## Posters and Presentations about SPOT4 (Take5) shown at RAQRS-IV

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The fourth edition of the RAQRS (Recent Advances in Quantitative Remote Sensing) Symposium took place in Valencia (Spain) two weeks ago. This symposium happens every forth year, is perfectly organised by J.Sobrino and his colleagues at University of Valencia. It  aims at showing the last advances in remote sensing for land surfaces, in all wavelengths and with either active or passive methods. We talked about low and high resolution time series of optical images, radar, passive microwaves, thermal infrared, paella, fluorescence, soil moisture, jamon, evapo-transpiration, biomass, orchata and biophysical variables...

The SPOT4 (Take5) was also present in a few posters and presentations, joined here. And it was the right place to announce SPOT5 (Take5) ! (If the document does not show up (it is too large), click on the icon on the upper right corner, if it still does not show up, use the download button)

Claire Marais Sicre      and         Marjorie Battude

## High cloud detection using the cirrus band of LANDSAT 8 or Sentinel-2

Atmospheric absorption : in blue, the surface reflectance of a vegetation pixel, as a function of wavelength. In red, the reflectance of the same pixel at the top of atmosphere.  For a wavelength of 1.38 µm, water vapour totally absorbs the light that comes from the earth surface at sea level.

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The LANDSAT-8 and SENTINEL-2 satellites have a spectral band centered on the 1.38µm wavelength, which is designed to allow the detection of high altitude clouds. This spectral band corresponds to a strong absorption band of water vapour. its absorption is so strong that a photon emitted by the sun in this wavelength has nearly no chance to reach the earth surface, and even less to reach the satellite after that without being absorbed. The consequence is therefore that the surface is usually not visible on the images taken for the 1.38 µm channel.

However, as water vapour is concentrated in the lower layers of the atmosphere, the photons reflected by high clouds have much less chances to be absorbed. The 1.38 µm images display the higher parts of the atmosphere, and can be used to screen the high clouds, as it may be seen on the image below, on which a very large number of plane contrails may be observed (I counted 35, what about you ?)

This spectral band is therefore useful to detect these thin cirrus clouds which, without this band, were usually difficult to spot and used to degrade our reflectances time series.

LANDSAT 8 image taken over Paris in April 2013. On the left, the RGB color composite, and on the right the 1.38µm channel. The plane contrails can be easily detected, and given their number, one can see that we might have to choose whether to fly or to observe...

It is just sad that a simple threshold cannot do the detection with a 100% accuracy (but if it was the case, cloud detection would be easy for everyone, and we would not be useful anymore !)

First of all, the low clouds and the fog are very close to the surface and are not visible in that band. One has to use other criteria to detect them. Moreover, some mountains may emerge from the absorbing layers, all the more when the atmosphere is dry. A thresholding to detect high clouds must take into account the surface altitude, and for a better accuuracy, should take into account the waper vapour quantity and vertical repartition, which may be predicted using weather analyses.

Landsat 8 image taken above the center of Madagascar, in September 2013. On the left, the RGB color composite, and on the rght, the 1.38 µm channel. There is nearly no cloud on this image, but the surface reflectance is much greater than zero, because part or the region has an altitude above 1500m, and because the atmosphere was particularly dry on that day..

Finally, the 1.38µm channel is efficient to detect high clouds and especially thin cirrus, but has to be used with some precautions to avoid that all mountains be classified as high clouds. This is how we proceed in the MACCS processor.

## Take 5 : a happy end for SPOT5

In a classical Hollywood thriller, in which the hero nearly dies several times, a last sequence of the film is usually dedicated to the happy ending with a swinging music. The same happens with the SPOT5 (Take5) story : after several periods with very little hopes of success, the SPOT5 (TAKE5) experiment is finally close to be decided (at least with a 90% likelihood...)

Despite an intensive campaign to show that repeating the experiment would still be useful, with very motivated users writing that the experiment would be useful (thanks to all !),  it quickly turned out that CNES could not afford the full cost of the experiment a second time alone. Therefore the support of ESA was sought, as the experiment is considered to fall in the core mandate of ESA's Third Party Mission Earthnet Program. ESA's decision to takeover CNES external costs still has to be approved by its Member States in autumn, however funding is already set aside. And finally, CNES as agreed to hold the experiment thanks to ESA's decision to take charge of CNES external costs : CNES has contracted several companies to operate its satellites and ground segments and these expenses for the experiment will be covered by ESA, while CNES will fund its internal costs, and provide the satellite.

