This post intends to answer a question about Sentinel-HR, that we have had quite often inside or outside CNES :
Securing continuity in a critical timeseries requires user community foresight, Programme justification, 10 yrs Agency prep, and finances. Whilst Sentinel-HR may well be an excellent/valid idea, I’m left asking why certain Copernicus contributing missions couldn’t plug this gap?
— Mark Drinkwater (@kryosat) April 29, 2021
Sentinel- HR is a mission project, currently studied in phase zero, for a free and open, global, high resolution (~2m), repetitive (~20 days), systematic earth observation mission in optics. Sentinel-HR would also provide, in one pass, a stereoscopic observation with a low difference in angle, but allowing a terrain restitution with an accuracy of 3 to 4 meters. For instance, this capability could be useful to monitor the evolution of worldwide glaciers, as in this study recently published in Nature. The latter study was mostly based on a sensor, ASTER, that acquired a vast and open archive of stereo images since late 1999. To our knowledge, no replacement is planned for ASTER.
"Copernicus contributing missions", in the European glossary, are commercial satellite missions (sometimes funded by the member states), whose data can be bought by Copernicus to serve the interests of European users. The table below shows a few examples of high resolution satellite constellations. The number of satellites in this category is of course much greater, but the satellites identified below are emblematic examples of what is possible.
These satellites may be stored in two categories :
- on demand acquisition or tasked satellites (such as Pleiades , Pleiades Neo , Maxar Workdview 3)
- systematic acquisition satellites (or coverage satellites) (such as planet, and maybe someday UrtheDaily)
Our Sentinel-HR mission would of course be a coverage mission.
VHR tasked satelllite missions
|Mission||N° of satellites||Swath Width (km)||Multi-spectral Resolution (m)||Surface/day (M.km²)|
|Worldview 2 & 3||2||13||1.2||1.2|
A few examples of VHR satellites with on-demand acquisition (we did not include the Skysat satellites in our analysis below because half of them are not heliosynchronous, and others have a different overpass time).
There are 150 M km² of emerged lands, so in order to fulfill its mission needs (20 days revisit at 2 meters resolution), Sentinel-HR should acquire 8 M km² daily (accounting for 20% overlap required for mosaics, cloud and shadow detection, etc.). Accomplishing this with tasked VHR satellites would require to dedicate 100% of the capacity of the satellites in the table above. Excluding SPOT6/7, which does not have the necessary resolution, this would even amount to about 200%. If we now consider their stereoscopic capabilities, the daily coverage from tasked VHR satellites would drop drastically. Even if reserving this amount of capacity was possible and affordable (and we think it is not), there are still loads of issues with sensors, lifetimes, orbits, resolutions, swaths, viewing angles and spectral bands difference which will make it very difficult to derive consistent, long term datasets. If we let those issues aside and admit that maybe only 20% of the total capability can be dedicated to a public observation service, then we will need to start making choices of what area is observed and what area is not. And inevitably, those data will miss someone, sometime, somewhere that we can not foresee.
VHR coverage satelllite missions
|Mission||N° of satellites||Swath Width (km)||Multi-spectral Resolution (m)||Surface/day (M.km²/sat)|
|Urthedaily||8||360 ?||5||25 ?|
A few examples of VHR satellites with systematic acquisition (until the limits of their capacity is reached). We call these missions "coverage satellites". For Urthedaily, although a launch is announced in 2023, there is not much literature, and we had to guess the swath width and acquisition capacity.
The planet constellation is closer to the type of mission we consider for Sentinel-HR, allowing for a daily revisit, at 4m resolution, which is the double of what is expected for Sentinel-HR. The issue with planet is the data quality. As the optics have a very small aperture, the acquisitions are made with very broad spectral bands, that overlap each other. The signal to noise ratio is also not in the range of the Sentinels. Another difficulty lies in the very small field of view, even if the most recent model of the constellation has reached almost 20 kilometers. As at least a few kilometers of overlap is necessary between the adjacent swaths, the world is in fact covered by a huge amount of spaghetti stitched together. The orbit of the satellites is not maintained so users will have to handle different overpass times when processing time series, which degrades the data quality.
On paper, the Urthedaily constellation of 8 satellites could be much more interesting, even if its resolution is also twice lower than the one needed by Sentinel HR. But there is not much information on this constellation on the internet which stays hypothetical, although announced in 2022.
Both missions (Planet and Urthedaily) do not provide stereoscopy, which is one additional reason why they do not fulfill the objectives of Sentinel-HR.
Our review may have missed newly launched or planned missions, including the Chinese missions for instance, and we do not claim having the complete overview of very high resolution missions in mind. But the tasked missions, with their off-nadir acquisition capabilities, are not adapted to make a systematic coverage, and too many expensive satellites would be required to fulfill the Sentinel-HR mission. So far, the only existing high resolution coverage mission, Planet, does not meet the standards of data quality from Sentinels, and would need an enhancement of a factor 2 of its resolution. Its stereo capabilities also remain modest and not systematic.
Their daily revisit is of course a plus compared to Sentinel-HR, but we believe a Very High Resolution mission does not require such a high revisit, for a lot of applications (urban, infrastructures, forests, glacier evolution, coasts, rivers, hedges...) and that it is more reasonable to try to merge, low revisit and high resolution mission such as Sentinel-HR with a frequent revisit and high resolution mission such as Sentinel-2 or Sentinel-2 NG. Moreover, neither Planet or Earthdaily will provide the stereoscopic mission included in Sentinel-HR.
Finally, let's recall that when we were preparing Sentinel-2, we often heard that the SPOT and SPOT-like satellites already provided this kind of data, and that a Sentinel-2 mission was therefore not necessary, and would kill the earth observation private missions. 20 years after that, the commercial earth observation is thriving, with a lot of different very high resolution missions, while the Sentinel-2 mission is an extraordinary success which proved useful to hundreds of thousands of people.
Moreover, we absolutely don't know how the landscape of VHR earth observation will be in the 2030's, and maybe a private owned mission could answer our needs some day. But, if we want its data to be free and open, it will mean that its entire image archive will have to be bought by the public sector (EU for instance) and provided to the community. How can we be sure it will be cheaper and correspond better to our needs than a mission designed by a space agency ?
The post was prepared by Olivier Hagolle, Julien Michel (CESBIO) and Etienne Berthier (LEGOS)