3.2.2 Onboard Calibration Hardware
One of the most severe requirements on MERIS is posed by calibration. A very accurate band-to-band calibration relative to the solar spectral irradiance is required for ocean colour applications. The imaging principle of MERIS imposes, in addition, a need for detector elements to be accurately normalised.
The basic hardware requirements for an onboard calibration system offering both a uniform reference signal over a large field of view and a stable absolute spectral reference signal has led to the preferred solution which utilises diffuser plates illuminated by the Sun.
The calibration hardware is implemented on a selection disk. A stepper motor allows the selection of any of the five positions of the disk as required by the instrument mission requirements.
A diaphragm is introduced in the field of view.
Radiometric Gain Calibration
A Sun-illuminated white diffuser plate is inserted into the field of view of MERIS at the cross-over point of the five cameras' fields of view. The diffuser provides a reflectance standard across the entire spectral range and field of view. In this way a full aperture instrument calibration which follows the same optical path as in the observation modes is realised. During the calibration the signal is recorded for 512 frames. Each frame is corrected for offset and smear components. The averaged signal is then divided by the BRDF of the diffuser obtained from on-ground characterisation and stored onboard, yielding the absolute radiometric gains. If the on-ground calibration mode has been selected, the onboard processing is limited to the averaging over the 512 frames. The gain coefficients are calculated on ground.
Figure 3.6 - Calibration hardware.
Diffuser Degradation Monitoring
The radiometric diffuser is exposed to the Sun for a cumulated period of about 1 hour during the MERIS lifetime. Some limited degradation caused by radiation exposure may be expected. A second diffuser is therefore provided to evaluate changes in the BRDF of the commonly used diffuser. This diffuser is used infrequently and thus does not degrade as quickly as the first diffuser. The ageing of the radiometric diffuser is monitored by comparing the data acquired with both diffusers.
This is achieved by using the wavelength diffuser featuring well-characterised absorption minima. MERIS is reprogrammed to sample adequately the absorption features. As a result, the spectral distribution law is obtained, which relates spectral bands to their wavelengths. It is also envisaged to use as an alternative the solar Fraunhoffer line absorption seen when observing the radiometric diffuser. The wavelength diffuser is inserted into the field of view of MERIS. A precise wavelength calibration of the O2A absorption band is performed by comparing the diffuser measurements of the O2A band, normalised by a baseline through adjacent bands, with pre-calculated values.
The Earth and Sun optical paths are blocked by a shutter.
Following extensive environmental tests, SpectralonTM has been selected as the diffuser material. Diffusers manufactured with this material offer the required uniformity over the field of view and a remarkable stability. A diffuser plate doped with a rare Earth oxide is used for the spectral calibration.
The onboard calibration sequence calculates the calibration coefficients for all FR pixels of the 15 spectral bands, stores them on board for further use during full processed observation mode.