• FLEX and Sentinel-3C are scheduled to launch together on Vega-C from French Guiana at 01:21 UTC on 15 September
  • Sentinel-3C is intended to replace Sentinel-3A after commissioning, while FLEX will add measurements of vegetation fluorescence

The fact

FLEX and Copernicus Sentinel-3C are scheduled to launch together aboard a Vega-C rocket from French Guiana at 22:21 local time on 14 September, or 01:21 UTC on 15 September. The timetable remains a launch plan until the flight is completed. Sentinel-3C is intended to continue the Copernicus Sentinel-3 observation record. After commissioning, it is expected to replace Sentinel-3A and operate alongside Sentinel-3B, providing measurements of the oceans and land surface.

FLEX has a different role. Its instrument will measure the faint fluorescence emitted by vegetation, which ESA says can provide information about photosynthetic activity. The missions are flying together so their observations can be used alongside each other.

Launch will not make either satellite a routine data source immediately. Commissioning follows, with the spacecraft and instruments checked before their measurements enter regular use. EUMETSAT is due to take responsibility for Sentinel-3C's routine operations after handover.

The assessment

Sentinel-3C is intended to take over from Sentinel-3A, but users will first need to know that its measurements line up with the record they already rely on. That is especially important for services that compare conditions over months or years, where differences between instruments could otherwise be mistaken for changes on the ground.

FLEX brings something new rather than replacing an existing sensor. Its fluorescence measurements could add another way to assess vegetation activity alongside Sentinel-3 data. Before providers build those measurements into products, they will need to see how the data is processed, how consistent it is and how well it works alongside existing observations.

For BTW readers, the launch is only the start. These missions become more useful once Sentinel-3C has been calibrated against the existing record and FLEX is producing validated data that service providers can use with confidence.

What to watch

Watch commissioning results for both spacecraft and the first validated data products. For Sentinel-3C, calibration against the existing Sentinel-3 record will show whether the handover can preserve continuity. For FLEX, early product validation will show when its fluorescence measurements are ready for routine analysis.