THEIA : A new French Data Centre dedicated to Land Surfaces

(French Version)

The THEIA Land Data Centre is a French inter-agency initiative designed to promote the use of satellite data, primarily for environmental research on land surfaces but also for public policy monitoring and for management of environmental resources. Its objective is to foster the use of remote sensing data to measure the impact of human pressure and climate on ecosystems and local areas, to observe, quantify and model water and carbon cycles, to follow the evolution of societies and of their activities, including agricultural practices, and to understand the dynamics of biodiversity.

 

Within the Land Data Centre, CNES set up a production centre named MUSCATE. This centre aims are providing users with ready-to-use products derived from time series of images acquired over large areas. Sentinel-2 will of course be the spearhead of the production centre, but before the launch of the Sentinel-2, MUSCATE will already begin to produce data from the SPOT4 (Take 5) experiment. At the same time, the processing centre also prepares the production of all Landsat data acquired over mainland France from 2009 to 2011.

 

MUSCATE production centre already exists in the form of a prototype developed by CNES with strong support from Cap Gemini. This prototype is already able to handle LANDSAT, SPOT, FORMOSAT-2, Sentinel-2 and Venμs data, using processors developed by CNES for geometric processing [1], and developed by CESBIO for cloud detection [2] and for atmospheric correction [3]. Simultaneously, the development of an operational production facility is being specified.

Products provided by the MUSCATE Centre are:

Simulations of SPOT4(Take5) products from Formosat-2 data
  • Level 1C (orthorectified reflectance at the top of the atmosphere)
  • Level 2A (ortho-rectified surface reflectance after atmospheric correction, along with a mask of clouds and their shadows, as well as a mask of water and snow).
  • Level 3A (bi-monthly or monthly composite products of surface reflectances, obtained as the weighted average surface reflectance of non-cloudy pixels obtained during the period). Up to now, Level 3A chain is only available for Venμs satellite.

The data produced by MUSCATE will be freely distributed to research laboratories on the one hand, and to the French public institutions on the other, they will be if possible distributed freely to a wider community. The Land Data Center is also building a distribution server to make all these data available.

 

Further reading about these products :

[1]: Baillarin, S., P. Gigord, et O. Hagolle. 2008. « Automatic Registration of Optical Images, a Stake for Future Missions: Application to Ortho-Rectification, Time Series and Mosaic Products ». In Geoscience and Remote Sensing Symposium, 2008, 2:II‑1112‑II‑1115. doi:10.1109/IGARSS.2008.4779194.

[2]: Hagolle, Olivier, Mireille Huc, David Villa Pascual, et Gérard Dedieu. 2010. « A multi-temporal method for cloud detection, applied to FORMOSAT-2, VENµS, LANDSAT and SENTINEL-2 images ». Remote Sensing of Environment 114 (8) (août 16): 1747‑1755. doi:10.1016/j.rse.2010.03.002.

[3]: Hagolle, O, G Dedieu, B Mougenot, V Debaecker, B Duchemin, et A Meygret. 2008. « Correction of aerosol effects on multi-temporal images acquired with constant viewing angles: Application to Formosat-2 images ». REMOTE SENSING OF ENVIRONMENT 112 (4) (avril 15): 1689‑1701. doi:10.1016/j.rse.2007.08.016.

 

Selection of SPOT4 (Take 5) sites

(French Version)

Choosing the sites observed for SPOT4 (Take5) was not easy. The schedule was very tight, we felt that demand would be strong and we did not want to be blamed for favoritism or cronyism (and we did not even accept gifts (1)).

 

We processed differently for sites for French laboratories and organizations on the one hand, and sites for other space agencies on the other hand.

 

SPOT4 (Take 5) sites in France and Belgium

For the French sites, we launched a call for proposals for sites, with a very short notice (one month), and CNES TOSCA committee made the selection. This call was a great success : 20 proposals were received, with the participation of 81 laboratories and agencies.

 

For international sites, we contacted space and research agencies involved in Sentinel-2 project, namely ESA, JRC, NASA or the CSRC. Here, the charge of favoritism could be retained (but we have not received gifts),as we did not have time to issue a call for proposals and set up a jury for selection. However, the participation of these agencies implies their financial contribution to purchase Level 1A images from Astrium Geo (formerly Spot Image).

 

SPOT4(Take5) sites in Africa.

Of course, we hope that the acquired images for SPOT4 (Take5) experiment will be widely distributed. The images will be made available to anyone upon request, for non-commercial use in the framework of Sentinel-2 preparation. Please send your requests to the Land Thematic Data Center (or Pôle Thématique Surfaces Continentales (PTSC).)

 

(1) But now that the sites are chosen...

Choix des sites observés pour SPOT4 (Take 5)
Choix des sites observés pour SPOT4 (Take 5)

Changing SPOT4 orbit : easy ?

