Several calibration entities are time dependent, i.e., change on a
time-scale given by the typical duration of an observation (ksecs).
Each corresponding CAL subroutine receives a time argument which
specifies a point in time represented as ellapsed number of seconds
since the fixed mission reference time
1998-01-01T00:00:00.00 TT = 1997-12-31T23:58.56.816 UTC