[About EXOSAT] [Archive] [Software] [Gallery] [Publications]

MHER6 OBC Program Anomaly

Under certain, rare circumstances the packet reference times (PRT's) contained in I6 packets can be in error by 1 software cycle (31.25 msec) when compared with their expected values. Normally the difference in PRT's between two I6 packets during an observation is constant and given by:

Workspace Parameter (4) X 16384 X No. of Samples/packet

Energy Sampling Rate (Hz)
This anomaly can only occur when MHER6 is executed in slots 5 or 6. The slot numbers in use are contained within the Housekeeping data and can be obtained using (see FOTH Ch.

D190Prog # in slot 18420161
D191Prog # in slot 28440161
D192Prog # in slot 38460161
D193Prog # in slot 48480161
D194Prog # in slot 584100161
D195Prog # in slot 684120161
Note that MHER6 is application program number 70 in this context and not 105 as on FOT's. The data contained within the affected packets is unaffected i.e. the PRT calculated using the above formula is the one to use in all data processing applications and not the PRT read from telemetry. The number of records with incorrect PRT's when MHER6 is executed in slots 5 or 6 is dependent on OBC load and is typically ~5% of the total.

In principle, a similar error can occur if any high time resolution program eg. GDIR, MDIR (or MHER7) is executed in slot 5 or 6 and, these programs will therfore in future be executed only in program slots 1-4.

A.N. Parmar

[About EXOSAT] [Archive] [Software] [Gallery] [Publications]