Re: [Orekit Users] Question about Frames

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

Le 06/11/2014 17:18, Zachary DeLuca a écrit :
> Hi,

Hi Zach,

> 
> I have ECEF data I would like to convert to J2000. I have read the docs
> and I know how to transform, but I don't know what to use as the source
> frame. Is ITRF the same or very close? And if so how do I determine
> which of the many ITRF configurations is most appropriate for the data I
> have?

ECEF is not really a precise definition, and several people will give
different answers.

ITRF is an ECEF, GTOD and TIRF are other, different ones.

The main driver for selecting one frame or the other is what effects
have been taken into account in your data. If your original data is
really Earth based (for example it was measured from a ground base
system and not transformed), then you should consider your data
naturally includes all the small motions of Earth itself, and hence that
you should use the frame that also includes all these effects and which
is ITRF. If on the other hand your original data is inertial based (for
example related to an orbit) and was converted to Earth by someone
taking some effects into account during the computation and ignoring
other, then you should ask what effects are really included and what are
excluded. Typically, if the conversion was done using precession,
nutation and rotation, but without taking the about 15m pole wandering,
then you should use an Earth frame that takes precession, nutation and
rotation but stops before applying pole wandering : GTOD or TIRF (which
are in fact similar).

If you use ITRF itself, you do not need to bother much about the
convention, just use the latest one (IERS 2010). When all corrections
are applied, the end frames (EME2000 in one end and ITRF in the other
end), the frames are really close, down to centimeter level. The various
conventions are mostly important when you deal with intermediate frames
like MOD/TOD, which may be completely different.


hope this helps,

best regards,
Luc

> 
> Thanks,
> Zach
>