Data Products

Data Product Overview
Product structure

MERIS Level 1b data products
MERIS Browse

MERIS Level 2: Geophysical Data Product
The MERIS Level 2 algorithms theoretical basis document

 Data Product Overview 

The MERIS products will be available at different time scales -FR and RR- near real time and consolidated processing - at different spatial scales depending on the geographical location - global and regional products - and at different levels of processing (Level 1b and Level2)

Near Real Time and Consolidated Processing 
MERIS data products will be available both in Near Real Time (NRT) and in the so-called consolidated form. NRT services will provide data after 3 hours of data acquisition. Consolidation is the gradual process by which more accurate external (auxiliary) data becomes available through time. The consolidated data products will be available typically within a timescale of three weeks. The format of the data products available at these different times do not change, only the quality of the auxiliary data used in their processing is improved.

Full and Reduced Resolution Products
MERIS products will be available at two spatial resolutions:

Full Resolution (FR) with a resolution at sub-satellite point 300m and Reduced Resolution (RR) with a resolution at sub-satellite point 1200m

Global and Regional Products
MERIS products will be available in the FR mode, and simultaneously in the RR mode. The RR data is stored onboard on tape recorders and then transmitted to ground for the entire daytime orbit. This globally acquired data is processed systematically for all orbits. The FR data will be available on average for 20 minutes per orbit. This FR data will be either stored onboard on solid state recorders or transmitted via Data Relay Satellite or X-band receiving stations when in view of the satellite. The FR data represents the regional mission in which data is processed only on request from users. 

Data Product Levels
ESA has defined the following data product levels relevant to MERIS data products:

Level 0 Product:
Level 0: is demultiplexed instrument source packet data. This is the data in its most raw format and will not generally be available to the users. 

Level 1b Products: 
Level 1b: is annotated engineering data which has been calibrated radiometrically and geolocated. For MERIS this is Top-Of-the-Atmosphere (TOA) calibrated radiances measured in mWm-²sr-¹nm-¹.

Level 2 Products: 
Level 2: contains both geolocated geophysical parameters and surface radiance/reflectance depending on the surface type.
For more information on Algorithms Theoretical Basis Documents (ATBD)

Browse:
The browse product will be processed systematically to a 4.8 km resolution for the entire daytime orbit segment. It will consist of a RGB image of TOA radiances enhanced for the best visual representation of land, sea, and cloud features. 

Segment concept
Product partition is performed by segment of 43.5 minutes for the MERIS reduced resolution RR, which consist of that part of the orbit for which the sun zenithal angle is below 80⁰. A MERIS RR segment corresponds to 17,400 km along track. 
The MERIS FR segment corresponds to the same sun illumination limitations as for the reduced-resolution mode, however the acquired data is not necessarily continuous. 

Scene concept
For purpose of distribution, the MERIS product will be packaged in multiple of scenes of 1150 km ×1150 km for the reduced-resolution product, and in scenes of 575 km × 575 km or 296 km × 296 km for the full-resolution products. 

RR scenes contain 71 × 71 tie points for an 1150 km × 1150 km image 

FR-1 scenes contain 36 × 36 tie points for an 575 km × 575 km image; FR-2 scenes contain 18 × 18 tie points for an 296 km × 296 km image.

The user will be able to request full resolution scenes centered anywhere within the swath of MERIS to within a separation of 19km.

Radiometric and geometric continuity will be kept between the scenes, which will allow users to produce high quality mosaics.

Distributed product concept
The MERIS Level 2 products are called distributed because the geophysical quantity found in the data sets varies depending on the surface measured. The different product groups are: ocean colour products, land products and cloud products.

Product structure 

The MERIS product structure is as follows: 

MPH: generic product identifier 

SPH: The specific product header for MERIS will contain all information applicable to the complete product (DSD). 

DS: The data set will be composed of an annotation data set (ADS) and a number of measurement data sets MDSs. 

ADS: The annotation data set is a set of equally sized annotation data set records ADSR. 

ADSR: The annotation data set record contains the annotation information provided at the tie points. The tie points are associated to the pixels of the measurement product grid at a spacing of 16 pixels for RR and of 64 pixels for FR. The same separation is applied between MDSR (lines). 

MDS: The measurement data set is a set of equally sized measurement data set records. The last data set of both the Level 1b and the Level 2 products is the flag measurement data set. 

MDSR: The measurement data set records contain the measurement information provided at each product pixel. The size of the pixel content can be different for different data sets. 

This table below presents the MERIS products.
  

For more information on MERIS Products, go to Product Spreadsheets
 
 
 
 

MERIS Level 1b data products

The MERIS Level1b data products consist of calibrated top of the atmosphere radiances, geolocated and resampled on a regular grid.

The product will be structured by measurement data sets consisting of a complete orbital segment of a single spectral band.

