3.1.1 The MERIS instrument
MERIS is a programmable, medium-spectral resolution, imaging spectrometer operating in the solar reflective spectral range. Fifteen (15) spectral bands can be selected by ground command, each of which has a programmable width and a programmable location in the 390 nm to 1040 nm spectral range.
The instrument scans the Earth's surface by the so called 'push broom' method. CCDs arrays provide spatial sampling in the across track direction, while the satellite's motion provides scanning in the along-track direction.
MERIS is designed so that it can acquire data over the Earth whenever illumination conditions are suitable. The instrument's 68.5° field of view around nadir covers a swath width of 1150 km. This wide field of view is shared between five identical optical modules arranged in a fan shape configuration (see Figure 3.3 - ). In the calibration mode, correction parameters such as offset and gain are generated, which are then used to correct the recorded spectra. This correction can be carried out either on board or on the ground.
The Earth is imaged with a spatial resolution of 300 m (at nadir). This resolution is reduced to 1200 m by the on board combination of four adjacent samples across track over four successive lines.
The scene is imaged simultaneously across the entire spectral range, through a dispersing system, onto the CCD array. Signals read out from the CCD pass through several processing steps in order to achieve the required image quality. These CCD processing tasks include dumping of spectral information from unwanted bands, and spectral integration to obtain the required bandwidth. On-board analogue electronics perform pre-amplification of the signal and correlated double sampling and gain adjustment before digitisation. The on-board digital electronics has three major functions: it completes the spectral integration, performs offset and gain corrections in full processed mode, and creates the reduced-resolution data when required.
The calibration of MERIS is performed at the orbital South pole, where the calibration diffuser is illuminated by the Sun by rotating a calibration mechanism.
Figure 3.1 - Observation / calibration cycle.
The engineering requirements on the instrument, which have been derived from the Envisat mission requirements, are as follows:
- Spectral range: 390 nm to 1040 nm
· Spectral resolution: 1.8 nm
· Band transmission capability: Up to 15 spectral bands, programmable in position and width
· Band-to-band registration: Less than 0.1 pixel
· Band-centre knowledge accuracy: Less than 1 nm
· Polarisation sensitivity: Less than 0.3%
· Radiometric accuracy: Less than 2% of detected signal, relative to sun
· Band-to-band accuracy: Less than 0.1%
· Dynamic range: Up to albedo 1.0
· Field of view: 68.5o
· Spatial resolution: 300 m at nadir