OGSE absolute radiometric calibration in VIS and NIR

Abstract

For Meteosat Third Generation (MTG), the new generation of geostationary meteorology satellites, the Flexible Combined Imager (FCI) instrument will bring a new step in image accuracy. The FCI is an imaging radiometer which will be able to provide Earth images with unprecedented absolute radiometric levels in its whole working spectrum, ranging from visible, near infrared to infrared. ESA has chosen Thales Alenia Space as MTG Prime contractor and responsible for the Assembly Integration and Test of the FCI instrument. The absolute radiometric calibration of the instrument needs to be better than 3%, an absolute accuracy level never reached before. To achieve this challenge, Thales Alenia Space has developed, along with Bertin Technologies, a new Optical Ground Support Equipment (OGSE) dedicated to FCI tests and calibration, which is called MOTA : Multi Optical Test Assembly. Thales Alenia Space has developed a rigorous metrology chain from metrology primary standard to the instrument detection chain, passing through several steps, to ensure absolute radiometric calibration of the FCI instrument. Thales Alenia Space has a long history of experience in radiometric tests and metrology, and this calibration method lies on all our previous experiences gathered in the achievement of numerous programs. This paper will describe historical calibration method, and then explain all the optimisations and evolutions brought by the new method, regarding secondary standard, spectro-radiometric monitoring, environment management, and a focus will be made on importance of the hardware quality. To achieve this challenge, Thales Alenia Space has developed, along with BERTIN Technologie, a new Optical Ground Support equipment dedicated to FCI tests and calibration, which is called MOTA : Multi Optical Test Assembly. Absolute radiometric calibration of the FCI instrument follows a rigorous metrology chain from NIST primary standard to the instrument detection chain, passing through many steps. Thales Alenia Space has developed this metrology chain, covering the whole spectrum from visible to near infrared at 2.5µm. The calibration procedure is based on a secondary standard sphere, which is calibrated in absolute at NIST. Then the calibration is passed through MOTA source using a highly stable spectro-radiometer, with a detector thermally controlled, and a spectral resolution down to 3 nm. The metrology steps are done on the spectro-radiometer in order to validate its linearity and stability. MOTA monitoring is realised using the spectro-radiometer (SR4500 of Spectral Evolution) itself. This is a breakthrough with regards to previous methods which relied on radiometry source with a simpler monitoring based on photodiode. The MOTA monitoring has then a spectro-radiometric extension covering the whole spectrum (from 0.4µm to 2.25µm) with a large dynamic range (radiance is calibrated at all levels from minimal to maximum radiance). Such a monitoring also allows to take into account the variations due to transition of MOTA configuration from air to vacuum, and of lamps ageing. Thales Alenia Space has a long history of experience in radiometric tests and metrology, and this calibration method lies on all our previous experiences gathered in the achievement of numerous programs (MERIS, OLCI, IASI, COROTEL…). This paper will describe the MOTA OGSE design and performances and will show how the final FCI instrument on-ground calibration and characterisation are performed using the Thales Alenia Space expertise in radiometry and metrology. Note: This work is performed under an ESA contract to Thales Alenia Space