The Science Data Analysis System (SDAS) at JPL received the scientific and housekeeping data taken by the satellite from the ground operations facility at Rutherford Appleton Laboratory in Chilton, England. The data were obtained and processed in units of a Satellite Operations Plan (SOP) containing 10-14 hours of data (Section III.C.1). A single SOP was further divided into 10-60 observations or scans, each of 3-50 minutes duration. Survey observations obtained the data for the all-sky survey, while calibration observations were made to determine, monitor and verify the survey photometry.
The survey strategy and observation constraints produced scans that were generally along meridians of ecliptic longitude. At the ecliptic plane, the scans followed meridians exactly, while at higher latitudes, deviations from meridians could become pronounced. The rectangular aspect of the infrared detectors (Section II.C.4.) combined with the survey strategy means that there are important differences between quantities, such as source size and positional uncertainties, measured in the in-scan direction (roughly ecliptic latitude) and those in the cross-scan direction (roughly ecliptic longitude). In extreme cases, whether a source is found in the point or small extended source catalogs could depend on whether it was extended in the in-scan direction.