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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:

  1. bytes 78 and 85 of every ME observation directory record are set to binary zero (previously they contained the ME drive status).

  2. 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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Last modified: Thursday, 26-Jun-2003 13:48:32 EDT