The experiment will start in April 2015 and stop at the end of August, the exact start date still needs to be determined and will depend on the finally selected orbit. Sentinel-2 launch is expected between the end of April 2015 and the end of June 2015, but the "ramp-up phase" will take a couple of months to start the routine acquisitions, probably too late to monitor the main part of 2015 growing season in the Northern Hemisphere routinely. For a number of selected sites, the SPOT5 (TAKE5) experiment will be useful to enable several teams to start building their applications for 2015 growing season, without having to wait for an additional year. Early Sentinel-2 images might also be taken on these sites.

This time, the selection of the sites will be lead by ESA, who will issue a call for sites proposal this autumn, in a very short time frame, as the final site list needs to be determined before the end of the year. If you are interested by proposing a site, get ready to answer and stay tuned on this blog  !

(Thanks to Sylvia Sylvander (CNES) and Bianca Hoersch  (ESA) for their contributions to this post)

## Directional effect correction for Sentinel-2 composites.

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#### Sentinel-2 orbits

Swaths observed by Sentinel-2A, for day 1 (green), 4 (Blue), 7 (grey), 8 (Pink). For Sentinel-2B, we will have to shift that by 5 days. Distance between swaths was computed so that a little overlap is available at the equator.

The Sentinel-2 orbit was set so that the swaths observed by the satellite have a little overlap at the equator. The width of the overlap increases quickly at higher latitudes. For instance, at France latitude (45 degrees), about half of the surface will be observed twice per satellite cycle, from two adjacent swaths.

It is not very fair, since it will always be the same places that will be observed twice and the rest of the world will only be observed once (The Cesbio site is well located !)

Same as above, with a zoom over France. Here, half of the land will be observed twice per cycle (red segments), and the other half (yellow segments) once per cycle.

#### Directional correction for composites.

Well, the issue is that each point within the overlap zone will be observed twice, but under two different viewing angles, and therefore will have different reflectances in each swath, due to the directional effects. The users of our data often ask for monthly syntheses as cloud free as possible, that merge the data observed from different orbits.To obtain such products, a directional correction is therefore necessary.

Monthly syntheses in Toulouse region, without directional correction on the left,  with a directional correction on the right.

N.B.. The scattered green points you may see are invalid points due to saturated pixels (saturations are often observed with SPOT? which will not be the case for Sentinel-2).

To do that, directional models have been developed, such as the ones of Roujean or Ross-Li, that model the directional variations as a function of viewing angles and solar angles, with a rather good accuracy for most types of surfaces. Here is how they look like :

$\rho= \rho_0 (1 + K_1. F_1(angles), + K_2. F_2 (angles))$

$\rho$ is the reflectance for the actual viewing and solar angles $\rho_0$ is the reflectance for a given angular condition chosen to standardise the data (for instance viewing at nadir and solar angle at 45 degrees), F1 and F2 are the directional functions that depend on the angles, and $K_1 and K_2$ are the coefficients of the directional model, that depend on the observe pixel type of surface.

Fortunately, in the case of S2, the angle differences are low, no more than 20 degrees. We have tried, as a first test; to find mean coefficient that could work more or less for all surfaces. Tu compute these coefficients, we used the SPOT4 (Take5) sites which have been observed under two viewing directions. These are Maricopa (In the USA), and Midi-Pyrénées, Bretagne and Provence in France. They show very different landscapes, with desert and irrigated crops in Maricopa, a very diverse agricultural landscape in Bretagne and Midi-Pyrénées, and Mediterranean forests and vineyards in Provence. We have used all the available couples of clear images separated by less than 5 days and we searched for the coefficients K_1 and K_2 that allow to minimise differences.

Finally, these coefficients were used to correct the data and produce composites. The monthly syntheses are finally obtained by computing  a weighted mean value of the reflectance of cloud free pixels obtained during a period of 42 days. The images above or below show the results obtained by M. Kadiri on the French sites (Maricopa is still running), with on the left the synthesis without directional correction, and on the right the one with directional correction. The shading observed from right to left on the image without correction almost disappears on the images with correction. It is the same for all 3 sites and the chosen images are the ones which show the highest differences. Knowing that the angle difference is greater for SPOT4 (Take5) than for Sentinel-2, we have good hopes that this simple method could work for Sentinel-2.

