The Sentinel-4 mission builds upon the heritage of a series of spectrometer instruments which have been or are currently measuring atmospheric properties since 1995; namely:
- The Global Ozone Monitoring Experiment (GOME) on ESA's ERS-2 satellite which operated between 1995 and 2011.
- The GOME-2 instrument on-board EUMETSAT's MetOp-A satellite, launched in 2006, still in service. A second GOME-2 is operating since 2012 on MetOp-B.
- The SCanning Imaging Absorption spectroMeter for Atmospheric CartograpHY (SCIAMACHY) instrument onboard ESA's Envisat mission which operated between 2002 and 2012.
- The Ozone Monitoring Instrument (OMI) instrument onboard NASA's AURA spacecraft, operating since 2004 and still in service.
- The Tropospheric Ozone Monitoring (TROPOMI) instrument onboard ESA's Sentinel-5P spacecraft expected to be launched in 2016.
As shown in the table, among these five heritage instruments, the one which has the best global performances (e.g. spatial resolution, Signal-to-Noise Ratio, spectral coverage) is TROPOMI as it reunites several of the best characteristics of its predecessors, even going beyond them like for example an unprecedented on-ground spatial resolution of 7 x 7 km.
All Sentinel-4 heritage missions are passive backscatter spectrometers operating in Low Earth Orbits (LEO) which allow performing Earth global coverage measurements with a daily revisit time.
Sentinel-4 on the other hand will be the first imaging spectrometer instrument which will be embarked on-board a satellite in a geostationary (GEO) orbit.
The GEO orbit, although limiting the geographic coverage (for Sentinel-4 only to Europe and parts of North Africa and the Atlantic), will allow for the first time an extremely frequent (about 1hour) delivery of accurate atmospheric data: twenty to hundred times better than the LEO missions.
In terms of spatial sampling, Sentinel-4 will be roughly equivalent to Sentinel-5 and Sentinel-5P.
The wide spectral range covered by Sentinel-4 is narrower as compared to Sentinel-5 and Sentinel-5P, and has been chosen to cover the signatures of fast varying tropospheric constituents.
The main characteristics of the five heritage instruments are summarised in the table.
|Instrument||Technical Concept||Spectral Range||Spatial resolution (km x km)||Earth Coverage||Revisit time||Operational|
|GOME||Whisk-broom (scanning)||UV-VIS-NIR (240-790 nm)||320 x 40||Global||1 ½ day||1995-2011|
|GOME-2||Whisk-broom (scanning)||UV-VIS-NIR (240-790 nm)||80 x 40||Global||1 ½ day||2006-present|
|SCIAMACHY||Whisk-broom (scanning)||UV to SWIR (240-2400 nm)||30 x 215||Global||6 days||2002-2012|
|OMI||Push-broom (staring)||UV-VIS (270-500 nm)||13 x 24||Global||1 day||2004-present|
|TROPOMI||Push-broom (staring)||UV-VIS-NIR-SWIR (270 – 2385 nm)||7 x 7||Global||1 day||Launch scheduled in 2016|
|Sentinel-4/UVN||Push-Broom (scanning)||UV-VIS-NIR (305- 775 nm)||8 x 8||Europe + parts of North Africa and the Atlantic||1 hour||Launch scheduled in 2021|