4 Final Report - Emits - ESA

4 Final
Report
4.3.3.8 Calibration
Challenging absolute (Goal: 1%, Threshold: 2%) and relative (0,2% inter-band) radiometric accuracy
requirements impose careful calibration of absolute and inter-band offsets and gains. The calibration
process shall enable to recover the scene reflectance values from instrument measurements. This
implies a complete calibration of radiances/irradiances according to following calibration process:
• Corrections of detectors systematic errors (Offset, Dark signal, Dark Signal Non Uniformity
(DSNU), Pixel response non uniformity (PRNU), Dead/bad pixels)
• Correction of absolute value and variation of the overall detection gain (optical transmission,
detector efficiency, electronics gain).
• Possibly, stray light correction based on ground characterisation
To reach the above accuracy, in-orbit calibration is required, using a calibration device with properties
well characterised on the ground and stable over the mission lifetime.
For PAN & UV-VNIR channels, a retractable Sun diffuser will be used. The preferred candidate
diffuser technology are QVD (Quasi Volumic Diffuser) and perforated plates for their low sensitivity to
GEO environment. Since sighting the Sun with the instrument is not possible for thermal reasons and
full pupil diffuser implementation at telescope entrance is not feasible because of large aperture, two
complementary Sun diffusers are proposed:
• A full pupil diffuser implemented near the intermediate focus, sighting the Sun through a
window in the baffle. This diffuser does not monitor M1 & M2 possible degradation.
• A small pupil diffuser implemented near the M2 spider to calibrate M1 & M2 transmission
(local degradation of the mirrors outside the small pupil area covered by the diffuser is not
monitored).
In both cases, calibration is performed during the 4h interruption of measurements around midnight,
thus do not interfere with the mission imaging capability. The Sun avoidance manoeuvre performed to
keep the Sun-to-LoS angle larger than 30 deg is used to sequentially orient each Sun diffuser towards
the Sun. The Sun diffuser LoS (defined by the window in the Sun shield) the has an offset from the
instrument LoS equal to the magnitude of the Sun avoidance manoeuvre, and the spacecraft is rotated
about the instrument LoS, as illustrated in the following figure.
GEO Sun diffuser
θman − βsun
North
Figure 4.3-14: Geometry of Sun diffuser sighting during Sun avoidance manoeuvre
For IR channels, calibration can be made against two stable radiometric references, a small black
body mounted on a flip-flop mechanism or periodic sighting to the cold space
Doc. No: GOC-ASG-RP-002 Page 4-41
Issue: 2
Date: 13.05.2009 Astrium GmbH
βsun
Equator