Problems with SPICE ephemeris

General help that does not fall into a more specific category

Problems with SPICE ephemeris

Postby astrojuanlu » Sat Sep 01, 2018 11:11 am

I was doing some experiments in GMAT printing the ICRF position of Mars in the reference epoch and noticed some inconsistencies between the results of GMAT and what I can get from other sources using the same SPK ephemeris. Here you can find my GMAT script, the results from HORIZONS, and a Python script using Astropy to read the ephemeris: ... fe17b8ea71

If I run the Python script, I get this:

Code: Select all
Compare with HORIZONS
GMAT (DE421?)              =  [ 2.06980543e+08 -1.86369976e+05 -5.66721298e+06]
jplephem (DE430)           =  [ 2.06980542e+08 -1.86369762e+05 -5.66723342e+06]
HORIZONS (DE430)           =  [ 2.06980542e+08 -1.86369762e+05 -5.66723342e+06]
Compare with GMAT
jplephem (DE421 from GMAT) =  [ 2.06980542e+08 -1.86369836e+05 -5.66723310e+06]
GMAT (DE421?)              =  [ 2.06980543e+08 -1.86369976e+05 -5.66721298e+06]

The differences between GMAT with DE421 and jplephem/HORIZONS with DE430 are natural. However, in the second case, I can't explain the differences. I already ruled out the Mars vs Mars barycenter and the differences in time scales (TDB vs TT vs TAI produce errors in the order of meters, not kilometers).

Also, I tried to set the ephemerides in GMAT to DE430 like this:

Code: Select all
GMAT SolarSystem.EphemerisSource = 'SPICE';
GMAT SolarSystem.SPKFilename = '../data/planetary_ephem/spk/de430_1850-2150.bsp';

but I got the following error:

Code: Select all
Toolkit version: N0065
Insufficient ephemeris data has been loaded to compute the state of 499 (MARS)
relative to 399 (EARTH) at the ephemeris epoch 2000 JAN 01 11:59:59.999.
A traceback follows.  The name of the highest level module is first.
spkgeo_c --> SPKGEO

Am I doing something wrong here?
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