Stefan Livens

PROBA-V mission for global vegetation monitoring: standard products and image quality

PROBA-V is a new global vegetation monitoring mission, to be launched in the second quarter of 2013. PROBA-V has been developed to show a consistent performance with SPOT-VEGETATION (SPOT-VGT) data, with similar spectral bands but with an improved spatial resolution of 1/3 km. The innovative mission concept has led to several key research topics related to image quality, which are discussed in this article. To support the existing VEGETATION user community, the data products for PROBA-V continue to provide daily top of canopy synthesis (S1-TOC) and 10 day synthesis products (S10-TOC). In addition, the new top of atmosphere daily synthesis (S1-TOA) products and a radiometrically/geometrically corrected (level 1C) product in raw resolution will also be provided for scientific users.

The PROBA-V mission: image processing and calibration

With the launch of PROBA-V (Project for On-Board Autonomy – Vegetation) in 2013, the continuity and availability of global land-coverage data in four multispectral bands are ensured for the SPOT (Système Pour l’Observation de la Terre)-VEGETATION user community. This community has been served for already more than 14 years with high-quality 1 kilometre-resolution data. To guarantee continuation of this high quality over the full lifetime of PROBA-V, an operational processing platform and in-flight calibration algorithms have to be in place, which fully consider the specific PROBA-V platform and instrument design characteristics. Data quality has to be ensured for all available product levels, i.e. from the radiometrically corrected radiance data to the 10 day global synthesis. In this article we first focus on some specific design characteristics, which impose some challenges for data processing and calibration. Next, a technical description is given for all the processing steps such as mapping, cloud masking, atmospheric correction, and compositing. The functioning of the Image Quality Centre (IQC) is described. The IQC is in charge of the assessment of the PROBA-V performance, the analysis of the image quality, and the radiometric and geometric calibration after launch. Finally information is given on the distribution of the various products to the user community.

Rayleigh, Deep Convective Clouds, and Cross-Sensor Desert Vicarious Calibration Validation for the PROBA-V Mission

PROBA-V is a remote sensing satellite mission for global monitoring of vegetation. It is designed to offer almost daily coverage of all land masses and to provide data continuity with the VGT2 instrument aboard SPOT-5. Accurate radiometric calibration is key to the success of the mission; therefore, a comprehensive system for in-flight radiometric calibration has been developed. Without no onboard calibration devices, this in-flight calibration will rely fully on vicarious methods. In total nine techniques for vicarious calibration have been implemented and tested in order to meet the radiometric mission requirements. Three key methods that contribute largely to the calibration performance are presented in this paper: Rayleigh, deep convective clouds, and cross-sensor calibration over stable desert sites. As the PROBA-V sensor has still to be launched, calibration algorithm verification is performed using data from the spectrally very similar SPOT-VGT1 and SPOT-VGT2 sensors.

PROBA-V geometric calibration

A system is described for the geometrical calibration of the planned PROBA-V multi spectral earth observation satellite. The largest challenge lies in the complex thermo-elastic distortion due to the absence of active thermal control onboard. The system is based on a 2 step (first by scene, then by global trend), weighted and constrained least squares inversion model. The geometric reference dataset consists in a set of automatically selected image chips from the Geocover 2000 dataset. Results show that the sub-pixel absolute localization accuracy requirements should be reached.

Spatially variable filters — Expanding the spectral dimension of compact cameras for remotely piloted aircraft systems

Recent advances in hyperspectral imaging techniques using spectral filters deposited directly onto an image sensor chip, and suitable for RPAS platforms, are reported in this paper. New filter configurations for a compact spectral camera are described and a prototype of the compact hyperspectral payload developed for small RPAS systems, as well as the first data acquired with the new camera are presented. The spectral range of the payload was optimized for earth observations such as vegetation monitoring or water quality studies. Although the spatially variable filters have to date only been used in small satellite sensors, this technology has a clear potential for RPAS platforms.

