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## Reference Frames question

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Topic author
lucaspazio
Posts: 7
Joined: 10.02.2010
With us: 9 years 8 months

### Reference Frames question

Let me first introduce myself. My name is Luca and I'm using Celestia for visualization purposes of spacecrafts. I need the help of the community to solve an issue I have. I'm trying to add a spacecraft around another planet than Earth. I read the wikibook about reference frames and I wonder if I can use the Equatorial J2000 et EclipticJ2000 around any planet of the solar system or if I have to use the MeanEquator property.

This because for the first two the wikibook states
"EclipticJ2000 is a reference frame with the Earth ecliptic [...]"
"EquatorJ2000 is a reference frame with the Earth equator [...]"

does this mean the Earth is taken also for other planets?

While for the MeanEquator
"MeanEquator defines a reference frame in which the body's equator of date is the xy-plane"
there is no Earth in the definition.

thank you to anybody willing to help me in clarifying the subject.

selden
Developer
Posts: 10069
Joined: 04.09.2002
With us: 17 years 1 month
Location: NY, USA

### Re: Reference Frames question

Which reference frame you use is determined by what you're trying to accomplish.

The Equatorial and Ecliptic frames are not necessarily centered on the Earth. In particular, you do not have to specify xyz offsets relative to the Earth. The offsets are handled automatically when you specify a Center. You can use them as coordinate systems centered wherever you need them to be -- the sun, a planet or a spacecraft, for example.

The orbital parameters of the interplanetary trajectories of real spacecraft usually are specified relative to the Ecliptic coordinate system centered on the Sun or (less often) to the Equatorial coordinate system. When a spacecraft has entered orbit around another body, it's usually more convenient to use a reference frame which is relative to the body it's orbiting around, like the MeanEquator coordinate system. Celestia uses the MeanEquator coordinate system if you specify EllipticalOrbit for an object orbiting around a planet but do not explicitly specify an OrbitFrame.
Selden

Topic author
lucaspazio
Posts: 7
Joined: 10.02.2010
With us: 9 years 8 months

### Re: Reference Frames question

Thank you for the help, even though it does not make me to advance.
I'm not mission analyst, but I know a bit of Orbital Mechanics. Its just that I have an xyzv file (representing a drifting elliptic orbit), that was produced by a mission analysis tool, but the outcomes in Celestia looks not the same. The reference frame in which those were computed is an Inertial Reference Frame centered on the Planet.

In the ssc file I used the "EquatorJ2000" reference frame and said to use the positions in the xyzv file.
In the outcomes it looks as the inclination is correct, therefore an equatorial frame is correct, but that the apoaxis is not where it should be. I suppose therefore a problem in the apsides line of the reference frame, as the X axis of the reference frame would not be the correct one. For such a reason I asked previousely if the EquatorJ2000 and EclipticJ2000 are not simply referenced to the Earth. They could be applicable to every planet, but keeping in mind that the X axis is always oriented toward the Aries point, which is, obviously, valid only for Earth.

I may check in using the MeanEquator at the date in order to see if this solves the issue.