in the MSO coordinate system. But the data format has not been confirmed yet. It's just TBD now. 2014-11-24: Removed "vss" keyword, because information of the spacecraft velocity is included in default.