However, our sampling of 4 sites is not sufficient, we will have to prove that theses results still hold for other types of surfaces. We could do that with SPOT5 (Take5) or with the first Sentinel-2 data (which should come soon !).

Same as above, for Provence-Languedoc.

Same as above, for Bretagne

Fauchée d'un instrument : c'est la surface observée par un satellite au cours d'un passage.

## The second SPOT4 (Take5) swinging workshop

Rappel : Le CNES invite tous les utilisateurs des données SPOT4 (Take5) à un atelier de deux jours dans les locaux du CNES à Toulouse, du 18 au 19 novembre 2014. Un an et demi après l'expérience SPOT (Take5), et un an après la première réunion des utilisateurs, ce sera l'occasion de faire le point sur les résultats obtenus pendant l'expérience SPOT (Take5), et de conclure sur l'adéquation des produits distribués par le pôle THEIA et sur l'adaptation des futures données Sentinel-2 aux besoins.

N'oubliez donc pas de noter cette date sur vos agendas, et si vous souhaitez participer, d'envoyer un message à sylvia.sylvander@cnes.fr, avant le 15 Septembre, en lui précisant si vous souhaitez faire une présentation (avec le titre et quelques lignes d'explications).

##### Numéro Spécial SPOT4 (Take5)

Par ailleurs, le journal Remote Sensing a accepté de publier un numéro Spécial consacré à SPOT4 (Take5). Toutes les informations sont fournies ici, et la date limite pour soumettre les papiers est le 28 février 2015.

Reminder : CNES is inviting all the users of SPOT4-(Take5) data to a 2 days meeting in CNES Toulouse,  on the 18th and 19th of November. One year and a half after the experiment took place, and one year after the first SPOT4 (Take5) users meeting, it will be time to summarize the results obtained by SPOT4 (Take5) users and to conclude on the suitability of THEIA's products for users, and on the applicability of Sentinel-2 data depending on the applications.

So please save the date, and send an email to sylvia.sylvander@cnes.fr before the 15th of September, about whether you intend to participate and whether you intend to give a talk (with a title and a very short abstract).

##### SPOT4 (Take5) Special Issue

Morover, the Remote Sensing journal accepted to publish a special issue about the SPOT4(Take5) experiment. All the information is here, and the deadline for submitting scientific papers is the February the 28th, 2015.

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Those who know me know that I work with Linux and that I like to use free softwares, even if it is not always easy at CNES where everything is around windows . But before I started this blog, I had never contributed with free software. A few months ago, I published here two pieces of code, one is SMAC (I just did a translation into Python), to do easy but approximate atmospheric corrections, the other is Landsat Download, to automatically download Landsat data from USGS website. These two codes are among the pages most often read in this blog.

A few weeks ago, one user of Landsat-Download tool suggested to host the code on GitHub, and I did it at the end of July (I published a piece of code for the first time !). And it was a pleasure to discover that a few days later, I had two contributions to this code, one from Jake Brinkmann, who added a nice progress bar, and one from Michel Lepage from CESBIO, which contributes to an interface simplification that I just implemented in the new version.

In brief, the module I released has been nicely enhanced, and as I am one of the main users, I benefit from it every day. Well, I know that my colleagues who have been producing free softwares since their childhood will say that I just re-invented the wheel....

And finally, remember that, if you need well processed LANDSAT data over France, with cloud masks and atmospheric correction, you might also try the products provided by THEIA.

## SPOT5 (Take5) : a step forward / un pas en avant

CNES just gave a green light to go on with the studies to repeat the Take5 experiment with SPOT5. These studies will determine the exact cost of the experiment and see if it fits with what could be available, accounting for ESA's help. If everything goes well, the experiment would start around April 2015, until the end of August 2015. It will provide the opportunity, for several sites, to anticipate the launch of Sentinel-2. I do not know yet how the sites would be chosen, but I will keep you posted.