Operational band set

MERIS was operated with a fixed set of bands, recommended by the Science Advisory Group (SAG) and frozen before launch. The level 2 ESA products are being developed and will be validated for this set of bands.

It will be possible to use alternative band sets for experimental campaigns of a few weeks duration. Initial requirements for such campaigns can be submitted through the Announcement of Opportunity

In support to the experimental campaigns, ESA is commited to produce Level 1b products of the same high quality as that produced for the MERIS operational band set.

Flags
Flags will be provided on a pixel by pixel basis, the list of flags are:

• Bright clouds - based on a radiometric test in the blue part of the spectrum. 
• Coastline - based on an a priori atlas. 
• Land/sea - based on an a priori atlas. 
• Sun glint risk - determined by the solar elevation angle at tie points. 
• Invalid. 
• Suspect - due to tranmission errors. 
• Cosmetic - filling of defective detection elements. 
• Duplicated - due to resampling using the nearest neighboring method. 

Annotation data set
All annotation data will be provided at equally spaced tie points every 16 by 16 pixels in Reduced Resolution and every 64 by 64 Full Resolution pixels, which is equivalent to a 19km by 19 km grid on ground.

The following meteorological data provided by ECMWF will be resampled to the tie point grid: 

• Mean sea level atmospheric pressure
• Surface wind speed
• Relative Humidity
• Total column ozone 

The following geographical information will be provided with respect to the earth reference model WGS084:

• Latitude, Longitude 
• Altitude and topographically corrected localisation 
• Bathymetry. 

The following geometrical information will be provided with respect to the earth reference model WGS084 :

• Solar illumination zenithal angle 
• Solar illumination azimuthal angle 
• Viewing zenithal angle 
• Viewing azimuthal angle 

The annotation data povided in the near real time products will consist of meteorological forecasts and, geographical and geometric information computed using a predicted satellite orbit state vector.

The annotation data provided in the off-line (consolidated) products will consist of meteorological analysis data and, geographical and geometric information computed using a restituted satellite orbit state vector.

Resampling 
Resampling will be performed at Level 1b. Considering the acquisition geometry of the instrument, and in order to generate a corresponding "image " with a registration of a fraction of a pixel, the product image is built acquisition by acquisition using the nearest neighbour method in the across track dimension.

Calibration
The radiometric calibration measurements will be performed every two weeks and the results analyzed systematically. The calibration tables will be provided at a time interval sufficient to guarantee a consistent calibration quality and will be referenced in the product.

The radiometric calibration is based on data acquired when the onboard calibration diffuser is illuminated by the sun. The processing accounts for the diffused signal angular and spectral variation through a spectral BRDF model based on on-ground characterization measurements. This model will be validated for a number of points with the use of characterized ocean and land based ground targets and further monitored using established vicarious calibration sites.

The calibration tables generation process will include corrections for the instruments residual nonlinear response, the inherent straylight of the instrument and will allow for the cosmetic filling of all the detector elements identified as defective.
 
 

 Browse 

The browse will be a three-band colour product derived from Level 1b data where three of the MERIS bands will be chosen for the best visualization of the land, sea, ice and cloud features. 

The browse product is generated as an orbital segment from the MERIS RR product. The browse will enable the user to choose the position of the floating scene of interest. 

User tools will be provided to allow for the selection and ordering of a floating scene of 1150 km × 1150 km for reduced resolution products, and of 575 km × 575 km or 296 km × 296 km for full resolution product. 

A compression factor of 20 is foreseen for the MERIS browse with the use of a public domain compression software. Contrast enhancement technique will be applied on a band basis to improve the visual aspect of the data. 

For further details regarding the MERIS Level 1b data products, see Product Spreadsheets.

Level 2: Geophysical Data Products 

MERIS Level 2 reference product
The MERIS Level 2 product are a distributed mixture of geophysical products and reflectances. 

The product format is called distributed because the geophysical quantity found in the different data sets changes according to the different surfaces measured. 

The different product groups are: ocean colour products, land and cloud products.

The classification between the different product groups, ocean, land and cloud will be made on a pixel by pixel basis by using the a priori determination of Level 1b further consolidated by the use of radiometric tests at level 2.

The Level 2 classification information will be provided on a pixel by pixel basis in the flag measurement data set.  

The scientific background to the MERIS products can be found in a MERIS special issue of the International Journal of Remote Sensing Volume 20, Number 9, 15th June 1999.

Product description
For each measurement data set, the products will be distributed according to the table below.
 

