The PLEIADES-HR Earth observing satellites, under CNES development, combine a 0.7m resolution panchromatic channel, and a multispectral channel allowing a 2.8 m resolution, in 4 spectral bands. The 2 satellites will be placed on a sun-synchronous orbit at an altitude of 695 km. The camera operates in push broom mode, providing images across a 20 km swath. This paper focuses on the specifications, design and performance of the TDI detectors developed by e2v technologies under CNES contract for the panchromatic channel. Design drivers, derived from the mission and satellite requirements, architecture of the sensor and measurement results for key performances of the first prototypes are presented.
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.
PLEIADES is an earth observing system conducted by the French National Space Agency, CNES. It consists of two satellites launched on December 2011 (PHR-1A) and December 2012 (PHR-1B), both designed to provide optical pushbroom imagery on five spectral bands to civilian and defense users, with ground sample distance up to 70 cm. During inflight image quality commissioning, radiometric activities included inter-detector normalization coefficients computation, refocusing operations, MTF assessment and estimation of signal to noise ratios. This paper presents inflight results for both satellites. It focuses on several innovative methods that were implemented, taking advantage of the satellite platform great agility. These methods are based on processing images obtained through dedicated exotic guidance. In particular, slow-motion steering enables an efficient estimation of the instrumental noise model, since during acquisition each detector has been viewing a stable ground target along different time samples. Conversely, rotated retina guidance is used to guarantee that all different elementary detectors have successively viewed the same set of landscape samples during acquisition. Non-uniformity of detector sensitivities can then be characterized, and on-board coefficients used prior to compression can be calibrated in order to prevent vertical striping effects on operational images. Defocus control and Point Spread Function estimation can be easily obtained through processing acquisitions of stars associated to various spectral characteristics, for different adjustments of the refocusing system. All these methods allow an accurate estimation of radiometric performance on the whole range of specified spectral radiances, while drastically reducing the number of required acquisitions on natural targets.
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 is the highest resolution civilian earth observing system ever developed in Europe. This imagery programme is conducted by the French National Space Agency, CNES. It will operate in 2008-2009 two agile satellites designed to provide optical images to civilian and defence users. Images will be simultaneously acquired in Panchromatic (PA) and multispectral (XS) mode, which allows, in Nadir acquisition condition, to deliver 20 km wide, false or natural colored scenes with a 70 cm ground sampling distance after PA+XS fusion. Imaging capabilities have been highly optimized in order to acquire along-track mosaics, stereo pairs and triplets, and multi-targets. To fulfill the operational requirements and ensure quick access to information, ground processing has to automatically perform the radiometrical and geometrical corrections. Since ground processing capabilities have been taken into account very early in the programme development, it has been possible to relax some costly on-board components requirements, in order to achieve a cost effective on-board/ground compromise. Starting from an overview of the system characteristics, this paper deals with the image products definition (raw level, perfect sensor, orthoimage and along-track orthomosaics), and the main processing steps. It shows how each system performance is a result of the satellite performance followed by an appropriate ground processing. Finally, it focuses on the radiometrical performances of final products which are intimately linked to the following processing steps : radiometrical corrections, PA restoration, image resampling and PAN-sharpening.
Optical remote sensing images are usually acquired according to the classical pushbroom principle. A linear array of CCD detectors, placed in the focal plane of the telescope, acquires a scanline over an integration time. The satellite's motion along its orbit, which is perpendicular to the linear array, ensures acquisition of successive lines. Because of inter-detector sensitivity differences, the image of a uniform landscape is striped vertically. Detector normalization aims at correcting these relative sensitivities and delivering uniform images of uniform areas. Determination of inter-detector coefficients requires observation of one uniform landscape, provided that each detector behaves linearly.
High resolution optical satellites like the future French PLEIADES-HR have to face a lack of signal, which moves the useful signal range towards the non-linear part of the detector response. For such designs, normalization has to be run with a non-linear model : this is a cost-effective way to improve image quality at low radiances and relax detector sorting. Regarding in-flight operations, non-linear parameters identification requires observation of several uniform landscapes and may be actually very difficult to run, because of the uniformity constraint.
An efficient way to bypass the quest of uniformity is to use the satellite agility in order to align the ground projection of the scanline on the ground velocity. This weird viewing principle allows all the detectors to view the same landscape. Thus, non-linear normalization coefficients can be computed by a histogram matching method.
The goal of this paper is to present the Pleiades-HR non-linear normalization model, the suited steered mode and the method to compute the coefficients within Pleiades-HR context.
Since SPOT1 launch in February 1986 and until SPOT5 launch in May 2002, the methods and means to insure the best quality of the images delivered to SPOT IMAGE customers have been continuously improved and updated. The quality of the corrected images is quantified through several figures of merit, including, for radiometric quality, the Signal-to-Noise Ratio (SNR).
Radiometric noise is due to two separate phenomena:
-column-wise noise: it represents on-board image chain performances.
-line-wise noise: normalization defects (radiometric model deviations) may lead to visible “columns” on a uniform landscape.
For each image, these two noises are combined in an “image noise” that quantifies the variations of the digital numbers on a uniform landscape.
Different techniques can be used to assess these different noises in-flight. For SPOT1 to SPOT4, the on-board lamp is used for both normalization and SNR assessment. For SPOT5, without lamp unit, we use images acquired over the quasi-uniform landscapes of Antarctic and Greenland for normalization. However, uniformity of these landscapes is not sufficient to accurately measure the SNR. So, a new method experimented during SPOT3 in-flight commissioning phase and operational for SPOT5 is applied. It consists in using two images of the same landscape acquired simultaneously to eliminate the landscape contribution.
This paper focuses on the presentation of this new method and compares its accuracy to the other methods. Finally, a comparison between flight measurements and ground measurements before launch is given.
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.
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. SPOT4's 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.