Re: [Orekit Users] Veis 1950 vs. B1950

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

Le 19/10/2011 23:04, Rick Adams a écrit :
> Orekit team (Luc?),

Hi Rick,

I am not the only one in the Orekit team, only the most talkative ;-)

> 
> Could you confirm whether or not the Veis 1950 frame is the same as the B1950 coordinate system?

First, I am sorry for this late answer, I simply forgot.

The Veis 1950 frame is not the same as the B1950 frame. It is a legacy
frame used by some reference programs we know of, mainly at CNES. Even
there it has different incompatible definitions. We opted for the
following one: the Veis 1950 frame as implemented in Orekit and in
several other programs is based on an Earth frame using classical
equinox-based paradigm, namely the GTOD frame and rotated back around
the Z axis using the reverse of the Veis sidereal angle. This means it
is close to an inertial frame, with  pole motion added and has the
interesting property (at the time it was heavily used) that transforming
between this frame and the Earth frame was a simple pure Z rotation.

I think the B1950 frame on the other hand is a really inertial frame,
using classical equinox-based paradigm and frozen at 1950.0 epoch. If
this definition is good, then you can get this frame by using something
like:

  Frame gcrf  = FramesFactory.getGCRF();
  frame mod   = FramesFactory.getMOD(true);
  Frame b1950 = mod.getFrozenFrame(gcrf,
                                   AbsoluteDate.FIFTIES_EPOCH,
                                   "B1950");

Beware, this should be validated!
If someone else could confirm this definition works, we would add this
frame to the set of Orekit predefined frames so it is easier to retrieve.

best regards,
Luc

> 
> Many thanks!
> 
> Rick Adams
>