Multiple vicarious calibration using combined accuracy estimation

Radiometric calibration often employs several independent vicarious calibration techniques to increase robustness and accuracy. We present a statistical methodology for combining results in a hierarchical scheme. The method, developed for the PROBA-V remote sensing mission, is based on handling and propagating of accuracies in accordance with the ISO GUM. Robust estimation is performed and outliers removed. Results over different sites are combined using weighted averaging. Weighted linear regression is used for temporal averaging. Results from different methods are combined taking into account possible bias. Finally an operational update strategy is proposed which relies on a significance criterion.

Compression of Remote Sensing Images for the PROBA-V Satellite Mission

We investigate compression of remote sensing images with a special geometry of non-square pixels. Two fundamentally different data reduction strategies are compared: a combination of pixel binning with near lossless compression and a method operating at higher compression rates. To measure the real impact of the compression, the image processing flow upto final products is included in the experiments. The effects on sensor non-uniformities and their corrections are explicitly modeled and measured. We conclude that it is preferable to apply higher compression rates than to rely on pixel binning, even if the derived images have lower resolutions.

Radiometric calibration of the cosi hyperspectral RPAS camera

The COSI hyperspectral imaging system, suitable for small RPAS, is able to produce high resolution hyperspectral data products. By extensive inflight testing, we have identified the main challenges for achieving reliable high quality results. Based on these insights, we propose a refined radiometric calibration strategy. It uses a set of three reference targets, two grey and one colored target, which are to be measured inflight. We present on-ground measurements of the targets with COSI, as in flight measurements, demonstrating the merits of the approach are still ongoing.

Linear variable filters — A camera system requirement analysis for hyperspectral imaging sensors onboard small Remotely Piloted Aircraft Systems

Improving the spectral detail of earth observation imaging from Remotely Piloted Aircraft Systems (RPAS) can greatly expand its potential for use in vegetation monitoring and specifically in precision agriculture. Spatially variable interference filters which can be placed very close to the image sensor offer an excellent opportunity for reducing the size, mass and complexity of hyperspectral imagers, allowing them to be mounted onboard small RPAS. Recent advances in filter deposition techniques allow to directly deposit interference filters on an image sensor. The monolithic integration of optical hyperspectral filters on top of a standard CMOS image sensor has been demonstrated by IMEC. Compared to the more conventional deposition of filters onto an external glass substrate, this new approach offers advantages in terms of cost, alignment accuracy, straylight, etc. A hyperspectral camera prototype compatible with small RPAS has been developed by VITO to demonstrate the potential of LVF-based compact spectral cameras. Whereas application of the filter technology offers major advantages for RPAS systems, it still faces some important challenges. The prototype system specifications need to fit a fixed wing RPAS platform that is able to cover several km2 in a single flight with hyperspectral geo-information. It remains challenging to make a sufficiently compact camera system, achieve precise spectral band registration, handle the amount of data to be processed and cope with limited integration times possible during acquisition.

Designing an in-flight airborne calibration site using experience from vicarious radiometric satellite calibration

Laboratory calibration of electro-optical sensors is preferably complemented by regular in-flight verification. This checks whether the lab calibration parameters remain valid or recalibration is necessary. In-flight verification can be achieved by vicarious calibration using in-flight measurements of calibration targets. We intend to identify and design a set of suitable radiometric calibration targets. For this, we borrow from expertise gained with the PROBA-V satellite calibration system, which uses multiple vicarious methods relying on diverse natural on-ground targets. Besides reflectance based calibration using ground measurements, the PROBA-V calibration methods are unproven for use in airborne calibration. The selected targets should be suitable for the calibration of both multispectral and hyperspectral imagers. We start from general requirements for radiometric targets and investigate their applicability to airborne calibration. From this we identify two possible sets of natural calibration sites in Belgium. One set, located in the Campine region, contains small water bodies and sandy lakesides. Another set is located in the Westhoek region near the Belgian coast. It offers better suitable water bodies, as well as sandy areas, grass fields and dark targets. Airborne calibration lends itself to the use of smaller artifical targets. We propose to complement the natural targets with a portable target consisting of agricultural nets with different densities. The definition of sets of calibration targets, both natural and artificial can facilitate the investigation of the usability of vicarious targets and method for inflight radiometric verification.