* SM - YS: Suspended Matter & Yellow Substance 
MGVI: MERIS global vegetation index 
PAR: Photosynthetically Active Radiation for chlorophyll fluorescence applications 
Aerosol t & e: Aerosol optical Thickness & Aerosol epsilon factor 
RS: Normalised water leaving radiance/reflectance
Chlorophyll Map of Eastern Mediterranean,
Image courtesy of the Laboratoire de Physique et Chimie Marines (F)
(Composite time series using CZCS data)

The open ocean data products consists of chlorophyll concentrations and several atmospheric parameters. In coastal zones more products are produced such as Total Suspended Matter and Yellow Substance Concentration. The land surface product contains MERIS global Vegetation Index (MGVI) and atmospheric parameters. In addition, surface reflectances will be made available, allowing the user to derive further products. Three cloud products are foreseen: cloud top height, cloud optical thickness and cloud albedo.

Each of the above candidate level 2 parameters is the subject of continuing scientific research commissioned by ESA. The algorithms for each of the parameters will be subject to examination by ESA to determine whether they should be implemented in the ground segment.

The current status of development is the following: 
 
 

Ocean products

Normalized water leaving radiance / reflectance
The MERIS Level 2 radiometric unit is atmospherically corrected water leaving reflectance. The atmospheric correction applied assumes that the water is absorbing in the NIR, and include a correction for those sediment loaded waters where this assumption fails. 
For more information, see ATBD 2.7, and ATBD 2.6 for the bright water correction. 

Algal Pigment Index I 
The MERIS algal pigment index is a measurement of the concentration in Log₁₀(mg/m³) of phytoplankton (algae) in the water. The concentration is derived by the direct relationship between the ratio of the blue and green signal leaving the water surface and the concentration of algal pigments. The relationship, based on published data, is valid over clear waters and spans a concentration range from mg/m³ to tens of g/m³ .For more information, see ATBD 2.9

Algal Pigment Index II 
The second MERIS algal pigment index is also a measurement of the concentation in Log₁₀(mg/m³) of phytoplankton (algae) in the water but, is part of a suite of oceanic products derived by inverting a model of the optical properties of the ocean by the use of a neural network. The other oceanic products are suspended matter and yellow substance. 
For more information, see ATBD 2.12

Suspended matter 
The MERIS suspended matter products is a measurement of the suspended sediments concentration in Log₁₀(g/m³). As described above, it is derived by inverting a model of the optical properties of the ocean by the use of a neural network. The model describe suspended matter as a scattering particle with very little absorption for which a more appropriate name would be "total suspended mineralic matter concentration" 
For more information, see ATBD 2.12

Yellow substance
The MERIS yellow substance product is a measurement of the Gelbstoff absorption in m-1. As described above, it is derived by inverting a model of the optical properties of the ocean by the use of a neural network. The model describes yellow substance as non-scattering absorbing matter. Yellow substance or "Gelbstoff" is decayed organic material that has been dissolved in the marine waters and is usually transported into the sea by rivers.
For more information, see ATBD 2.12

Photosynthetically Active Radiation (PAR) 
The MERIS PAR product is a measurement the amount of radiation in mEinstein.m² available to the Photosynthetically Active oceanic flora. It will be computed from atmospherically corrected irradiances using the method proposed by Calder et al. This product is intended for the study of chlorophyll fluorescence and should be used with the 681.25 nm band selected for this purpose.
For more information, see ATBD 2.18

Aerosol optical thickness 
The MERIS aerosol optical thickness is a measurement the opacity of the aerosol layers at 865 nm. It is measured by assuming that the ocean is a black surface in the NIR (see Normalized water leaving radiance / reflectance). 
For more information, see ATBD 2.7

The MERIS Aerosol Epsilon Factor 
The MERIS aerosol Epsilon Factor is a description of the aerosol assemblage detected over water bodies. It is expressed as the spectral slope of the Aerosol Reflectance in the near Infra Red, Complemented by two flags which identifies whether they are absorbing or not.
For more information, see ATBD 2.7 

Cloud products

Cloud optical thickness 
The MERIS cloud optical thickness is a measurement the opacity of the cloud. 
For more information, see ATBD 2.1

Cloud albedo 
The MERIS cloud albedo is a measurement the amount of radiation reflected by clouds.
For more information, see ATBD 2.2

Cloud top pressure 
The MERIS cloud top pressure is a measurement of pressure in hPa based on the absorption of solar radiation by oxygen. It is derived by inverting a model of the optical properties of the atmosphere and clouds by the use of a neural network. 
For more information, see ATBD 2.3

Cloud Type 
The MERIS cloud type product is determined by using a simple relationship between the cloud properties - optical thickness and height - and its type, as established by the International Satellite Cloud Climatology Project (ISCCP).

