Calibration is an essential part of a space instrument. Without calibration the data from the instrument cannot be interpreted and science is not possible. TNO has a leading position in providing calibration technologies for earth observation instruments. These technologies involve onboard calibration hardware for space instruments as well as on-ground calibration of components (MERIS diffusers) or complete instruments (SCIAMACHY, OMI, GOME-2).
As space instrument designs can be different so is the calibration. That means that in many cases a dedicated stimulus must be designed and built. For the on-ground calibration of a space instrument TNO has developed several optical stimuli to be able to acquire complete knowledge of the instrument behavior. Main aspects that are calibrated are:
Many of the instrument characteristics depend on the environment in which the instrument is operated. In case of space instruments this means in vacuum, within a certain temperature range and at zero gravity. Calibration is performed in a dedicated Thermal Vacuum Chamber at the Van Leeuwenhoek Laboratory of TNO in Delft.
GOME-2 was one of the most extended calibration campaigns within TNO. The measurements took more than five months. Within this period the instrument was calibrated at three different optical bench temperatures and two different detector temperatures. Special effort was put into the radiometric calibration and the slit-function characterization. For the radiometric calibration NASA and NPL were invited to calibrate their sources alongside of the TNO sources. The slit-function was measured using a dedicated source, which enabled a characterization at sub-pixel level. This was a large improvement over the previous method were only a few points of the slit-function where measured. This improved slit-function knowledge is a major improvement for the retrieval of science products. TNO also collaborated with the scientist of the University of Bremen for a three week measurement campaign with their Cat-Gas set-up. With this set-up the instrument 'looked' at a controlled atmospheric condition to characterize instrument behavior for different atmospheric compositions. These measurements will be used for retrieval algorithms.
Diffusers are at the heart of the radiometric calibration of the instrument. In orbit, the instrument uses the diffuser to look at the sun to acquire the sun irradiance for absolute calibration. The accuracy of the calibration is predominantly determined by the accuracy of the diffuser characterization.
During on-ground calibration of the SCIAMACHY instrument, an unexpected noise like feature was superimposed on the spectrum when using the diffuser. These so called 'spectral features' were caused by a speckle phenomenon in the entrance slit of the spectrometer. An extra diffuser was implemented in the instrument which could be rotated with respect to the sun to average out the features. TNO has developed a new type of diffuser called the Quasi Volume Diffuser (QVD), which produces smaller speckles and has a better scattering performance. This type of diffuser is implemented in the OMI and GOME-2 instruments. TNO has also developed a model to predict the feature behavior of a diffuser design and is currently looking at even more advanced diffuser designs.