Re: [Orekit Users] Reference frame of a S/Cs attitude

[Raffaello.Lorenzo.Mancini@esa.int]

Dear Luc,

first of all, thanks a lot for your quick and helpful answer! Is still 
have a few questions.

> If you want to get just one-time values, yes your approach is correct.
> If on the other hand you want to get the values on a regular time grid 
> (say to produce some ephemeris), then you should rather move all the 
> computation statements in a handleStep method that will be called 
> automatically for you during the propagation. That is I would 
> recommend using master mode instead of slave mode (see 
> <https://www.orekit.org/forge/projects/orekit/wiki/Propagation> and 
> see 
> <https://www.orekit.org/forge/projects/orekit/wiki/
> PropagationTutorial#Master-mode>).

I will do this once I get all the geometry right. I will create a custom 
pass detector that checks if a list of unequalities described as strings 
(e.g. "e > 5.0") are all true for the S/C to be considered "visible". For 
now I've written the pass detection code myself.

> > Could anyone please give me a hint at how to get the S/C->G/S 
unit-vector
> > in the frame of reference of the S/C with it's attitude applied 
(parameter
> > 4)? I would like to use this vector to extract two angles relative to 
an
> > antenna in order to evaluate the antenna gain in direction of the G/S. 
My
> > basic problem is that I don't really know how I can get the S/C-bound
> > Frame that is oriented according to the attitude of the S/C. The only
> > S/C-bound frames I can produce are TNW, VNC, QSW/VLVH and VVLH which 
are
> > all not related to attitudes.
> 
> The simplest way to do it is as follows, starting from a 
> SpecacraftState instance:
> 
>   final SpacecraftState state = ...;
> 
>   // compose transforms from ground station to spacecraft
>   final Transform inertialToSC = state.toTransform();
>   final Transform gsToInertial =
>     gsTopoFrame.getTransformTo(state.getFrame(), state.getDate());
>   final Transform gsToSC = new Transform(state.getDate(),
>                                          gsToInertial,
>                                          inertialToSC);
> 
>   // ground station is at position (0,0,0) in ground station frame
>   // see where this point is in spacecraft frame
>   Vector3D gsSeenFromSC = gsToSC.transformPosition(Vector3D.ZERO);
>   Vector3D unitVector = gsSeenFromSC.normalize();

Does this code take into account the attitude specified in the propagator? 
What attitude is used when no attitude is specified in the propagator 
constructor? In general, are attitudes only handled at the level of the 
propagator?

I think my understanding of attitudes is still quite rough at the moment, 
especially when it comes to specifying degrees of freedom that are not 
explicitely specified by the law. How would I for example specify the 
following attitudes?

1. Inertial attitude with a specific Rotation at a specified point in time
2. Z-axis pointing nadir with the x-axis colinear to the earth rotation 
axis
3. Target pointing to some earth G/S with additional requirements on 
either x- or y-axis

> > In order to get the S/C-G/S-Sun angle (parameter 3), I guess it would 
be
> > enough to get the unit vectors a = G/S->S/C and b = G/S->Sun in a 
common
> > reference frame (e.g. the G/Ss topocentric frame) and then set the 
angle
> > to be equal acos(a.normalize().dotProduct(b.normalize())). Is that
> > correct?
> 
> It would not be accurate for small angles, as acos behaves badly near 
> 0. You should rather use Vector3D.angle(a, b)

Ah, I haven't thought about that I've changed it in my code.

Best regards,
Raffaello Mancini

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