The performance stability of CCD detectors and video electronics during life time is an important issue for most of space missions. Several items are concerned, such as CCD dark signal increase, induced by space radiation environment (dose effects, proton hits, etc... ). Ground tests are performed to predict on-board behaviour and end-of-life performance. But generaly this approach cannot achieve a rigorous representation of mission conditions.
Experience feedback from in-flight measurements is therefore very useful in order to infer what really occurs and to allow comparison between actual findings and ground tests.
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.
MODIS is the key instrument for the NASA’s EOS Terra and Aqua missions, launched in late 1999 and early 2002, respectively. MODIS has 20 reflective solar bands (RSB) and 16 thermal emissive bands (TEB). MODIS RSB are calibrated on-orbit using an on-board solar diffuser and regularly scheduled lunar observations. For each instrument, the scheduled lunar observations are made through its space view (SV) port at nearly identical lunar phase angles via spacecraft roll maneuvers. Occasionally, unscheduled lunar observations at different phase angles are also collected by both Terra and Aqua MODIS. The PLEIADES system is composed of two satellites, PLEIADES-1A launched at the end of 2011 and PLEIADES-1B a year later. The PLEIADES has 5 reflective solar bands or channels (blue, green, red, nearinfrared, and panchromatic) that are calibrated on-orbit using observations of Pseudo Invariant Calibration Sites (PICS). Since launch, more than 1000 lunar images covering the phase angle range of ±115° have been acquired by PLEIADES- 1B for its on-orbit calibration and sensitivity study of lunar calibration methods. This paper provides an overview of MODIS and PLEIADES lunar observations and an assessment of their calibration difference based on lunar observations made over a range of phase angles. Also discussed in this paper are strategies and future effort that could potentially benefit other earth observing sensors and improve the calibration accuracy and consistency of existing lunar model(s).
PLEIADES is a dual Earth observation system composed of two satellites, PLEIADES-1A and PLEIADES-1B, respectively launched at the end of 2011 and 2012. This imagery system, led by CNES, has four spectral bands, blue, green, red and near infrared, with a spatial resolution of 2.8 m and a panchromatic band with a resolution of 0.7 m in vertical viewing. Its swath is about 20 km. In the framework of the PLEIADES radiometric calibration, studies took place in order to determine the calibration precision that could be reached from the acquisitions realized on the Moon. Indeed, the precisions reached from observations of calibration sites on Earth (African deserts, Antarctica, clouds, instrumented sites) are about 2-3% for most of the spectral bands in the visible and the near infrared spectra. It is very difficult to further improve this precision down to 1% because each method has its own limitations, generally due to atmospheric disturbances. In this context, the Moon seems to be an ideal calibration site: there is no atmosphere and its surface properties – thus its optical properties - are perfectly stable. Taking advantage of the high level of agility of PLEIADES, we performed an intensive observation campaign of the Moon in addition to the nominal acquisitions – when the Moon phase angle is about 40°. This intensive observation of the Moon, named POLO for Pleiades Orbital Lunar Observations, consists of a thousand acquisitions covering the phase angle range ±115 deg. The Moon was acquired as frequently as once every orbit, which represents acquisitions every 100 minutes. This paper provides an overview of these lunar experiments and an assessment of the variation of the irradiance of the Moon with phase angle. This paper also discusses a way to improve the phase angle dependence of existing lunar models.
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.
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.
PLEIADES earth observing system consists of two satellites designed to provide optical 70cm resolution images to civilian and defense users. The first Pleiades satellite 1A was launched on December 2011 while the second satellite Pleiades 1B was placed on orbit, one year after, on December 2012. The calibration operations and the assessment of the image of the two satellites have been performed by CNES Image Quality team during the called commissioning phase which took place after each launch and lasted each time less than 6 months. The geometric commissioning activities consist in assessing and improving the geometric quality of the images in order to meet very demanding requirements. This paper deals with the means used and methods applied, mainly the innovative ones, in order to manage these activities. It describes both their accuracy and their operational interest. Finally it gives the main results for geometric image quality performances of the PHR system.
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.
The SPOT5 remote sensing satellite was launched in May 2002. It provides SPOT service continuity above and beyond SPOT4 operation but the SPOT5 system also significantly improves the SPOT service with the new characteristics of its two HRG (High Resolution Geometry) cameras and its HRS (High Resolution Stereo) camera. SPOT5's first two months of life in orbit were dedicated to instrument calibration and the assessment of image quality performances. During this period, the CNES team used specific target programming to compute image correction parameters and estimate the performance of the image processing chain, at system level. This paper focuses on the relative radiometric performances of the different spectral bands for the three instruments, deduced from in-flight measurements. For each spectral band, a radiometric model gives the ratio between detector response and input radiance. This model takes the architecture of the onboard image chain into account. Calibration provides the normalisation parameters (dark currents and relative inter-detector sensitivities) used to correct the images. The quality of the corrected images is quantified through several signal-to-noise ratio measurements based on different techniques. These methods are presented and their accuracy is discussed. Finally, a comparison is given between flight measurements and ground measurements.