[Spice_discussion] Re: Spice_discussion Digest, Vol 46, Issue 2

[Jon Giorgini]

>... when I ask Horizons for an ephemeris of state vectors
>I get different values to when I ask it for an SPK and then use
>spkez_c to retrieve the state vectors at the same epoch.  For example,
>2006 RH120 at 2010-Jan-01 gives me:
>
>from spk 72324528.908090 121285770.538049 52886682.629751 -26.346082
>13.091303 6.010543
>horizons 72326272.589071 121284904.093955 52886284.823933 -26.345897
>13.091614 6.010679

The two outputs are in two different time-scales, so aren't directly
comparable.

The "from spk" values are for >UTC< 2010-Jan-01. The Horizons vector
output is for CT/TDB 2010-Jan-01. [Horizons vector tables are deliberately
limited to this dynamical scale since you almost never want to be dealing 
with solar system vectors in a non-uniform scale like UTC. More precisely,
we don't want to be responsible for it ...] 

When reading the SPK file in your software, you probably need to explicitly 
specify the query time as TDB ("2010-Jan-01 TDB"). Then it will match 
the Horizons dynamical vector table at the sub-meter fidelity level of the 
small-body SPK.

Regards,
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Jon Giorgini                       |  Navigation & Mission Design Section
Senior Analyst                     |  Solar System Dynamics Group
Jon.Giorgini at jpl.nasa.gov          |  Jet Propulsion Laboratory
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