Aime Meygret

In-flight refocusing and MTF assessment of SPOT5 HRG and HRS cameras

The MTF (Modulation Transfer Function) is a means of characterizing the spatial resolution of the instruments. So, the MTFs of HRG and HRS cameras are parts of image quality parameters assessed during the in-flight commissioning phase. Vibrations during the launch and transition from air to vacuum may defocus the HRG cameras and degrade their MTF. Therefore, SPOT5 HRG cameras are refocused before measuring their MTF. The paper first describes the HRG focusing procedure that uses both cameras viewing the same landscape: the focus of one camera is changed while the other is fixed and used as a reference. Results are given for each camera in terms of best focus and focus variation in the field of view. These results are compared to those provided by an autotest system, on-board each HRG camera, that images a high frequency periodic pattern while the focus is changed. Then, MTF measurements are presented. The MTF of HRG cameras is measured by imaging a spotlight that aimed at the satellite; the results are compared with pre-flight measurements. Besides, the MTF of HRS cameras is assessed by imaging landscapes with edge patterns; the main objective is to compare the two HRS cameras.

Calibration of SPOT4 HRVIR and Vegetation cameras over Rayleigh scattering

The Rayleigh scattering over a clear ocean is a target which radiance is very well modeled and which enables to calibrate the short wavelengths of remote sensing instruments. But the quality of the calibration strongly depends on the evaluation of the other contributors to the observed Top Of Atmosphere radiance i. e. aerosol scattering and reflection over the sea surface (water color, foam, glint...). However these contributors can be reduced by appropriate viewing conditions. This technique is used to calibrate B1 (051-0.59 µm) and B2 (0.61-0.68µm) channels of HRVIR camera, and B0 (0.4-0.5µm) and B2 channels of VEGETATION camera both of which are aboard SPOT4. This article presents the calibration results obtained during the satellite two years in orbit. The results are compared to: - pre-flight results (integrating sphere) - in-flight results. The in-flight results are provided by: - on board calibration system (lamp and sun sensor) - vicarious calibration over test sites (White Sands, La Crau) - calibration over stable deserts - calibration over the sun glint The analysis of the sensitivity of the calibration to the different parameters used to model the TOA radiance shows the accuracy of such a technique.

ROSAS: a robotic station for atmosphere and surface characterization dedicated to on-orbit calibration

La Crau test site is used by CNES since 1987 for vicarious calibration of SPOT cameras. The former calibration activities were conducted during field campaigns devoted to the characterization of the atmosphere and the site reflectances. Since 1997, au automatic photometric station (ROSAS) was set up on the site on a 10m height pole. This station measures at different wavelengths, the solar extinction and the sky radiances to fully characterize the optical properties of the atmosphere. It also measures the upwelling radiance over the ground to fully characterize the surface reflectance properties. The photometer samples the spectrum from 380nm to 1600nm with 9 narrow bands. Every non cloudy days the photometer automatically and sequentially performs its measurements. Data are transmitted by GSM (Global System for Mobile communications) to CNES and processed. The photometer is calibrated in situ over the sun for irradiance and cross-band calibration, and over the Rayleigh scattering for the short wavelengths radiance calibration. The data are processed by an operational software which calibrates the photometer, estimates the atmosphere properties, computes the bidirectional reflectance distribution function of the site, then simulates the top of atmosphere radiance seen by any sensor over-passing the site and calibrates it. This paper describes the instrument, its measurement protocol and its calibration principle. Calibration results are discussed and compared to laboratory calibration. It details the surface reflectance characterization and presents SPOT4 calibration results deduced from the estimated TOA radiance. The results are compared to the official calibration.

VENµS (Vegetation and environment monitoring on a new micro satellite)

VENµS is an Earth observation demonstration mission developed in cooperation between FRANCE and ISRAEL.

Overview Of Sentinel-2

GMES is a joint initiative of the European Commission (EC) and the European Space Agency (ESA), designed to establish a European capacity for the provision and use of operational monitoring information for environment and security applications. ESAs role in GMES is to provide the definition and the development of the space- and ground-related system elements. GMES Sentinel-2 mission provides continuity to services relying on multi-spectral high-resolution optical observations over global terrestrial surfaces. The key mission objectives for Sentinel-2 are: (1) to provide systematic global acquisitions of high-resolution multi-spectral imagery with a high revisit frequency, (2) to provide enhanced continuity of multi-spectral imagery provided by the SPOT series of satellites, and (3) to provide observations for the next generation of operational products such as land-cover maps, land change detection maps, and geophysical variables. Consequently, Sentinel-2 will directly contribute to the Land Monitoring, Emergency Response, and Security services. The corresponding user requirements have driven the design towards a dependable multi-spectral Earth-observation system featuring the MSI with 13 spectral bands spanning from the visible and the near infrared to the short wave infrared. The spatial resolution varies from 10 m to 60 m depending on the spectral band with a 290 km field of view. This unique combination of high spatial resolution, wide field of view and large spectral coverage will represent a major step forward compared to current multi-spectral missions. The mission foresees a series of satellites, each having a 7.25-year lifetime (extendable to 12 years) over a 20-year period starting with the launch of Sentinel-2A foreseen by mid-2014. During full operations two identical satellites will be maintained in the same sun synchronous orbit with a phase delay of 180° providing a revisit time of five days at the equator.

