After reprocessing the entire survey time-ordered
detector data, the scans were broken into segments and re-ordered
based on sky position. All scans that cover an area of sky
were grouped into one database called an ISSA field. A field is an
accumulation of all the time-ordered IRAS data that cross over
the region of the sky defined by the field boundaries. The
scans defined within an ISSA field were used to make an ISSA image.