Scans are observing modes which also perform observations on a regular one dimensional grid, like rasters, but which are differential measurements. The chopper is used for oversampling between individual spacecraft positions, where chopping is performed along the scan line. This means that all these observations are performed using the chopper and several spacecraft positions, which is in contrast to the staring raster mode. The same celestial position is observed during several raster pointings allowing for elimination of temporal changes in detector response (cf. Fig. 17).
The scanning direction is parallel to the spacecraft Y-axis. Its orientation with respect to equatorial coordinates depends on the date of observation and will not be known before the observation is scheduled. As a fixed orientation of the scan line implies strong scheduling constraints due to a fixed observing time, a tolerance angle for the actual scan line as large as possible has to be provided (cf. Sect. 6.2).
Scans are used to obtain flux density profiles, or to observe bright sources with Nyquist sampling.
Maps are composed of a series of parallel scans which may overlap and are dedicated to observe extended sources or complex far infrared sources with the optimum spatial resolution. As in scans, chopping is only performed in scan direction, the sampling in the orthogonal direction is obtained by stepping the spacecraft.