The offset is the raw signal [V] in the limit of an exposure time of zero. Two versions of the offset are given, which should have negligible differences.
- Official – Nominal version, derived with ICM & L01b processor using radiance background measurements with exposure times between 210 and 540 ms.
- Beta-nominal – Development version, using the same data as nominal version.
Until orbit 1740 (2018-02-13 00:37:44 UTC), the background measurements were performed during eclipse looking at the Earth. Afterwards all radiance background measurements are performed with the folding mirror closed. At the same time the duration of the background measurements has been reduced, and therefore the offset and dark-flux derived with the beta-algorithm are performed every 45 orbits during E2, instead of the nominal 15 orbits. The current beta-algorithm is part of release 2 of the ICM & L01b software, but applied on 15 orbits.
Starting from orbit 9388 (2019-08-06 02:20:06 UTC), the exposure times of the radiance measurements have been reduced to obtain smaller ground-pixels. Therefore, the radiance background measurements are performed with exposure times between 166 and 840 ms.
The offset deviation is expressed as a percentage of the 0.21 V reference signal due to spectral radiance at 2313 nm (without atmospheric absorption), albedo 0.05, solar zenith angle 70 degrees, exposure time 0.54 s.