Some examples of the use of structure functions in the analysis of satellite images of the ocean

International audienceThis paper deals with the use of the structure function, also called a variogram, to analyze satellite images of the ocean. The structure function is a powerful tool for the description of two-dimensional random fields. Its characteristics are used in two different examples. First, the behavior of the structure function close to its origin gives the variance of salt and pepper noise within an image. Such a method has been applied to various spaceborne sensors. Second, fitting the experimental structure function by a power law demonstrated the way the turbulent energy at the surface of the ocean is transferred from scales to scales

Potentials of images from geostationary satellite for the assessment of solar energy parameters

International audienceImages taken by meteorological geostationary satellites are currently used to map global radiation. Several methods exist which process these images. Among them, the Heliosat method ranks as one of the most accurate and one of the easiest to use. Typical uncertainties (rms.) of such assessment are about 10 % for the monthly mean global irradiance on the ground, about 10 % for the daily value, and about 20 % for the hourly value. Use of ground measurements and proper processing (e. g. kriging) increase the accuracy of the estimates in global radiation. Splitting the global radiation into its direct and diffuse components is not currently made on an operational basis. This requires additional information, such as turbidity in clear skies and geometrical and optical properties of the clouds, which is not available from the current geostationary satellites. However, some methods have been proposed which make use of the sole satellite data and which rely on assumptions replacing the missing information. In assessing the components of the global radiation, one should take care of the space-time scales he is dealing with: they are of paramount importance in designing a method, of its usefulness, as well as in the evaluation of its accuracy. It is emphasized that satellite images are measurements taken by a radiometer and as such obey the theory of signal processing, particularly space-time sampling constraints. The application of such images and of their processing is limited to time scales equal or greater than 1 hour, and to space scale equal or greater than two pixels (about 20 km). Therefore, we cannot treat the case of fragmented cloud coverage, unless additional external information is available. An important aspect to be taken into account when satellite-derived and ground-measured information are to be compared, either for calibration or validation, is the fact that satellite information is a snapshot over a large area, while ground information is a time-integrated pinpoint measurement. Of course practical problems to be solved are strongly dependent upon the particular applications, with major effects of space and time samplings. For example, in daylighting for a peculiar building, detailed spatial distribution is required for a very accurate assessment of the luminance. The large size of a pixel prevents from having this information, and the best and most useful that can be got is likely a sky class together with some relevant probabilities. In daylighting as well as in many other applications in solar energy, it is necessary to split the global radiation into its diffuse and direct components in order for example to be able to compute values on tilted surfaces. Keeping in mind the above-mentioned limitations, several works have been made to infer diffuse radiation from satellite images. Some of these works are briefly presented, their principles and their uncertainties. Then future tracks are discussed which rely heavily on numerical models of the radiative transfer within the cloudy or cloud-free atmosphere. The advantages and drawbacks, as well as the pending questions are presented. Such approaches require additional information that are not available by the sole use of the data of the geostationary satellites. The supply of these data, the robustness of the method to these data are discussed, too. It is shown that some properties, such as turbidity, geometrical and optical properties of the clouds, are predominant for the assessment of the diffuse and direct components. These properties are best estimated by some other satellites and it is recommended to use them in order to gain in accuracy. However this leads to a large increase in complexity of the processing chain as well of the fundamental problems to be solved with respect to the space-time characteristics

The present achievements of the EARSeL - SIG "data fusion"

International audienceData fusion is becoming of paramount importance in Earth observation. A Special Interest Group "data fusion" has been created within EARSeL and SEE to better understand data fusion for a better exploitation of the synergy between all instruments observing and measuring our environment. The SIG also intends to make recommendations to agencies, industries and the scientific community. The communication presents the SIG and its objectives. The achievements already accomplished are: i) statement of the importance of data fusion in Earth observation, ii) dissemination of the fundamentals and properties of data fusion, iii) concept, definition and terms of reference, and iv) assessment of methods for the synthesis of images at higher spatial resolution. Further tracks are indicated. Several publications have been made, resulting from the SIG meetings. The SIG has a wide audience world-wide, benefiting from the use of Internet. It has also launched a series of high-quality international conferences in Cote d'Azur

Quality of high resolution synthesised images: Is there a simple criterion ?

