1.1.3 Principles of Measurement
MERIS is a passive imaging spectrometer, which performs simultaneously spatial and spectral imaging of the Earth, by looking in the nadir direction.
The most outstanding characteristics of MERIS, detailed below, are:
· MERIS is a push-broom instrument.
· The InFOV is 68° +1°/-0.1°, which equates to a swath width of 1150 km centred around the subsatellite point.
· The 15 observed spectral bands are all programmable in position and width.
· Two spatial resolutions can be selected.
· Onboard processing can be performed on the image data.
· The polarisation sensitivity of MERIS is very low.
· MERIS has a high radiometric and spectrometric performance.
The InFOV is divided into five segments, each of which is imaged by one of the corresponding five cameras. A slight overlap exists between the FOVs of adjacent optical cameras. An area Charge-Coupled Device (CCD) detector is used, with an instantaneous detector element FOV of 1.149 arcmin.
Spatial imaging is achieved using the push-broom principle: the across-track sampling is performed electronically and the along-track sampling is made thanks to the satellite motion. (See the figure below.)
A spatially bi-dimensional image is obtained by the gathering and the on-ground processing of subsequent images as ENVISAT moves along track.
MERIS measures the reflected solar radiation from the Earth's surface and clouds, in the visible and near-infrared spectral regions. Therefore, observation is nominally limited to the day side of the Earth, in particular the angular observation range is limited to a Sun zenith angle of less than 80 degrees at the subsatellite point. Figure 1.5 illustrates the instrument's FOV, swath dimension and camera tracks:
Figure 1.6 - MERIS FOV, camera tracks, pixel enumeration and swath dimension
The observation is performed simultaneously in 15 programmable spectral bands, ranging from the visible to the near infrared (390 nm to 1040 nm). Each of these 15 bands is programmable in position and in width.
MERIS is able to deliver:
· Reduced spatial resolution data
· Reduced and full spatial resolution data simultaneously
These two spatial resolutions, for the nominal orbit are:
· for full spatial resolution: 290 m × 260 m at subsatellite point
· for reduced spatial resolution: 1.2 km × 1.04 km at subsatellite point
An reduced spatial resolution pixel is obtained by averaging the signal of 16 full spatial resolution pixels. More precisely, 4 adjacent pixels across-track for 4 successive pixel lines along-track are used.