(French Version)

When we submitted the SPOT4(Take 5) experiment to CNES, we knew that CNES would not accept it easily, since a similar proposal made by Gérard Dedieu before SPOT2 de-orbitation had been rejected. But we did not imagine the amount of work we were requesting from our colleagues at CNES. To show the project feasability, our CNES colleagues had to :

  • find a project manager who coordinated the study: Sylvia Sylvander
  • choose the new orbit (2 to 6 days repeat cycle) minimizing fuel consumption : We must keep enough fuel to be able to reduce the altitude of the satellite, so that it burns in the atmosphere after a period of less than 25 years. The finally chosen orbit provides a 5 days repeat cycle, resulting in a very low fuel consumption. This orbit also provides the exact repeatability of the two satellites Sentinel-2.
  • choose the strategy change orbit. The date of optimal maneuver is January 29, but it corresponds to the end of a full moon, and it is prohibited to maneuver SPOT4 during the full moon. It is not due to superstition, but only because of a potential blinding of star sensors used to determine the orientation of the satellite. But a detailed analysis of recent full moons in the same period last year showed that the maneuver could still be executed on January 29 without any risk.
  • check that the ground segment (designed 15 years ago) can handle the new orbit. The ground segment programs the satellite and the acquisitions, manages the old tape recorders, coordinates the data download to the receiving station, while avoiding interference with other satellites. Because the satellite is no longer on its nominal orbit, all the conditions of interference are to be recalculated.
  • check that the ground segment is able to ingest and process products. The products are usually referenced by their orbit number, which will be different...
  • test the system interfaces: a one-week trial on a simulator of the spacecraft and its system showed that everything should work fine
  • the usual programming system at SpotImage will not work on this orbit, we will have to use CNES programming system, more flexible but less automated. It takes one hour and a half to program the 42 sites observed over 5 days, but someone will have to do it every 5 days.
  • find internal and external staff (and budgets) to extend the life of SPOT4 for 5 months.
  • negotiate with SpotImage (Astrium Geo), the cost of producing Level 1A products and maintaining the cloud notation
  • prepare the MUSCATE production center that will provide level 1C and 2A users. This production facility will be implemented within the Land Data Centre.

Many thanks to Didier Roumiguières, Sylvia Sylvander, Laurence Houpert, Jean-Marc Walter, Jordane Sarda (CS-SI), Aurélie Moussy-Soffys, Frédéric Daniaud (CS-SI), Michel Moulin, Benoît Boissin,  Selma Cherchali, Françoise Schiavon, Marc Leroy, Jerôme Bijac (Astrium geo) and to all CNES and Astrium-Geo people who contributed to the acceptation SPOT4(Take5) experiment.

Sentinel-2, Spot-4, Take-5 (English)

Paul Desmond's Take 5 jazz standard was written in an unusual 5 beat rythm.

(French version)

At the end of life of each satellite, CNES issues a call for ideas for short-term experiments taking place before de-orbiting the satellite. CESBIO took the opportunity to set up the Take 5 experiment at the end of SPOT4' life : this experiment will use SPOT4 as a simulator to give us a hint of the time series that ESA's Sentinel-2 mission will provide.

 

The first Sentinel-2 satellite should be launched within the next two years, and the second satellite should follow 18 months later. Together, these satellites will provide us every fifth day with high-resolution images of all land areas... or of the clouds that cover them. Despite these clouds, users will be guaranteed access to cloud-free data at least once per month. The arrival of these data should therefore cause a revolution in the use of remote sensing data.

 

In order to avoid wasting time when Sentinel-2 is launched, it is necessary to prepare today for the use of these data. However, at present, there are no suitable data to perfectly simulate the features of Sentinel-2:

  • ESA is providing datasets simulating the spectral bands of the instrument, but these airborne data are not multi-temporal, and only cover small areas.
  • -CNES and the Israel Space Agency are developing the VENµS project, whose goal is to provide high repeatability time series, but its launch is only scheduled for late 2014.
  • CESBIO provided time series of FORMOSAT-2 images and LANDSAT, but in the first case, the data only cover areas of 20 * 20 km, whereas in the second case, the repeatability of the data is much lower than expected from Sentinel-2.

 

After 6 months of feasibility studies, CNES has just decided to launch Take 5 experiment. On January 29, the orbit will be lowered from SPOT4 by 3 kilometers to put it on a 5 day repeat cycle orbit, which means that the satellite will fly the same places on earth every 5 days. Spot4 will follow this orbit until the end of May 2013. During this period, 42 sites will be observed every 5 days, as in the case of Sentinel-2. The data will be processed and distributed by the "Pôle Thématique Surfaces Continentales" (PTSC) and distributed to users by the end of June 2013, they will be provided with the following two levels:

  • Level 1C (data orthorectified reflectance at the top of the atmosphere)
  • Level 2A (Data ortho-rectified surface reflectance after atmospheric correction, along with a mask of clouds and their shadows, as well as a mask of water and snow).