PROCEDURE: maven_orbit_makeeph PURPOSE: Generates a MAVEN spacecraft ephemeris using the SPICE Icy toolkit. The ephemeris is returned in a structure with the following tags: T Time (UTC): number of seconds since 1970-01-01 X X position coordinate (km) Y Y position coordinate (km) Z Z position coordinate (km) VX X velocity component (km/s) VY Y velocity component (km/s) VZ Z velocity component (km/s) The available coordinate frames are: IAU_MARS = body-fixed Mars geographic coordinates (non-inertial) X -> 0 deg E longitude, 0 deg latitude Y -> 90 deg E longitude, 0 deg latitude Z -> 90 deg N latitude (= X x Y) origin = center of Mars units = kilometers GEO = synonym (in this routine only) for IAU_MARS MSO = Mars-Sun-Orbit coordinates (approx. inertial) X -> from center of Mars to center of Sun Y -> opposite to Mars' orbital angular velocity vector Z = X x Y origin = center of Mars units = kilometers J2000 = Mean equator and equinox at J2000 epoch (inertial) X -> aligned with mean (vernal) equinox Z -> aligned with celestial north pole (Earth's rotation axis) Y = Z x X origin = center of Mars units = kilometers USAGE: maven_orbit_makeeph, frame=frame, eph=eph INPUTS: KEYWORDS: TSTEP: Time step (seconds). Default = 60. EPH: Named variable to hold the ephemeris structure. FRAME: Coordiante frame. Can be "J2000", "IAU_MARS", or "MSO". (Also accepts "GEO" as a synomym for "IAU_MARS".) $LastChangedBy: dmitchell $ $LastChangedDate: 2014-10-13 12:29:59 -0700 (Mon, 13 Oct 2014) $ $LastChangedRevision: 15983 $ $URL: svn+ssh://thmsvn@ambrosia.ssl.berkeley.edu/repos/spdsoft/trunk/projects/maven/maven_orbit_tplot/maven_orbit_makeeph.pro $ CREATED BY: David L. Mitchell 2014-10-13