Since the source and background positions are not observed simultaneously,
detector drifts influence the differential signal between source and
background. Drifts are present in most C100 measurements even at very low
flux level. The C100 drift curve consists of a highly non-linear transient
period in the beginning 
, followed by a slowly changing
part 
.
In addition, the steady and slow increase of the detector's response due
to cosmic hits along the orbit also changes the signal level. A maximum
responsivity increase of a factor 2 is observed in a period of 8 hours
for P3 and C100. For C200 the long term drift is a factor 1.3 over that
period.
Chopping eliminates both the long term drift component and the response change. However, the change due to the chopper mirror introduces short term transient drifts within each chopper plateau even at very low flux level. These transients cause an underestimation of the difference signal. Since the number of integration ramps per chopper plateau is usually low, it is difficult to fit an empirical drift curve to the data points.
Staring observations last usually 64-128 sec, and are more affected by the slowly changing part of the drift curve rather than the transients. Correction for these kind of drifts should be possible, because there are generally enough (8-16) integration ramps to fit an empirical drift curve to the data points. Shorter staring measurements of 32 sec or less are more difficult to correct.
A raster map is a long sequence of staring observations, where the dominant drift components are the long term drift and the detector response change. In principle the large number of ramps makes it easy to fit and subtract an average drift curve from the whole measurement provided that the raster map does not contain strong sources.
Oversampled maps are raster maps where on each raster position one or more sweeps with the chopper in sawtooth mode is performed. Besides the long term drift, which is similar to the normal raster map, a transient drift may be introduced when the chopper jumps from the sweep's end position, observing the on-source, to the start position, looking to the off-source area.