3.1.2 Instrument Concept
The instrument acquisition and processing chain can be separated into four sub systems:
- The Instrument Optics
- The Detection Focal Plane
- The Video Electronic Unit
- The Digital Processing Unit
Figure 3.2 - Instrument Concept.
22.214.171.124 Instrument optics
The instrument has a field of view of 68.5° divided between five identical cameras, each having a field of view of 14°. The cameras are arranged in a fan shape configuration in which the fields of view overlap slightly (see Figure 3.3 - ). The modular design has been specifically selected for MERIS to ensure high optical image quality over a large field of view. The output of each camera is processed separately in an analogue and digital processing unit.
Figure 3.3 - Arrangement of optical modules, folding mirror and Earth viewing windows.
The MERIS optics consists of an external window, a folding mirror, an off axis catadioptric ground imager and a spectrometer. A window scrambles the incident polarised light coming from the Earth, making the instrument less sensitive to changes in light polarisation. The window also plays the role of protecting the rest of the optical elements.
The ground imager consists of a three lenses aperture group, a concave primary mirror, a convex secondary mirror cemented on the third aperture lens, and a field lens cemented on the spectrometer. The dispersive element of the spectrometer is a low grooves density concave reflecting holographic grating. A blocking filter is inserted in the corrector block to suppress the second order of the grating.
126.96.36.199 Detection Focal Plane
The camera's detectors are CCD arrays specifically developed for MERIS. Thinned back side illuminated CCDs have been selected which offers the required responsivity in the blue part of the spectral range. The camera swath is imaged along the CCD line while the light dispersion takes place along the CCD column. Each pixel is 22.5 micron square. The CCD covers the spectral range with a nominal 1.25 nm spectral sampling interval. The CCD basic layout is illustrated below. The CCDs operate in a frame transfer mode. The frame period is 44 ms. After integration, the charges are rapidly transferred from the imaging zone to the storage zone. A frame transfer is followed by a new integration period in the imaging zone, while the store zone is read out.
Figure 3.4 - Basic layout of the CCD.
The programmed spectral width is obtained by summing the necessary number of CCD lines in the shift register. This process is termed spectral relaxation. The CCD lines which fall outside the 15 selected spectral bands are dumped at shift register level.
The width and position of the MERIS spectral bands can be modified in-flight by programming the CCD. Apart from allowing the selection of different sets of spectral bands during the mission, the CCD programming also serves the purpose compensating for any spectral drift occurring during launch or in flight.
188.8.131.52 Video Electronic Unit
Each camera has a dedicated image processing chain. The analogue processing is undertaken by the Video Electronic Unit, whose functions are:
- To extract the useful signal in 15 selected bands
- To compensate the offset variation by using the dark reference pixels
- To amplify the signal
- To digitize the video signal on 12 bits
The signal amplification is done by selecting one of the 12 fixed gains defined in the range 1 to 3.75. The selection of the amplification gain is done separately for each spectral bands. Thus the saturation level of any band can be optimised for the purpose of the band. For instance a spectral band used only for ocean applications can saturate over clouds, leaving the full 12 bit digitisation for the useful dynamic range.
184.108.40.206 Digital processing Unit
The digital output of the Video Electronic Unit is subsequently processed by the Digital Processing Unit in three major steps:
- Complete the spectral relaxation up to the required bandwidth
- Subtract the offset components and correct the gains in full processed mode
- Reduce the spatial resolution of the data to 1200 m for the global mission
Offset and gain correction are based on coefficients computed during the calibration sequences. These coefficients are stored on board as well as sent to the ground. The instrument design offers the flexibility to have these corrections applied either on board or on ground. In the latter case offset, smear and gain correction are bypassed in the on board processing flow. The overall instrument acquisition and on board processing is illustrated in MERIS instrument: camera (left) – spectro-imager camera (centre) – CCD (right).
The CCD is coupled to a Peltier cooler stabilizing the CCD temperature to -22°C.