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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. 3.4.2.1):
| Parameter | Description | Frame | Byte | Bit | Width | Commutation |
|---|
| | D190 | Prog # in slot
1 | 84 | 2 | 0 | 16 | 1 |
| D191 | Prog # in slot
2 | 84 | 4 | 0 | 16 | 1 |
| D192 | Prog # in slot
3 | 84 | 6 | 0 | 16 | 1 |
| D193 | Prog # in slot
4 | 84 | 8 | 0 | 16 | 1 |
| D194 | Prog # in slot
5 | 84 | 10 | 0 | 16 | 1 |
| D195 | Prog # in slot
6 | 84 | 12 | 0 | 16 | 1 |
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
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