ISSA Explanatory Supplement
APPENDIX B

Compression Algorithm

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The first step in processing the IRAS time-ordered scan data into the ISSA images was to smooth and resample the time-ordered detector data such that all detectors were sampled with a 0.5-second period. This resulted in a spatial sampling period of 1.93' at the IRAS scan rate of 3.85' s-1. A symmetrical Lanczos (1) single smoothed filter kernel was convolved with the full resolution detector streams to produce the smoothed output data. 

K(\tau)\cases{\displaystyle\left( {{\sin(\pi\tau)\over{\pi\tau \right)^2,
	&for -{1\over2 <\tau<{1\over2\cr\strut\displaystyle
	\;0\;, &otherwise.\cr \eqno(1)
where \tau is time in seconds.

The numerical values of the normalized filter kernels are shown in Table B.1. Convolution of these kernels with the 16, 16, 8 and 4 samples per second data from the 12, 25, 60 and 100 µm detectors, respectively, smoothed the data so that every eighth, fourth, or second sample were retained, producing the desired two samples per second output sets at each wavelength.

These compressed detector data were also used to create the IRAS Zodiacal History File (ZOHF).

Table B.1 Filter Kernels
tau
(sec)		12
µm		 25
µm		60
µm		 100
µm
-7/16 0.00268 0.00268   
-3/8 0.01247 0.01247 0.02454  
-5/16 0.03065 0.03065   
-1/4 0.05611 0.05611 0.11221 0.22384
-3/16 0.08515 0.08515   
-1/8 0.11223 0.11223 0.22442  
-1/16 0.13148 0.13148   
0 0.13845 0.13845 0.27687 0.55231
1/16 0.13148 0.13148   
1/8 0.11223 0.11223 0.22442  
3/16 0.08515 0.08515   
1/4 0.05611 0.05611 0.11221 0.22384
5/16 0.03065 0.03065   
3/8 0.01247 0.01247 0.02454  
7/16 0.00268 0.00268   

References

Hamming, R.W. 1977, Digital Filters, Prentice-Hall, Inc., Englewood Cliffs, NJ.

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