Daniel Greslou

PLEIADES-HR image quality commissioning foreseen methods

The new French high resolution earth observing satellite PLEIADES-HR will be launched in 2011. A specific design and new technologies have been embarked to provide great agility. These capabilities offer new methods to perform the image quality activities during the commissioning period. Some of them depend on dedicated guidance of the satellite platform and specific targets on the Earth or beyond. For instance, one concerns the so-called AMETHIST method to compute the normalization coefficients of the radiometric model. Another one is based on a controlled slow-motion to get a reference image line whose deviation along the image will give information on radiometric noise or attitude perturbations. Another uses the stars to measure the line-of-sight dynamic stability or the instrument refocusing needs. These originals methods and some more are briefly presented in this paper after an introduction of PLEIADES-HR capacities.

In-flight attitude perturbances estimation: application to PLEIADES-HR satellites

This paper deals with the problem of retrieving attitude perturbances in the framework of the PLEIADES-HR optical satellites. Thus, two complementary methods are compared. The first one uses the high agility capacity of satellites to acquire stars in an inertial steering mode. The second method exploits the fact that multispectral CCD arrays are shifted in the telescope focal plane in the velocity direction: for a same ground point, the resulting images are not affected by the same attitude perturbances. The resulting misregistrations can be exploited to deduce information about the attitude platform. Both methods have been applied to PLEIADES-HR satellites, during commissioning period.

First in-flight results of Pleiades 1A innovative methods for optical calibration

The PLEIADES program is a space Earth Observation system led by France, under the leadership of the French Space Agency (CNES). Since it was successfully launched on December 17th, 2011, Pleiades 1A high resolution optical satellite has been thoroughly tested and validated during the commissioning phase led by CNES. The whole system has been designed to deliver submetric optical images to users whose needs were taken into account very early in the design process. This satellite opens a new era in Europe since its off-nadir viewing capability delivers a worldwide 2- days access, and its great agility will make possible to image numerous targets, strips and stereo coverage from the same orbit. Its imaging capability of more than 450 images of 20 km x 20 km per day can fulfill a broad spectrum of applications for both civilian and defence users. For an earth observing satellite with no on-board calibration source, the commissioning phase is a critical quest of wellcharacterized earth landscapes and ground patterns that have to be imaged by the camera in order to compute or fit the parameters of the viewing models. It may take a long time to get the required scenes with no cloud, whilst atmosphere corrections need simultaneous measurements that are not always possible. The paper focuses on new in-flight calibration methods that were prepared before the launch in the framework of the PLEIADES program : they take advantage of the satellite agility that can deeply relax the operational constraints and may improve calibration accuracy. Many performances of the camera were assessed thanks to a dedicated innovative method that was successfully validated during the commissioning period : Modulation Transfer Function (MTF), refocusing, absolute calibration, line of sight stability were estimated on stars and on the Moon. Detectors normalization and radiometric noise were computed on specific pictures on Earth with a dedicated guidance profile. Geometric viewing frame was determined with a particular image acquisition combining different views of the same target. All these new methods are expected to play a key role in the future when active optics will need sophisticated in-flight calibration strategy.

Sentinel 2 global reference image

Sentinel-2 is a multispectral, high-resolution, optical imaging mission, developed by the European Space Agency (ESA) in the frame of the Copernicus program of the European Commission. In cooperation with ESA, the Centre National d’Etudes Spatiales (CNES) is responsible for the image quality of the project, and will ensure the CAL/VAL commissioning phase. Sentinel-2 mission is devoted the operational monitoring of land and coastal areas, and will provide a continuity of SPOT- and Landsat-type data. Sentinel-2 will also deliver information for emergency services. Launched in 2015 and 2016, there will be a constellation of 2 satellites on a polar sun-synchronous orbit, imaging systematically terrestrial surfaces with a revisit time of 5 days, in 13 spectral bands in visible and shortwave infra-red. Therefore, multi-temporal series of images, taken under the same viewing conditions, will be available. So as to ensure for the multi-temporal registration of the products, specified to be better than 0.3 pixels at 2σ, a Global Reference Image (GRI) will be produced during the CAL/VAL period. This GRI is composed of a set of Sentinel-2 acquisitions, which geometry has been corrected by bundle block adjustment. During L1B processing, Ground Control Points will be taken between this reference image and the sentinel-2 acquisition processed and the geometric model of the image corrected, so as to ensure the good multi-temporal registration. This paper first details the production of the reference during the CALVAL period, and then details the qualification and geolocation performance assessment of the GRI. It finally presents its use in the Level-1 processing chain and gives a first assessment of the multi-temporal registration.

