A.3 Failure Modes

The background swath fitting process is considered to fail a particular Pass 1 swath if any of the following conditions apply.

• Fewer than 20 valid background pixels exist in the swath.
• The first two valid background pixels in the swath are not continuous.
• There are no data gaps in the swath (continuum image only).
• The starting pixel for the IOR is less than spatial pixel number 3 in the swath (continuum sources only).
• The first and last pixel with usable background fluxes are on the same side of the IOR (continuum sources only).

When a solution for a Pass 1 swath fails any of these tests, the background solution is set to zero for the entire swath. If this occurs for an isolated swath, an interpolated solution based on the two neighboring Pass 1 swaths is substituted for the original solution. If two consecutive Pass 1 swaths, or a total of five, have null values based on these conditions, the background arrays for the entire image are set to zero, and BCKGRD has ``given up" on the image. This occurs for an exceedingly small number of images (e.g., about 13 out of 12,898 images in the GSFC high-dispersion archives), usually as a consequence of a major portion of the image being missing. This condition is documented in the processing history log.