The responsivity of the LWS detectors increased with time during each ISO half revolution. After each half revolution a bias boost was performed to return the detector responsivity to the nominal level. In L03 observations where a large range in wavelength was covered the drift causes a gradient in the continuum across the observation.
A routine to correct for the responsivity drift during L03 observations has
been added at the absolute responsivity correction stage in the LIA routine
FP_PROC.
This is based on the illuminator flashes performed before and after
every observation. These recorded the response of the ten detectors to a
standard illumination level and therefore traced the drift in responsivity.
The illuminator flashes from all LWS observations have been combined to
produce an
average gradient in responsivity during each half revolution
for use with the LWS parallel mode observations which did not have their
own illuminator flashes (Section 4.5).
This gradient is used to calculate a drift in responsivity for L03
observations by tying its absolute level to the observation's own
illuminators. This can produce much improved stitching of observations in
a long L03 dataset and means that each observation reflects the true
continuum slope more accurately. The responsivity drift correction is not
applied to L04 observations as they were made up from many repeated scans
over a small range in wavelength and therefore the responsivity drift of
the detectors did not affect their overall shape.