Jump to the updates of 20 Nov 2007
Pointing and live time history (FT2, LS-005)
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5/16/07: We'll stick with SC_DATA. This means that the Science Data Products File Format Document will need to be updated.
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Here is an additional item added on 20 Nov 3007
6. Attitude quaternions in FT2?
(Digel) In the FT2 file the attitude of the spacecraft is expressed via the directions (in RA, Dec) of the z and x axes. Gleam and the astro package in general use quaternions to represent the attitude, and the 'Magic 7' information from the spacecraft does express the attitude via a quaternion. Some users of the Science Tools are advocating that FT2 be extended to include the quaternions in addition to the RA, Dec of the x and z-axes.
This is an issue primarily of convenience, although Toby has noted that precision errors can decrease the accuracy of the attitude when the x and z-axis directions are derived from a quaternion. The errors in precision are small and might be most relevant to the case where Gleam is driven by an ASCII equivalent of a pointing history.
In terms of the Science Tools, I don't think that anything is fundamentally broken with how we are expressing the attitudes in FT2 files, and perhaps too often we consider FT2 files as input files for Gleam (to specify the attitudes) rather than as a pointing/livetime history useful for calculating exposures. That said, I don't have any objection to including the quaternion in the FT2 files.
Would single precision floating point provide adequate resolution?
I don't see any downside to including attitude quaternions, other than increased sizes for FT2 file, but I'm likewise not sure whether I see any big advantages either.