Heritage

Remote sensing of ocean colour from space began in 1978 with the launch of NASA's Coastal Zone Colour Scanner (CZCS). A number of other missions have been launched and more are planned by various space agencies. The International Ocean-Colour Coordinating Group (IOCCG) coordinates information relating to the various missions and instruments.

The key mission driver for the Sentinel-3 OLCI instrument is continuity of the ENVISAT MERIS instrument capability.

The SENTINEL-3 OLCI instrument is based on the opto-mechanical and imaging design of ENVISAT MERIS. The instrument is a quasi-autonomous, self-contained, visible push-broom imaging spectrometer and incorporates the following significant improvements when compared to MERIS:

  • an increase in the number of spectral bands (from 15 to 21)
  • improved SNR and a 14-bit analogue to digital converter
  • improved long-term radiometric stability
  • mitigation of sun-glint contamination by tilting cameras in a westerly direction
  • complete coverage over both land and ocean at 300 m full resolution (for MERIS the reduced resolution was on-board computed)
  • improved instrument characterisation including stray light, camera overlap and calibration diffusers
  • improved coverage of the global ocean (<4 days), land (<3 days with one satellite, ignoring the effect of clouds), where MERIS is approximately 15 days
  • improved data delivery time of 3 hours for Level-1B and Level-2 products
  • 100% overlap with SLSTR instrument swath and simultaneous acquisitions facilitating the use of OLCI and SLSTR in synergy.

OLCI bands are optimised to measure ocean colour over open ocean and coastal zones. A new channel at 1.02 μm has been included to improve atmospheric and aerosol correction capabilities. Two additional channels in the O2A absorption line (764.4 and 767.5 nm, in addition to the existing channel at 761.25 nm) are included for improved cloud top pressure (height) with an additional channel at 940 nm in the H2O absorption region, to improve water vapour retrieval. A channel at 673 nm has been added for improved chlorophyll fluorescence measurement.

The OLCI swath is not centred at nadir (as in the MERIS design) but the whole field-of-view is shifted across track by 12.6° away from the sun to minimise the impact of sun glint (sun glint contamination affects more than half of the MERIS observations at sub-tropical latitudes). In addition, the OLCI instrument is mounted on the satellite to allow a direct view of the Earth, removing the need for an additional fold mirror as used by MERIS.

Table 1: OLCI specification bands, Meris heritage in cyan, additional bands in yellow
Band # λ center Width Lmin Lref Lsat SNR @ Lref
nm nm W/(m2.sr.μm) W/(m2.sr.μm) W/(m2.sr.μm)
Oa1 400 15 21.60 62.95 413.5 2188
Oa2 412.5 10 25.93 74.14 501.3 2061
Oa3 442.5 10 23.96 65.61 466.1 1811
Oa4 490 10 19.78 51.21 483.3 1541
Oa5 510 10 17.45 44.39 449.6 1488
Oa6 560 10 12.73 31.49 524.5 1280
Oa7 620 10 8.86 21.14 397.9 997
Oa8 665 10 7.12 16.38 364.9 883
Oa9 673.25 7.5 6.87 15.70 443.1 707
Oa10 681.25 7.5 6.65 15.11 350.3 745
Oa11 708.75 10 5.66 12.73 332.4 785
Oa12 753.75 7.5 4.70 10.33 377.7 605
Oa13 761.25 2.5 2.53 6.09 369.5 232
Oa14 764.375 3.75 3.00 7.13 373.4 305
Oa15 767.5 2.5 3.27 7.58 250.0 330
Oa16 778.85 15 4.22 9.18 277.5 812
Oa17 865 20 2.88 6.17 229.5 666
Oa18 885 10 2.80 6.00 281.0 395
Oa19 900 10 2.05 4.73 237.6 308
Oa20 940 20 0.94 2.39 171.7 203
Oa21 1020 40 1.81 3.86 163.7 152

 

Source, Credit: The Global Monitoring for Environment and Security (GMES) SENTINEL-3 mission, C.Donlon et al.

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