Last revision November 27, 2001
The GRO clock, in the form of a 3-element array of 2-byte integers,
was found to be wrong for the early part of the mission. On TJD 8798
(June 25, 1992), the GRO clock was changed in two steps. Therefore,
corrections to the resulting times are required for the early part of
the mission after converting the three integers to truncated Julian
day (TJD) and seconds of day (SoD) in the usual simple way as
described below.
NOTE: these corrections have been made in all FITS files and no
further time corrections are required!
The clock was changed in two steps, first by exactly 2 seconds and
then by 0.042144 seconds. Therefore, the following corrections
would need to be applied to any uncorrected times:
If TJD/SoD (after usual simple conversion) is less than 8798/39208.5478,
then 2.042144 seconds should be subtracted from the time.
If TJD/SoD (after usual simple conversion) is greater than 8798/39208.5478 and
less than 8798/39325.3938657, then 0.042144 seconds should be subtracted
from the time.
If TJD/SoD (after usual simple conversion) is greater than 8798/39325.3938657,
then no adjustment is needed.
The meaning of the three GRO clock words (2 bytes each and 8 bits per byte)
is the following:
The least significant word is subseconds; units are 1/64000 second.
Valid range is 0 to 63999.
The middle word together with the least significant bit of the most
significant word inserted as the most significant bit of a number with
a total of seventeen bits is seconds of day (SoD); units are seconds.
Valid range is 0 to 86399.
The middle fourteen bits of the most signicant word (i.e., omitting the
least significant bit and the most significant bit) is the truncated
Julian day (TJD), which is the integer part of the Julian date minus
2440000.5; units are integer day numbers.
Valid range is for BATSE is 8361 to 11692.