CCSD3ZF0000100000001NJPL3IF0PDS200000001 = SFDU_LABEL
RECORD_TYPE                    = STREAM
SPACECRAFT_NAME                = GALILEO_ORBITER
INSTRUMENT_NAME                = "NEAR INFRARED MAPPING SPECTROMETER"
INSTRUMENT_ID                  = NIMS
OBJECT                         = TEXT
   NOTE                        = "Description of special processing
      applied to 'garbled' NIMS EDRs from the G1 encounter, pointed
      to from label of 'ungarbled' EDRs."
   PUBLICATION_DATE            = 1998-05-01
END_OBJECT                     = TEXT
END

At least two NIMS anomalies occurred during the G1 encounter.  Beginning
with the G1JNHOTMAP01 observation, near Galileo's closest approach to
Jupiter, NIMS data appeared 'garbled', though in a systematic way which
gave hopes of recovery.  Sometime after the G1INNSPEC_01 observation, the
data were still garbled, but no longer in a systematic way.  Subsequent
analysis concluded that the phase 2 NIMS RAM software had halted at that
point, presumably due to radiation, but that CDS was still picking up
garbage out of the NIMS buffer.  (A later reload of the RAM software
restored functionality.) 

The data acquired between the first and second anomalies was mostly
reconstructed, based on an analysis of the RAM processing and the
systematic appearance of the data.  It was concluded that radiation most
likely altered a variable in the RAM so that the formatter (*) lost sync
with the code.  An algorithm was developed involving bit shifts and byte
order re-arrangements, based on the assumption that the formatter was one
step ahead of the code (in a cycle of 4 steps, one for each mirror
position in an RTI).  Since the 'ungarbled' data appeared plausible to
NIMS scientists (especially off-limb data which could be assumed to be
dark) and since there was a reasonable scenario for the behavior of the
RAM software, the NIMS team feels justified in distributing the ungarbled
data with a high probability that it is correct. 

The algorithm mentioned above assumes that all wavelengths were selected,
as in the G1INNSPEC_01 observation.  In such cases, the original data could
be completely reconstructed, except for a couple of bits at one end of the
observation.  But when wavelength editing in the instrument was selected,
as in G1JNHOTMAP01 and most of the other garbled observations, this was
not entirely true.  Due to the interaction of the garbling mechanism and
the wavelength editing code, and depending on the edit pattern selected,
data from some wavelengths lost two bits of significance, and data from
some others were entirely lost. 

(*) See the NIMS instrument paper and VOLINFO.TXT for background to this
discussion, and see SPECPROC.TXT on the G1 Cube CD-ROM (forthcoming) for
further details of the algorithm and a complete description of limitations
on data accuracy for each of the 11 ungarbled observations.

[Prepared by R. Mehlman, 01 May 1998]