Planetary Ephemeris File Format (*.pe)

A planetary ephemeris file is an ASCII text file that is formatted for compatibility with STK and ends in a .pe extension. Planetary ephemeris files can contain data gathered in another program or data that is important to the analysis being performed using STK. In order for a planetary ephemeris file to be formatted correctly, it must contain the following elements:

Keywords used in every planetary ephemeris file

Keywords Description
stk.v.<major release number>.<minor release number> The version of STK software for which the file is formatted to be used. Files can be created in and imported to STK software versions consistent with the file version stamp or higher. For example, a file created in and stamped with stk.v.4.0 can be imported to STK software versions 4.0 through the most recent version.
BEGIN Ephemeris
END Ephemeris keywords		 Set off the beginning and end of the ephemeris data included in the file.
Units keyword phrase The type of units to be used when specifying ephemeris data. Formats available are:
  • au/day
  • km/sec
  • m/sec
NumberOfEphemerisPoints keyword phrase The number of ephemeris points to follow. For example, if you enter:
NumberOfEphemerisPoints 50
50 points of ephemeris data would be included. This entry must precede actual ephemeris data.
EphemerisJ2000SciJedPosVel keyword Signifies the beginning of the planetary data.

The structure of a generic planetary ephemeris file, then, looks something like:

Generic planetary ephemeris file structure

stk.v.6.0
BEGIN Ephemeris
NumberOfEphemerisPoints x
EphemerisJ2000SciJedPosVel
.
.
.
END Ephemeris

Data points following the EphemerisJ2000SciJedPosVel keyword are:

  1. <TimeInJED> - The time, in Julian Ephemeris Date (JED) format, corresponding to the planet's position and velocity data. JED is the Julian Date representation of Terrestrial Dynamical Time (TDT), which is equal to Atomic Time (TAI) plus 32.184 seconds. UTC differs from TAI by the number of leap seconds which have been accumulated. The conversion between the Julian Date based on UTC and Julian Ephemeris Date as of Jan 1, 1996, when 20 leap seconds have been accumulated with an initial 10 second offset, is:
    Julian Ephemeris Date = Julian Date + (32.184 + 30) / 86400.0
  2. <x> <y> <z> - Position of the planet specified in a Sun-centered inertial coordinate system for which the axes are in alignment with the J2000 coordinate system.
  3. <xDot> <yDot> <zDot> - Velocity of the planet.

Sample planetary ephemeris file format

stk.v.6.0
BEGIN Ephemeris
Units au/day
NumberOfEphemerisPoints 30
EphemerisJ2000SciJedPosVel
2449652.500708 -3.1997092e+00 -4.0216144e+00 -1.6458850e+00 8.0130291e-13 -5.0124296e-13 -2.3437952e-13
2449653.500708 -3.1937244e+00 -4.0253524e+00 -1.6476330e+00 8.0210979e-13 -5.0022715e-13 -2.3396376e-13
2449654.500708 -3.1877337e+00 -4.0290828e+00 -1.6493780e+00 8.0291527e-13 -4.9921028e-13 -2.3354751e-13
2449655.500708 -3.1817370e+00 -4.0328056e+00 -1.6511199e+00 8.0371935e-13 -4.9819235e-13 -2.3313078e-13
2449656.500708 -3.1757343e+00 -4.0365207e+00 -1.6528586e+00 8.0452202e-13 -4.9717336e-13 -2.3271355e-13
2449657.500708 -3.1697255e+00 -4.0402283e+00 -1.6545943e+00 8.0532330e-13 -4.9615331e-13 -2.3229584e-13
.
.
.
2449678.500708 -3.0421783e+00 -4.1163153e+00 -1.6903164e+00 8.2182182e-13 -4.7448882e-13 -2.2341157e-13
2449679.500708 -3.0360406e+00 -4.1198535e+00 -1.6919825e+00 8.2259166e-13 -4.7344570e-13 -2.2298320e-13
2449680.500708 -3.0298971e+00 -4.1233838e+00 -1.6936455e+00 8.2336006e-13 -4.7240156e-13 -2.2255436e-13
2449681.500708 -3.0237479e+00 -4.1269064e+00 -1.6953052e+00 8.2412700e-13 -4.7135638e-13 -2.2212504e-13
END Ephemeris