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Re: [Orekit Users] HEE or GSE Frames?
simon.spoerri@students.fhnw.ch a écrit :
Dear all,
Hi Simon,
I am developing a webbased visualization tool of in-situ data gathered by
various satellites. Now fyi i am not very proficient in astronometry or
astrophysics... Nevertheless I'd like to use orekit to consolidate
coordinates
from different coordinate systems provided by the data sources which
use mostly
HEE (Heliocentric Earth Ecliptic) or GSE (Geocentric Solar Ecliptic)
coordinates. Now i was wondering if there already are some helper classes or
predefined transforms that could be used for transformations to ICRF or GCRF
respectively from the aforementioned coordinate systems
Unfortunately, these coordinates systems are not supported (yet) by Orekit.
The only global solar system frame we have is ICRF, located at solar
system barycenter, and I even think it's orientation is slightly wrong
(the famous 18 mas offset between GCRF and EME2000 striked again ...).
By the way, could someone confirm me that GCRF and ICRF should be
aligned? If so, I'll fix our ICRF.
Implementing a new frame is mainly a matter of implementing the
TransformProvider interface which only declares one method:
getTransform(AbsoluteDate date) between the new frame and one existing
frame chosen as its parent. All the rest (inserting the frame in the
tree, providing cache and thread safety, combining transforms) is
already done by the rest of the framework.
Considering Heliocentric Earth Ecliptic, I would select ICRF as its
parent frame so there are no translations (both centered at solar
system barycenter). In order to compute the rotation, we need the
Earth motion, which is already available in Orekit using JPL
ephemerides (see the package bodies). So the first direction , from
solar system barycenter to Earth is easy to get in ICRF. The second
direction (normal to the ecliptic) can be computed at each time using
the cross product of position and velocity, but this would provide an
instantaneaous ecliptic. I don't know if the precise definition of
this frame use an instantaneous value or some mean value. If a mean
value is used, then there is probably a simple model for it, perhaps
even a fixed direction.
Considering Geocentric Solar Ecliptic, it seems to me it could be
defined either by a similar process as HEE but starting from GCRF
instead of ICRF (and reversing the X direction, of course), or defined
starting from HEE and performing a translation towards Earth and a 180
degrees rotation.
hope this helps,
Luc
help is very appreciated!
cheers,
Simon
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