[Spice_discussion] spkezr question

[Ed Wright]

On Jul 20, 2005, at 9:21 AM, Aimee Mostella wrote:

> Hello, 
>
> I'm using spkezr to get the position of a spacecraft (target) in the 
> J2000 reference frame with the observer as the Sun.  What coordinate 
> system is this based upon?  Is it Heliocentric Aries Ecliptic 
> HAEJ2000? 

The spkezr call returns the state vector relative to the J2000 
reference frame in Cartesian (rectangular) coordinates with units KM 
and KM/s. The definition of J2000 as used by NAIF:

J2000 (reference frame) – the standard epoch of the fundamental 
astronomical coordinate system is January 1 2000 at 12:00:00 TDB. The 
reference frame defined at this time is described by the location of 
the ascending node of the mean equator at epoch J2000.0 obtained from 
Lieske’s models for the precession of the pole and ecliptic.

> Ultimately, I need to get the lat and lon of the spacecraft based on 
> the Heliocentric of Date (HCD) coordinate system so that I can convert 
> to Heliocentric RTN (HGRTN).  I thought I could either get the 
> position of the spacecraft in HAEJ2000 and convert to HCD from there 
> or get the position in GEIJ2000 and convert from there.  However, when 
> I tried the GEIJ2000 route, my calculations were more off than they 
> were when I did it from scratch.  I tried the algorithm in the SPK 
> Required Reading but still didn't get a good answer.  Any 
> suggestions?  

Can you describe the basis of the HCD and HGRTN frames? How are they 
defined for your use?

As always,
Ed Wright
ed.wright at jpl.nasa.gov
1-818-354-0371

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