Re: [Orekit Developers] IERS 2010 & frames configuration

[Luc Maisonobe <Luc.Maisonobe@c-s.fr>]

Le 08/08/2012 18:21, Tanguy Yannick a écrit :
> Hi all,

Hi Yannick,

First of all, I'm sorry for the answer delay. I have been busy doing
other things recently.

> 
> In this mail, I try to sum up some some work we made on the IERS 2010
> frames. We started this development with the previous conception
> (each Frame has its own Transform) and before merging it in the new
> conception (see
> https://www.orekit.org/wws/arc/orekit-developers/2012-07/msg00009.html),
> we would like to discuss some technical aspects. Eventually, we wish
> to contribute this to Orekit before OREKIT 6.0 is released, and we'd
> like to know if you'd agree with this conception.
> 
> ** Why do we developed IERS 2010 convention ? ** OREKIT does (or
> did..) not implement every detail of the IERS convention : - UT1-UTC
> and pole motion were interpolated with a linear method (it's not the
> case now) - the libration correction was not taken into account ; it
> affects UT1-UTC and pole coordinates (U,V) - the dX & dY angles are
> not corrected thanks to nutation
> 
> -> So we had some difficulties to validate OREKIT frames
> transformation in regard of our reference tools which implement IERS
> 2010. Furthermore, some users want to configure frames transformation
> very precisely.
> 
> ** Frames configuration **
> 
> The implementation we propose allows the user to configure each part
> of the terrestrial-celestial transformation (precession & nutation,
> s' correction, libration correction, tidal models) using a
> FramesConfiguration object as a Frame attribute.

Is there a default configuration or should user always specify the
corrections they want to apply ?

> The main idea is
> that the user can configure the whole frame tree, by applying
> different conventions.

Is it possible to use several configurations at the same time and get
the transform between them? One use case that come to my mind is when
users want to assess the effect of some correction: they would build one
frame with the correction applied and the same frame without the
correction, then the transform between these two frames would be the
effect they want to study.

> The configuration contains three elements : -
> a polar motion model : it depends on S' correction, libration
> correction and the tidal corrections - a diurnal rotation model : it
> depends on libration correction and the tidal corrections. - a
> precession & nutation model : at the moment, the IERS 2003 precession
> & nutation is implemented

I fear too flexible configuration could lead to user errors. We have
already seen this with the good old MSLIB: people could mix effects
almost arbitrarily (using one theory with the constants from another).
The current frames designed has carefully tried to avoid asking people
to become frames transformation experts to use the frames. They
currently need to know only the frame name, not what theory is used
underneath and not how the frames are related to each other.

I would very much like to have this feature (frames accessible to
non-specialists) still available. does you design support this?


> 
> Each model correction respects a specific interface, so new models
> could be developed (for future conventions), or simplier models can
> also be developed ; for instance, there would be a NoLibration class
> to be compliant with old OREKIT transforms.
> 
> By default, each user will get frames compliant with the new IERS
> 2010 convention. But if the user wants to modify the frames tree
> convention, he can either create a frames configuration or get a
> predefined configuration (thanks to a FramesConfigurationFactory).

Could you explain this a little ore please?

best regards,
Luc

> 
> ** Integration in the new conception **
> 
> It works well in an older version of Orekit (few months ago) but we
> now have to adapt it to the new conception : we have yet to modify
> the various TransformProvider to use the different models (libration,
> precession & nutation effects, S' correction, tides correction) and
> the FramesFactory to use Frames Configuration.
> 
> If you want more information, we can eventually attach some class
> diagram to Jira.
> 
> Best regards,
> 
> Yannick
>