IRAS Explanatory Supplement
II. Satellite Description
Appendix II.1 - Data Compression
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In order to conserve space on the spacecraft digital tape recorder, the 16-bit output of the analog to digital was compressed to an 8-bit value. The current 8-bit data output represented the absolute difference between the current 16-bit analog to digital value for a detector and the same detector's reconstructed 16-bit value. The reconstructed value was derived from the previous 8-bit compressed difference value taken during the preceding data iteration.
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Figure II.Ap.1 a) The format of the compressed 8-bit data word (top); and
b) the meaning of portions of the code used for data compression (bottom). larger largest |
The compressed 8-bit value represented only the 4 to 5 most significant bits of the difference between the current analog to digital reading and the reconstructed value. The 8-bit compressed value consisted of the sign of the difference (positive or negative), three significant bits of the difference called the significand and a shift code indicating the most signficant bit of the difference as indicated in Fig. II.Ap.1.a.
Figure II.Ap.1.b shows the correlation between the shift code, the positions of the 3-bit significand and the most significant bits of the difference. The right column indicates the total number of significant bits of the difference that could be stored in the 8-bit compressed form.
Every 4 seconds the 16-bit value, called the key value,
for a detector was inserted as the first word of a data line.
The key value was provided as a means to moniter and synchronize
the data compression with the ground reconstruction software.