Automatic and generic mosaicing of satellite images

The CNES (the French Space Agency) has specified and developed a fully automatic mosaicing processing unit, in order to generate satellite image mosaics under operational conditions. This tool, called SIGMA, can automatically put each input image in a common geometry, homogenize the radiometry, and generate orthomosaics using stitching lines.

Geometric improvement for Earth observation applications

Since SPOT1, the French national space center (CNES) has worked on improving the geometry of Earth observation spacecrafts. The accuracy of sensor calibration is one of the main key points for any Earth observation application such as orthorectification, DEM generation or surface change detection [7]. Two families of methods have been developed by CNES for twenty years: absolute methods and relative methods. These methods are used to characterize a pushbroom acquisition along the detector line and the time line. By this way, the viewing directions are measured and the residual of the spacecrafts attitude angles (not restituted by the AOCS) are estimated. This information can complete the geometric model of all the scenes acquired by the instrument and is used in all geometric applications. We will first consider the absolute methods and then the relative methods.

Sentinel-2A image quality commissioning phase final results: geometric calibration and performances

In the frame of the Copernicus program of the European Commission, Sentinel-2 offers multispectral high-spatial-resolution optical images over global terrestrial surfaces. In cooperation with ESA, the Centre National d’Etudes Spatiales (CNES) is in charge of the image quality of the project, and so ensures the CAL/VAL commissioning phase during the months following the launch. Sentinel-2 is a constellation of 2 satellites on a polar sun-synchronous orbit with a revisit time of 5 days (with both satellites), a high field of view - 290km, 13 spectral bands in visible and shortwave infrared, and high spatial resolution - 10m, 20m and 60m. The Sentinel-2 mission offers a global coverage over terrestrial surfaces. The satellites acquire systematically terrestrial surfaces under the same viewing conditions in order to have temporal images stacks. The first satellite was launched in June 2015. Following the launch, the CAL/VAL commissioning phase is then lasting during 6 months for geometrical calibration. This paper will point on observations and results seen on Sentinel-2 images during commissioning phase. It will provide explanations about Sentinel-2 products delivered with geometric corrections. This paper will detail calibration sites, and the methods used for geometrical parameters calibration and will present linked results. The following topics will be presented: viewing frames orientation assessment, focal plane mapping for all spectral bands, results on geolocation assessment, and multispectral registration. There is a systematic images recalibration over a same reference which is a set of S2 images produced during the 6 months of CAL/VAL. This set of images will be presented as well as the geolocation performance and the multitemporal performance after refining over this ground reference.

PLEIADES satellites image quality commissioning

The CNES Pleiades-HR satellites have been launched December 17th 2011 and December 2nd 2012. They provide optical images to civilian and defense users with a resolution of 70 cm and a swath of 20 km in false or natural colors. Coverage is almost world-wide with a revisit interval of 24 h. The new capabilities offered by these satellites agility allowed imagine new methods of image calibration and performances assessment. This paper presents all the operations that were conducted by the CNES Image Quality Team during the commissioning phases and also give the main results for every image quality performance.

Pleiades-HR image products 5 years after launch

Pleiades-HR is a high resolution remote sensing system developed by the French Space Agency (CNES) for civil and military users. The constellation is composed of two identical satellites PHR1A launched on 2011, December 17th and PHR1B launched one year after, on 2012, December 2nd. More than 600 images can be daily acquired by each satellite in various viewing angles conditions: the satellites are able to target images with viewing angles greater than 47°. Since the launch of the first satellite, major improvements have been integrated in the ground processing to enhance the quality of products and offer new options. Starting from a Pleiades-HR system overview, this paper offers a description of the ground processing and presents an assessment of the different products available now, including image quality performances.