Christophe Latry

On board strip-based wavelet image coding for future space remote sensing missions

Spacecraft sensor obtained data has to be stored on-board until an opportunity arises for it to be transmitted to the ground. Compression algorithms used in space were based first on DPCM and then on DCT. Image quality evaluation determined however that for applications using the adaptive DCT, such as for SPOT 5, the maximum acceptable compression ratio was about 3 for high resolution observation and about 15 for scientific missions. Beyond that compression ratio, the occurrence of block effects in uniform areas and the loss of detail related to compression noise is no more acceptable for scientific use. To overcome these limits and to prepare future generations of Earth observation satellites and scientific missions, the authors propose a novel and efficient strip-based satellite image coder adapted for on board processing. The performances of their method are better than JPEG 2000 with a complexity lower than 60 operations per pixel. Furthermore, their method can be implemented for different image samplings. The authors quickly explain the general compression scheme used and the solution for on the fly DWT computation. They present the dynamic bit allocation and the regulation process chosen for rate control. Then, they evaluate the complexity and performances of their method comparatively to JPEG 2000.

SPOT5 THR mode

SPOT Earth observation satellites are based upon the PUSH- BROOM acquisition principle whereby a CCD linear array acquires images row by row, perpendicularly to the satellite track, with columnwise displacement being generated by the satellites motion on its orbit.

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.

Validation of an automatic image orthorectification processing

Following SPOT5 launch, Spotlmage and French National Geographic Institute (IGN) have decided to design a worldwide accurate database called Reference3D/sup TM/ using data from the High Resolution Stereoscopic SPOT5 instrument (HRS). Spotlmage and IGN have also decided to develop and commercialize a system called ANDORRE to produce orthorectifled images thanks to Reference3D/sup TM/ data. ANDORRE has been designed to take advantage of Reference3D/sup TM/ planimetric and altimetric accuracy to automatically register and rectify any image from SPOT satellites. In this framework, CNES is acting as prime contractor to realize and industrialize the algorithms following a preliminary study undertaken by IGN. This paper focuses on the validation of the geometric performances of the algorithms. A special attention is also paid to the processing time in order to achieve the rectification of a 24000/spl times/24000 image in less than one hour.

Staggered arrays for high resolution earth observing systems

Staggered arrays is the common rule for high resolution earth observing systems, particularly for panchromatic/multispectral acquisition: the linear arrays are shifted in the telescope focal plane along the satellite velocity direction, which means registration performances sensitivity to both terrain elevation and high frequency platform perturbances. This paper aims to show how this sensitivity may be transformed from a drawback to an asset, allowing to retrieve high frequency attitude perturbances ranging above the gyros cutoff frequency or refine the viewing directions, compute digital elevation model and apply superresolution technique with direct applications within the framework of SPOT5, Quickbird and Pleiades-HR satellites.

On-board optical image compression for future high-resolution remote sensing systems

Future high resolution instruments planned by CNES to succeed SPOT5 will lead to higher bit rates because of the increase in both resolution and number of bits per pixel, not compensated by the reduced swatch. Data compression is then needed, with compression ratio goals higher than the 2.81 SPOT5 value obtained with a JPEG like algorithm. Compression ratio should rise typically to 4 - 6 values, with artifacts remaining unnoticeable: SPOT5 algorithm performances have clearly to be outdone. On another hand, in the framework of optimized and low cost instruments, noise level will increase. Furthermore, the Modulation Transfer Function (MTF) and the sampling grid will be fitted together, to -- at least roughly -- satisfy Shannon requirements. As with the Supermode sampling scheme of the SPOT5 Panchromatic band, the images will have to be restored (deconvolution and denoising) and that renders the compression impact assessment much more complex. This paper is a synthesis of numerous studies evaluating several data compression algorithms, some of them supposing that the adaptation between sampling grid and MTF is obtained by the quincunx Supermode scheme. The following points are analyzed: compression decorrelator (DCT, LOT, wavelet, lifting), comparison with JPEG2000 for images acquired on a square grid, compression fitting to the quincunx sampling and on board restoration (before compression) versus on ground restoration. For each of them, we describe the proposed solutions, underlining the associated complexity and comparing them from a quantitative and qualitative point of view, giving the results of experts analyses.

Pleiades image quality: from users' needs to products definition

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.

Selection of the SPOT5 image compression algorithm

Improving the ground resolution of SPOT 5 compared with SPOT 4 involves multiplying the data rate by 4. This made it necessary to seek a new image compression algorithm able to significantly increase the compression ratio while complying with the image quality requirements of SPOT users. We finally selected a DCT based algorithm embedded in a regulation loop in order to obtain a constant rate at the compressor output. This algorithm was first tested using simulated images. Quantitative and qualitative analyses were carried out at different development stages. Finally, a wide range of images chosen from the SPOT image database was used for validation purpose. This led us to propose several optimizations which have now been thoroughly tested. The result is an almost lossless compression algorithm, which will be used on both SPOT5 and HELIOS II, the French space agencys forthcoming Earth observation missions.

PLEIADES-HR 1A&1B image quality commissioning: innovative radiometric calibration methods and results

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.

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.