MERIS

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MEdium Resolution Imaging Spectrometer (MERIS) is one of the main instruments on-board the European Space Agency (ESA)'s Envisat platform.

This instrument is composed of five cameras disposed side by side, each equipped with a pushbroom spectrometer. These spectrometers use two-dimensional CCDs. One of the sides of the detector is oriented perpendicular to the trajectory of the satellite and simultaneously collects, through the front optics, observations for a line of points at the Earth's surface (or in the atmosphere). The displacement of the platform along its orbit, combined with a short integration time, generate data that can later be used to create two-dimensional images. A light dispersing system separates the various wavelengths (colors) composing the incoming radiation at the entrance of the instrument and directs these on the detector along the second dimension, i.e., along track. These spectrometers acquire data in a large number of spectral bands, but, for technical reasons, only 16 of them are actually transmitted to the ground segment (one of which is required for the low-level processing of the raw data). This instrument thus provides useful data in 15 spectral bands, which are actually programmable in position, width and gain. In practice, these technical characteristics are kept constant most of the time to allow a large number of systematic or operational missions.

The intrinsic spatial resolution of the detectors provides for samples every 300 m near nadir at the Earth's surface, and the pushbroom design avoids or minimizes the distortions (e.g., bow tie effects) typical of scanning instruments. This is known as the 'Full Resolution (FR)' product. The more common 'Reduced Resolution (RR)' products are generated by aggregating the FR data to a nominal resolution of 1200 m. The total field of view of MERIS is 68.5 degrees around nadir (yielding a swath width of 1150 km), which is sufficient to collect data for the entire planet every 3 days (in equatorial regions). Polar regions are visited more frequently due to the convergence of orbits.

The primary objective of MERIS is to observe the color of the ocean, both in the open ocean (clear or Case I waters) and in coastal zones (turbid or Case II waters). These observations are used to derive estimates of the concentration of chlorophyll and sediments in suspension in the water, for instance. These measurements are useful to study the oceanic component of the global carbon cycle and the productivity of these regions, amongst other applications. The characterization of atmospheric properties (gaseous absorption and aerosol scattering) is essential to derive accurate information over the oceans because they contribute to the bulk of the signal measured (under clear skies) or simply because clouds prevent the observation of the underlying surface. Last but not least, this instrument is very useful to monitor the evolution of terrestrial environments, such as the fraction of the solar radiation effectively used by plants in the process of photosynthesis, amongst many others applications.