MERSS Simulator

The Medium Resolution Imaging Spectrometer (MERIS) belongs to a new generation of ocean colour sensors ; leading to an improved bio /geophysical characterisation of oceans and coastal zones, it also provides information in atmospheric and land applications.

MERIS is a push-broom instrument and measures the reflected radiation from the Earth's surface and from clouds through the atmosphere in the visible and near infrared range during daytime. The 1150 km wide swath is divided into 5 segments covered by 5 identical cameras having corresponding field of views with a slight overlap between adjacent cameras. The cameras view the Earth through 5 polarisation scrambling windows and five mirror elements. Each camera images the Earth on the entrance slit of a spectrometer. The area of the Earth seen instantaneously forms an across track stripe. This stripe is imaged on a CCD-line. The spectrum of each element in the Earth image is dispersed on a corresponding CCD column. The spatial resolution along track is determined by successive read-outs of the CCD-array. Full spatial resolution data, i.e. 300 m at nadir, will be transmitted over coastal zones and land surfaces. Reduced spatial resolution data i.e. 1200 m at nadir, will be transmitted continuously. These data will be processed in flight and on ground to provide spectral images of the Earth corrected for atmospheric influence. After correction, the main geo-physical parameters which can be derived from MERIS measurements are :

* Ocean colour parameters in open waters and coastal waters : chlorophyll, gelbstoffe, other pigments, suspended solid matter ;
* Simple qualitative parameters : presence of clouds, emerged land ;
* Atmosphere parameters : Aerosol optical thickness, Ångström exponent ; Cloud albedo, optical thickness, top pressure ; Water vapour column contents.
To meet the very stringent specifications, atmospheric effects and radiometric and geometric distortions induced by the image acquisition process must be corrected for. While the atmospheric correction is carried out during data processing on ground, instrument characteristics are partly rectified in flight during calibration. In this mode correction parameters such as offset and gain are generated to improve the digitised signal values by computation on board.

A typical set of MERIS spectral bands, as reviewed by the ESA advisory group at the workshop of Villefranche in 1991, is given in table I, together with their envisaged application. The centre frequencies and the bandwidths are programmable in flight, by telecommand. The main performance parameters of MERIS are presented in table II.

No. Band centre Band width Application
1 410 nm 10 nm Yellow substance and turbidity
2 445 nm 10 nm Chlorophyll absorption maximum
3 490 nm 10 nm High chlorophyll pigment concentration
4 520 nm 10 nm Turbidity / suspended sediment / red tides
5 565 nm 10 nm Chlorophyll absorption minimum
6 620 nm 10 nm Suspended sediment
7 665 nm 10 nm Chlorophyll absorption
8 682.5 nm 5 nm Chlorophyll fluorescence peak, red edge
9 710 nm 10 nm Red/NIR boundary, red edge transition
10 755 nm 10 nm atmospheric correction
11 765 nm 2.5 nm Oxygen band, cloud top observation
12 880 nm 10 nm Aerosols, vegetation reflectance
14 900 nm 10 nm Aerosols, cloud
14 960 nm 10 nm H2O absorption, vegetation max. reflectance
15 1022.5 nm 25 nm Aerosols, leaf morphology, snow grain size

Table I - The MERIS spectral bands

Swath width 1150 km
Spectral range 400 ... 1050 nm
Spectral sampling interval 1.25 nm
Spectral bands 15, centre frequencies programmable
Spectral bandwidth 2.5 ... 30 nm, programmable
Instrument field of view +/- 34.25 degrees
Detector element field of view + 0.01915 degrees
Frame time 0.044 sec
Error of absolute spectral radiance < 2% between 400 and 900 nm
Error of absolute localisation < 2000 m
Error of spectral position < 1 nm
Radiometric resolution 55 mW.m-2.sr-1.nm-1 at 900 nm ;
Dynamic range From noise level to albedo 1, i.e. about 40 dB
Polarisation sensitivity < 1%
Measurement modes : full resolution, 0.3 x 0.3 km2 reduced resolution, 1.2 x 1.2 km2
Calibration modes : dark current calibration spatial relative radiometric calibration absolute radiometric calibration inter-module relative calibration wavelength calibration

Table II - Performance parameters

MERSS : The MERIS System Simulator

The MERIS system simulator provides support for end-to-end performance evaluation of the MERIS instrument and on-ground processing. It does so by :
  1. managing a set of targets ;
  2. simulating the image formation process within the instrument ;
  3. performing the ground segment software functions which contribute to image production.
Performance evaluation is based on qualitative and quantitative evaluation of the difference between the original target parameters set, and the results of processing by instrument and ground segment sub-models.

The modular structure of the simulator allows to do such comparisons at several different stages of the image production process. This is intended to help evaluate the contribution of each stage : satellite, instrument, on-ground processing, to the overall system performance.

Intermediate results, in particular internal parameters of the instrument sub-model, are produced as intermediate results to help trouble-shooting.

The selected targets provide a sub-set of the possible scenes viewed by MERIS, which is meaningfull

* from the point of view of performance evaluation by referring to the evaluation conditions ;
* from the point of view of instrument calibration and testing on ground and in flight.

The complete imaging process is simulated and includes the flight of the instrument above the Earth, the pointing perturbations due to the Platform, the collection of light by the sensors and the electrical processing on board, and the digital data processing on board and on ground.

Several modes of simulation are provided in order to support :

  1. realistic simulation of the MERIS instrument operations : imaging, calibration ;
  2. investigation of local as well as global effects.