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The times that will come out of Level 1 processing for the events and the pointing/livetime history will be Mission Elapsed Time, seconds since a reference epoch. This reference time has been specified mission wide as midnight, January 1, 2001. In FITS files the conventional designation of a reference epoch is MJDREF, the Modified Julian Date for the reference. For GLAST this is 51910. It will not change.
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Regarding my note to scisoftlist on the reference time for the LAT, Steve Culp says that 'midnight, January 1, 2001' is meant to be interpreted as 00:00 UTC on January 1, 2001. The MJDREF corresponding to this time is 51910, in UTC. Also, Steve points out that Spectrum Astro is no longer planning to have GPS time converted to UTC onboard, so FSW will not have to handle leap seconds onboard (which was the original understanding), and we will not have to remove them in converting to TT in Level 1 processing. Seth Digel, 11 Oct 2005 |
The problems in checkout 3 arose in part because the tools did not check the value of MJDREF, and just assumed that all times were MET, i.e., with respect to MJD 51910. Unfortunately, this was broken when an FT2 file with a different MJDREF was used.
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