NEW DEFINITION OF ARRAY POSITION MONITORS ON ME FOTs
The position monitors, bytes 30/31 of each ME observation directory record, give
the orientations of the 2 array halves during the time period covered by that
record. Any change in array orientation therefore constitutes an observation
boundary. A few minutes' worth of data have sometimes been omitted from the FOT
because of the many status changes associated with an array swap. A change in the
FOT production software was implemented on April 1st 1985 to minimise the data
loss:
- bytes 78 and 85 of every ME observation directory record are set to binary
zero (previously they contained the ME drive status).
- each position monitor is set to binary zero, if all the Argon HT's for the
corresponding array half are below their nominal values.
Change a. has no consequences for observers' analysis systems because these two
bytes are irrelevant and should never be used by any software. Change b. has the
consequence that the value zero (exactly) for a position monitor means "unknown
and not useful". It does not mean any particular offset. If this change is
introduced into institutes' FOT decoding software, then "backward compatibility"
is assured for all FOTs generated before April 1985. There is no need for the
analysis software to be aware of the FOT generation date.
In this note all byte offsets are counted from zero, for consistency with the FOT
Handbook. Note that bytes 30 and 31 correspond to array halves 2 and 1
respectively.
J R. Sternberg
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Page authors: Lorella Angelini Jesse Allen
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