Cloud reflectance
The MERIS Level 2 cloud radiometric unit will consist of top of the atmosphere reflectance only. 
For more information, see ATBD 2.16

Land products

Reflectance 
The MERIS Level 2 land radiometric unit will consist of Rayleigh corrected reflectance. The atmospheric corrections over land does not include a correction for the aerosols contribution. 
For more information, see ATBD 2.15

Aerosol optical thickness 
The MERIS cloud optical thickness is a measurement the opacity of the aerosol layers at 865 nm. It is measured by assuming that Dense Dark Vegetation has a known dependent standard reflectance value in the visible. 
For more information, see ATBD 2.15

Aerosol Epsilon Factor 
The aerosol epsilon factor is description of the aerosol assemblage detected over dense dark vegetation. It is expressed as the spectral slope of the aerosol reflectance in the Near Infra Red.
For more information, see ATBD 2.15

Meris Global Vegetation Index 
The MERIS Global vegetation index is a measurement of the presence of healthy live green vegetation. It has been optimised to be robust to atmospheric conditions and surface reflectance and constrained to provided an estimate of the fraction of absorbed photosynthetically active radiation. 
For more information, see ATBD 2.10

Water Vapour products 

The MERIS water vapour product is a measurement of the concentration in g.m^-2 of water vapour found in the total atmospheric column. This product is of particular interest over land where the signal is high, but it will also be provided over water surfaces and clouds. 
For more information, see ATBD 2.4 

The MERIS Level 2 products are coded in such a way as to take the full advantage of the storage space allocated to them. As a consequence, in order to convert the data to geophysical units, an offset and gain correction shall be applied. 

In addition to the level 2 reference product which contains all data sets of the distributed product table, three smaller extracted products will be available:

Reduced resolution (1.2km) column water vapour content product. 

Reduced resolution (1.2km) vegetation product.

Low resolution (4.8km) column water vapour content product. 

Flags 

Flags will be provided on a pixel by pixel basis.

The list of flags is as follows:

Surface Classification:

• land product available
• cloud product available
• water product available

• validity for MDS 1 to 13
• validity for MDS 14
• validity for MDS 15
• validity for MDS 16
• validity for MDS 17
• validity for MDS 18
• validity for MDS 19

• Coastline: From Level 1b
• Cosmetic: From Level 1b
• Suspect: From Level 1b
• Continental absorbing aerosol
• Dust-Line absorbing aerosol
• Turbid water
• Anomalous scattering water
• Yellow substance loaded water
• Ice or high aerosol load
• Corrected for glint
• Contaminated by glint
• Dense dark vegetation
• Cloud Top Pressure and Surface pressure do not match
• Pressure product Lower than ECMWF pressure

Annotation data set
Annotation data provided in the Level 2 product are identical to those of the Level 1b product.

All annotation data will be provided at equally spaced tie points every 16 by 16 pixels in Reduced Resolution and every 64 by 64 Full Resolution pixels, which is equivalent to a 19km by 19 km grid on ground.

The following meteorological data provided by ECMWF will be resampled to the tie point grid: 

• Mean sea level atmospheric pressure
• Surface wind speed
• Reletive Humidity
• Total column ozone 

The following geographical information will be provided with respect to the earth reference model WGS084:

• Latitude, Longitude 
• Altitude and topographically corrected localisation 
• Bathymetry. 

The following geometrical information will be provided with respect to the earth reference model WGS084 :

• Solar illumination zenithal angle 
• Solar illumination azimuthal angle 
• Viewing zenithal angle 
• Viewing azimuthal angle 

The annotation data povided in the near real time products will consist of meteorological forcasts and, geographical and geometric information computed using a predicted satellite orbit state vector.

The annotation data povided in the off-line (consolidated) products will consist of meteorological analysis data and, geographical and geometric information computed using a restituted satellite orbit state vector.
For further details regarding the MERIS Level 2 data products see Product Spreadsheets.
 
 
 
 

 The MERIS Level 2 algorithms theoretical basis document 

The scientific description of the MERIS Level 2 algorithms are described in the algorithm theoretical basis documents (ATBD). 

The Level 2 processing architectural link between the different ATBDs is described in the chart below. Each processing step, symbolised by a rectangle (process) or diamond (decision), produces its own products (parallelogram symbols) which are reused by downstream steps, stored in the Level 2 product, or both. 

Each processing step corresponds to one or more ATBDs, with the numbers shown in each box. ATBDs with their number between parentheses describe algorithms relevant to the problem but not implemented in the MERIS operational baseline. Processing steps without a number will be described in a future ATBD issue. 
 

The following table contains the chapters of the ATBDs. All documents are available in Adobe PDF format and can be downloaded by clicking on the title found in the table below. (They can be viewed with the Adobe Acrobat Reader (see help in this site for more information on Adobe Acrobat), a free software available on the Adobe web site which allows users to view, navigate, and print PDF files on all major computing platforms.)
 

Keywords: ESA European Space Agency - Agence spatiale europeenne, observation de la terre, earth observation, satellite remote sensing, teledetection, geophysique, altimetrie, radar, chimique atmospherique, geophysics, altimetry, radar, atmospheric chemistry