MERIS
Encyclopedia
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 CCD
s. 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 degree
s 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 coast
al zones (turbid or Case II waters). These observations are used to derive estimates of the concentration of chlorophyll
and sediment
s 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.
European Space Agency
The European Space Agency , established in 1975, is an intergovernmental organisation dedicated to the exploration of space, currently with 18 member states...
(ESA)'s Envisat
Envisat
Envisat is an Earth-observing satellite. It was launched on 1 March 2002 aboard an Ariane 5 from the Guyana Space Centre in Kourou, French Guyana into a Sun synchronous polar orbit at an altitude of...
platform.
This instrument is composed of five cameras disposed side by side, each equipped with a pushbroom spectrometer
Spectrometer
A spectrometer is an instrument used to measure properties of light over a specific portion of the electromagnetic spectrum, typically used in spectroscopic analysis to identify materials. The variable measured is most often the light's intensity but could also, for instance, be the polarization...
. These spectrometers use two-dimensional CCD
Charge-coupled device
A charge-coupled device is a device for the movement of electrical charge, usually from within the device to an area where the charge can be manipulated, for example conversion into a digital value. This is achieved by "shifting" the signals between stages within the device one at a time...
s. 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
Nadir
The nadir is the direction pointing directly below a particular location; that is, it is one of two vertical directions at a specified location, orthogonal to a horizontal flat surface there. Since the concept of being below is itself somewhat vague, scientists define the nadir in more rigorous...
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 degree
Degree (angle)
A degree , usually denoted by ° , is a measurement of plane angle, representing 1⁄360 of a full rotation; one degree is equivalent to π/180 radians...
s 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
Ocean
An ocean is a major body of saline water, and a principal component of the hydrosphere. Approximately 71% of the Earth's surface is covered by ocean, a continuous body of water that is customarily divided into several principal oceans and smaller seas.More than half of this area is over 3,000...
, both in the open ocean (clear or Case I waters) and in coast
Coast
A coastline or seashore is the area where land meets the sea or ocean. A precise line that can be called a coastline cannot be determined due to the dynamic nature of tides. The term "coastal zone" can be used instead, which is a spatial zone where interaction of the sea and land processes occurs...
al zones (turbid or Case II waters). These observations are used to derive estimates of the concentration of chlorophyll
Chlorophyll
Chlorophyll is a green pigment found in almost all plants, algae, and cyanobacteria. Its name is derived from the Greek words χλωρος, chloros and φύλλον, phyllon . Chlorophyll is an extremely important biomolecule, critical in photosynthesis, which allows plants to obtain energy from light...
and sediment
Sediment
Sediment is naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of fluids such as wind, water, or ice, and/or by the force of gravity acting on the particle itself....
s in suspension in the water, for instance. These measurements are useful to study the oceanic component of the global carbon cycle
Carbon cycle
The carbon cycle is the biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and atmosphere of the Earth...
and the productivity of these regions, amongst other applications. The characterization of atmospheric properties (gaseous absorption and aerosol
Aerosol
Technically, an aerosol is a suspension of fine solid particles or liquid droplets in a gas. Examples are clouds, and air pollution such as smog and smoke. In general conversation, aerosol usually refers to an aerosol spray can or the output of such a can...
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
Photosynthesis
Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...
, amongst many others applications.
External links
- Earth Snapshot - Web Portal dedicated to Earth Observation. Includes commented satellite images, information on storms, hurricanes, fires and meteorological phenomena.
- Miravi - Meris Image Rapid Visualization. MIRAVI shows the gallery of images generated on the Level0 (raw data) Meris Full Resolution (300m) products, few seconds after their availability.
- SRRS - Satellite Rapid Response System. Like MIRAVI but including also ASAR, MERIS Full and Reduced Resolution and ALOS AVNIR2 images.
- ESA page on MERIS