Le CNES vient de donner son feu vert pour poursuivre les études de faisabilité d'une nouvelle expérience Take5 réalisée avec SPOT5. Ces études auront notamment pour but de déterminer le coût exact de l'expérience et de voir s'il correspond aux budgets qui peuvent être dégagés, avec l'aide de l'ESA. Si tout se passe bien, l'expérience démarrerait en Avril 2015 et durerait jusqu'à fin Août, elle permettrait donc d'anticiper le lancement de Sentinel-2 pour quelques sites. Je ne sais pas encore comment les sites seraient choisis, mais je vous tiens bien sûr au courant.

## And now, LANDSAT 8 2014

A partir de maintenant, le pôle THEIA mettra en ligne tous les mois un lot de données LANDSAT 8 supplémentaire, récemment acquises au dessus de la France. Le dernier lot publié va jusqu'au 31 mai 2014. Les données sont toujours à la même adresse : http://spirit.cnes.fr/resto/Landsat/

Nous vous conseillons toujours la lecture de la description de leur traitement, du guide d'utilisation du serveur de distribution, et de la méthode pour télécharger des lots de données d'un seul coup.

From now on, THEIA will release each month a new bunch of LANDSAT 8 products recently acquired over France. The most recent date is now the 31st of May 2014. The data can be downloaded from : http://spirit.cnes.fr/resto/Landsat/.

We still advise you also to read the processing description. the short user's guide for the distribution server, and a manual to download a lot of data at once.

## LANDSAT 5 et 7 data over France are on line !

C'est fait, les données Landsat 5 et 7 au niveau 2A sont en ligne sur le serveur de THEIA. Elles sont accessibles depuis cette adresse http://spirit.cnes.fr/resto/Landsat/

Nous vous conseillons aussi la lecture du guide d'utilisation du serveur de distribution, et la description de leur traitement.

The LANDSAT 5 and 7 data at level 2A are available on THEIA's server. You may access them from http://spirit.cnes.fr/resto/Landsat/.

We advise you also to read the short user's guide for the distribution server, as well as the processing description.

## Phased orbits, how do they work ?

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As we are working to set a new Take5 experiment with SPOT5, here are some explanations of how it is possible to change the repeat cycle of a satellite from 26 days to 5 days, by just changing the satellite altitude by a couple of kilometres. There is nothing complicated behind that, just some simple arithmetic.

A phased orbit is an orbit for which the satellite repeats the same trajectory periodically. From its orbit at an altitude of 822 km, SPOT5, like its predecessors, has a cycle of 26 days. Every 26 days, it overflies the same places on earth. In 26 days, SPOT5 makes 369 revolutions around the earth. In 24 hours, a SPOT satellite runs through 369/26=14.19 orbits. Lowering its altitude by 2 km, the satellite slows a little, but the length of the circle it has to run along is reduced. It takes a little less time to make a revolution around the earth. The satellite does exactly 14.2 orbits per day.

Here are some of the orbits of SPOT4 (Take5), with some of the sites observed in France and North Africa during the experiment. The satellite started with the Cyan track, then the green one on the day after, then the yellow one on the next day and so on. 5 days later, it came back to the cyan orbit. You may see that it was possible to acquire a site on on the green track from the adjacent one on the cyan track.

14.2 orbits per days, is equivalent to 71 orbits in 5 days. After 71 orbits and 5 days exactly  SPOT4 was always at the same place during the Take5 experiment, and its cycled was changed from 26 to 5 days.

I have been also asked how the initial 26 days repeat cycle of SPOT5 was defined. The CNES engineers who designed it wanted to make it possible to observe each point on the earth from the vertical. As the SPOT satellites had a field of view of 116 km using both instruments, with a 26 days repeat cycle we had 116x26x14.19 = 43000km, just a little more than earth equator length. However, it was quickly seen that users did not ask for exactly vertical images and that the instruments were programmed mostly independently looking in different directions. However, the 26 days cycle was kept for all the SPOT satellites just as the High Speed Trains rail separation is related to the width of the hindquarters of a horse.

Finally, nothing would prevent from using the SPOT satellites from a 5 days repeat cycle orbit, which would not really change the ability to use the images how they are used now, but would allow new possibilities thanks to the possibility to observe users from constant viewing angles.

It is a little funny to observe that SPOT6 and SPOT7 do not use the initial SPOT orbit, and only fly at an altitude of 694 km but still with a 26 days phased orbit, this time obtained with 379/26=14.58 orbits per day. However, the justification cannot be the field of view, as this field of view is only 60 km.  But just by rising the orbit by a few kilometers, a 5 days orbit could be obtained