SPOT5 radiometric image quality

The SPOT5 remote sensing satellite was launched in May 2002. It provides continuity of the SPOT service owing to new characteristics of its two HRG (High Resolution Geometry) cameras and its two HRS (High Resolution Stereo) cameras. The image quality performances were assessed during SPOT5s first two months of life (commissioning phase) and are still monitored during the commercial operations. To reach this goal, the CNES team uses a specific target programming in order to compute image correction parameters and estimate the performance of the system. This paper focuses on the radiometric performances of the different instruments, and on the corresponding tasks.

In-flight assessment of SPOT5 image quality

SPOT5, the fifth satellite of the SPOT remote sensing satellite family was successfully launched on the 4th of May 2002. SPOT5 is designed to ensure continuity of data acquisition and space image services but also to provide users with advanced products. It flies two identical cameras named HRG (High Resolution Geometry) providing a 2.5 m and a 5 m resolution in a panchromatic mode and a 10 m resolution in a multi-spectral mode, still keeping a 60-km ground field. Stereo application is part two of the SPOT5 mission; the satellite flies a specific High Resolution Stereo instrument (HRS) made up of two telescopes allowing a 20° fore view and a 20° aft view over a 120-km swath, sampling the landscape every 5m. VEGETATION2, a wide field of view imaging radiometer complements the mission thanks to its daily coverage of the earth. The paper presents the mission, the commissioning phase that followed the satellite launch, the assessment of the image quality and the first calibration results.

SPOT4 HRVIR first in-flight image quality results

The SPOT4 remote sensing satellite was successfully launched at the end of March 1998. It was designed first of all to guarantee continuity of SPOT services beyond the year 2000 but also to improve the mission. Its two cameras are now called HRVIR since a short-wave infrared (SWIR) spectral band has been added. Like their predecessor HRV cameras, they provide 20-meter multispectral and 10-meter monospectral images with a 60 km swath for nadir viewing. SPOT4s first two months of life in orbit were dedicated to the evaluation of its image quality performances. During this period of time, the CNES team used specific target programming in order to compute image correction parameters and estimate the performance, at system level, of the image processing chain. After a description of SPOT4 system requirements and new features of the HRVIR cameras, this paper focuses on the performance deduced from in-flight measurements, methods used and their accuracy: MTF measurements, refocusing, absolute calibration, signal-to-noise Ratio, location, focal plane cartography, dynamic disturbances.

SPOT4 : First in flight absolute calibration results

SPOT4, the fourth satellite of the SPOT family remote sensing satellites, was launched on the 20th of March 1998. During the first months, we calibrate the two identical on-board cameras named HRVIR (because of the added Mid Infra-Red channel) and VEGETATION, a wide field of view radiometer providing 1.15 kilometers resolution measurements in the same designed channels as HRVIR (B2, B3 and MIR), and we evaluate the quality of the images. Radiometric calibration results are presented in this paper. Different methods are applied based on the experience gained with SPOT1, 2, 3 and POLDER: (1) pre- launch measurements, (2) on-board calibration system, (3) vicarious calibration over test sites, (4) inter-SPOT calibration over desert areas, (5) calibration over the molecular scattering, (6) inter-cameras calibration between HRVIR1 and HRVIR2, (7) inter-cameras calibration between HRVIR and VEGETATION. The accuracy of each calibration procedure is estimated. The measurements are combined in a model that minimizes errors and provides the camera sensitivity as a function of time.

Vegetation: in-flight multiangular calibration

In the characterization of a space-borne wide field-of-view sensor, like Vegetation, the multi-angular calibration is strongly complementary to the absolute calibration. It is defined as the process of estimating the sensitivity variations at different points of the Vegetation wide field-of-view. This effect has to be integrated in the data processing. Pre-flight measurements were performed before launch, but because of heavy irradiations and aging of the different part of the sensor, it is necessary after launch to check and/or adjust the multi-angular calibration coefficients, gp. For this, the gp coefficients were split into three terms which required different methods: i/ first, the low-frequency term (gpLF) which refer to variation of the optic transmission which slightly decreases when viewing angle increases. The gpLF were verified using acquisitions over 20 desert sites for which TOA reflectances are accurately characterized (from ground measurements and POLDER/ADEOS-1 measurements). No in-flight variation of the gpLF were detected. ii/ second, the high-frequency term (gpHF) which refer to variation of the sensitivity of the elementary detectors. The gpHF were verified statistically using acquisitions over the Antarctica site and were accurately checked for the 4 spectral bands. ii/ third, the medium-frequency term (gpMF) which refer to various kinds of variation (optics, detectors...). The gpMF were verified during the 9pJpWDWLRQ like using the on-board calibration device (lamp profiles) and some small variations were identified (< 0.5% for B0, B2, B3 and ~1% for MIR). This aspect is still under investigation using acquisitions over Antarctica.