measuring LAN at the moon

[caningo]

Hi guys,

I'd like to know how does Orbiter measure LAN at the moon. I know that on Earth, the vernal equinox line is used.

I'm assuming a good reference point would be the Earth-Moon line (0 longitude??) ... however I'm not 100% :lol:

Any help is appreciated.

 

[jarmonik]

Hi guys,

I'd like to know how does Orbiter measure LAN at the moon. I know that on Earth, the vernal equinox line is used.

I'm assuming a good reference point would be the Earth-Moon line (0 longitude??) ... however I'm not 100% :lol:

It is computed the same way in every planet and moon. Orbit MFD is using two different reference frames ECL = Ecliptic and EQU = Equator of the reference body.

In ECL frame the reference point is always the vernal equinox. (even in an orbit around the Moon)
In EQU frame the reference point is a cross-product of the ecliptic normal and a rotation axis of a planet.

---------- Post added at 19:57 ---------- Previous post was at 19:28 ----------

In EQU frame the reference point is a cross-product of the ecliptic normal and a rotation axis of a planet.

No, actually that was the case in Orbiter 2006 but it's obsollete now.

The reference axis for equatorial coordinate system is calculated:

MATRIX3 Mat;
oapiGetPlanetObliquityMatrix(hRef, &Mat);
VECTOR3 RefAxis = mul(Mat, _V(1,0,0));

That's the only valid method.

 

[hribek]

So it's not above 0deg longitude in EQU mode? If it was, you could sort of align your orbit with a base just using the orbit MFD and knowing the base's coordinates.

More like, is the direction in a non-rotating equatorial reference frame?

 

[tblaxland]

More like, is the direction in a non-rotating equatorial reference frame?

The reference frame rotates but not with the body itself. Rather it rotates with the same period as the precession period (~18.6 years in the specific case of the Moon).