International audienceMethods exist which are synthesizing high spatial resolution high spectral content images from a set of low spatial resolution high spectral content images and high spatial resolution low spectral content images. There is a need to quantify the quality of these synthesized images for both the producers and customers of such fused products. Some protocols are discussed. The main concern is on the definition of a parameter characterizing the quality. The requirements on this parameter are discussed. A review of published parameters is made and a new one is proposed. From published works, it is shown that a good quality is achieved when the parameter is less than 3

A conceptual approach to the fusion of earth observation data

International audienceData fusion is a subject becoming increasingly relevant as scientists try to extract more and more information from remotely sensed data using their synergy. A definition of data fusion is proposed, which allows to set up a conceptual approach to the fusion of Earth observation data by putting an emphasis on the framework and on the fundamentals in remote sensing underlying data fusion instead of on the tools and means themselves, as is done usually. Further definitions are given, which describe the objects intervening in any problem of data fusion. Fusion may be performed at different levels, simultaneously: measurement level (also improperly called pixel level), at attribute level, and at rule, or decision, level. It is shown that any process of fusion should deal with the selection of the representation space, the level of fusion and the processing to be applied onto the sources of information. The various architectures of fusion systems are presented. Their properties are discussed, including aspects in accuracy, time-consuming, operational constraints. From these basic architectures, more complex systems can be built, which are suitable to a given application

HelioClim-1: 21-years of daily values in solar radiation in one-click

International audienceThe HelioClim-1 database offers daily means of surface solar irradiance for the period 1985-2005 and has been cre-ated from archives of images of the Meteosat First Generation satellites. Expectations of users regarding access to similar data were carefully analyzed, especially regarding dissemination of and access to data, and were taken at the heart of the design of the database. Efforts were made to deliver time-series spanning over 21 years very rapidly on the Web with a limited number of clicks. The soundness of the approach by MINES ParisTech is now rewarded by the large number of access to HelioClim-1, approximately 400 per workday, and by the number of scientific publications using these data. The Global Earth Observation System of Systems (GEOSS) has declared HelioClim-1 as a Data Collection of Open Resources for Everyone (GEOSS Data-CORE) in November 2011. A Web processing service (WPS) obeying the OGC (Open Geospatial Consortium) standard has been developed. Being interoperable, it can be invoked in operational routines, such as those under development in the European funded ENDORSE and MACC projects

A European proposal for terms of reference in data fusion

International audienceData fusion is a subject becoming increasingly relevant as scientists try to extract more and more information from remotely sensed data. Archives are growing, as well as the number of space missions devoted to Earth observation. It is generally correct to assume that improvements in terms of classification error probability, rejection rate, and interpretation robustness, can only be achieved at the expenses of additional independent data delivered by more separate sensors. Sensor data fusion allows to formalise the combination of these measurements, as well as to monitor the quality of information in the course of the fusion process. A Special Group of Interest ‘data fusion' has been established jointly within the European Association of the Remote Sensing Laboratories (EARSeL) and the French Society for Electricity and Electronics (SEE). This Group has defined several major tasks to be handled in order to increase our understanding and use of data fusion. One of these tasks is the establishment of terms of reference that are accepted by both the scientific and the industrial communities at least in Europe. A definition of the data fusion is proposed, which allows to set up a conceptual approach to the fusion of Earth observation data by putting an emphasis on the framework and on the fundamentals in remote sensing underlying data fusion. Several other definitions are given which are useful to describe any problem of data fusion

In-flight interband calibration on AVHRR data by a cloud-viewing technique

ISBN 90-5410-933-5International audienceA significant degradation in the responsivity of the AVHRR radiometers aboard the NOAA satellite series, affects the index vegetation (NDVI), which is an important source of information for monitoring vegetation conditions on regional and global scales. Many studies have been carried out which use the viewing Earth calibration approach in order to provide accurate calibration correction coefficients for the computation of the vegetation index using the visible and near-infrared spectral channels 1 and 2 of AVHRR. This study deals with the interband calibration of AVHRR visible and near-infrared data by means of an cloud-viewing technique. This technique is simple to implement and can be used in real-time. It is also well-suited to the processing of large time-series of data. Results are presented for various NOAA satellites and are in full agreement with the calibration degradation model proposed by NOAA and various authors

An overview of concepts in fusion of Earth data

International audienceA definition of the data fusion is proposed, which allows to set up a conceptual approach to the fusion of Earth observation data by putting an emphasis on the framework and on the fundamentals in remote sensing underlying data fusion. Further definitions are given which describe the information intervening in any problem of data fusion. Fusion may be performed at different levels: at measurements level, at attribute level, and at rule or decision level. Several problems are to be solved prior to any process of fusion. They deal with either the selection of the representation space and the level of fusion, or with the processing to be applied onto the data. A formalism is discussed which sketches a fusion process. Several examples of fusion processes are given using this formalism

The ARSIS concept in image fusion: an answer to users needs

International audienceThe ARSIS concept (from its French name "amélioration de la résolution spatiale par injection de structures" that means improvement of the spatial resolution by structure injection) is briefly presented. It offers a framework for the synthesis of multi-modality images at highest spatial resolution by fusion of two sets of images. This concept is comprised of three types of models. These different types are introduced and a set of solutions for implementation proposed. Four solutions are detailed and applied to a satellite Ikonos image of the city of Hasselt, Belgium. The fusion products are analyzed visually and quantitatively. These analyses enhance the benefits of offering a set of solutions to remote sensing end-users in order to fulfill their needs