Petra D'Odorico

APEX - the hyperspectral ESA Airborne Prism Experiment

The airborne ESA-APEX (Airborne Prism Experiment) hyperspectral mission simulator is described with its distinct specifications to provide high quality remote sensing data. The concept of an automatic calibration, performed in the Calibration Home Base (CHB) by using the Control Test Master (CTM), the In-Flight Calibration facility (IFC), quality flagging (QF) and specific processing in a dedicated Processing and Archiving Facility (PAF), and vicarious calibration experiments are presented. A preview on major applications and the corresponding development efforts to provide scientific data products up to level 2/3 to the user is presented for limnology, vegetation, aerosols, general classification routines and rapid mapping tasks. BRDF (Bidirectional Reflectance Distribution Function) issues are discussed and the spectral database SPECCHIO (Spectral Input/Output) introduced. The optical performance as well as the dedicated software utilities make APEX a state-of-the-art hyperspectral sensor, capable of (a) satisfying the needs of several research communities and (b) helping the understanding of the Earths complex mechanisms.

APEX - current status, performance and validation concept

The Airborne Prism EXperiment (APEX) is an airborne pushbroom imaging spectrometer for Earth observation. Its products will become available in 2011. APEX is currently prepared for final acceptance configuration completing final hardware upgrades, refined calibration methodologies and test flights. APEX is composed of an airborne dispersive pushbroom imaging spectrometer, a Calibration Home Base (CHB) for instrument calibration and a data Processing and Archiving Facility (PAF) for operational product generation and delivery. A unique In-Flight Characterization (IFC) unit is integrated within the sensor optical head, providing pre- and post- data-acquisition characterization monitoring the instruments spectral and radiometric stability. This paper outlines the activities performed with a special focus on system calibration and validation procedures, as well as preliminary measurement results.

Performance assessment of onboard and scene-based methods for Airborne Prism Experiment spectral characterization

Accurate spectral calibration of airborne and spaceborne imaging spectrometers is essential for proper preprocessing and scientific exploitation of high spectral resolution measurements of the land and atmosphere. A systematic performance assessment of onboard and scene-based methods for in-flight monitoring of instrument spectral calibration is presented for the first time in this paper. Onboard and ground imaging data were collected at several flight altitudes using the Airborne Prism Experiment (APEX) imaging spectrometer. APEX is equipped with an in-flight characterization (IFC) facility allowing the evaluation of radiometric, spectral, and geometric system properties, both in-flight and on-ground for the full field of view. Atmospheric and onboard filter spectral features present in at-sensor radiances are compared with the same features in reference transmittances convolved to varying instrument spectral configurations. A spectrum-matching algorithm, taking advantage of the high sensitivity of measurements around sharp spectral features toward spectrometer spectral performance, is used to retrieve channel center wavelength and bandwidth parameters. Results showed good agreement between spectral parameters estimated using onboard IFC and ground imaging data. The average difference between estimates obtained using the O(2) and H(2)O features and those obtained using the corresponding filter features amounted to about 0.3 nm (0.05 of a spectral pixel). A deviation from the nominal laboratory instrument spectral calibration and an altitude-dependent performance was additionally identified. The relatively good agreement between estimates obtained by the two approaches in similar spectral windows suggests they can be used in a complementary fashion: while the method relying on atmospheric features can be applied without the need for dedicated calibration acquisitions, the IFC allows assessment at user-selectable wavelength positions by custom filters as well as for the system on-ground.

Global parameterization and validation of a two-leaf light use efficiency model for predicting gross primary production across FLUXNET sites

Light use efficiency (LUE) models are widely used to simulate gross primary production (GPP). However, the treatment of the plant canopy as a big leaf by these models can introduce large uncertainties in simulated GPP. Recently, a two-leaf light use efficiency (TL-LUE) model was developed to simulate GPP separately for sunlit and shaded leaves and has been shown to outperform the big-leaf MOD17 model at six FLUX sites in China. In this study we investigated the performance of the TL-LUE model for a wider range of biomes. For this we optimized the parameters and tested the TL-LUE model using data from 98 FLUXNET sites which are distributed across the globe. The results showed that the TL-LUE model performed in general better than the MOD17 model in simulating 8 day GPP. Optimized maximum light use efficiency of shaded leaves (epsilon(msh)) was 2.63 to 4.59 times that of sunlit leaves (epsilon(msu)). Generally, the relationships of epsilon(msh) and epsilon(msu) with epsilon(max) were well described by linear equations, indicating the existence of general patterns across biomes. GPP simulated by the TL-LUE model was much less sensitive to biases in the photosynthetically active radiation (PAR) input than the MOD17 model. The results of this study suggest that the proposed TL-LUE model has the potential for simulating regional and global GPP of terrestrial ecosystems, and it is more robust with regard to usual biases in input data than existing approaches which neglect the bimodal within-canopy distribution of PAR.

In-flight spectral performance monitoring of the Airborne Prism Experiment

Spectral performance of an airborne dispersive pushbroom imaging spectrometer cannot be assumed to be stable over a whole flight season given the environmental stresses present during flight. Spectral performance monitoring during flight is commonly accomplished by looking at selected absorption features present in the Sun, atmosphere, or ground, and their stability. The assessment of instrument performance in two different environments, e.g., laboratory and airborne, using precisely the same calibration reference, has not been possible so far. The Airborne Prism Experiment (APEX), an airborne dispersive pushbroom imaging spectrometer, uses an onboard in-flight characterization (IFC) facility, which makes it possible to monitor the sensors performance in terms of spectral, radiometric, and geometric stability in flight and in the laboratory. We discuss in detail a new method for the monitoring of spectral instrument performance. The method relies on the monitoring of spectral shifts by comparing instrument-induced movements of absorption features on ground and in flight. Absorption lines originate from spectral filters, which intercept the full field of view (FOV) illuminated using an internal light source. A feature-fitting algorithm is used for the shift estimation based on Pearsons correlation coefficient. Environmental parameter monitoring, coregistered on board with the image and calibration data, revealed that differential pressure and temperature in the baffle compartment are the main driving parameters explaining the trend in spectral performance deviations in the time and the space (across-track) domains, respectively. The results presented in this paper show that the system in its current setup needs further improvements to reach a stable performance. Findings provided useful guidelines for the instrument revision currently under way. The main aim of the revision is the stabilization of the instrument for a range of temperature and pressure conditions to be encountered during operation.

Underestimated role of East Atlantic-West Russia pattern on Amazon vegetation productivity

The Amazon forest experienced severe droughts in 2005 and 2010; however, the extent to which precipitation anomaly affects the vegetation productivity remains controversial (1, 2). Hilker et al. (3) report significant correlations (P < 0.05) between the El Nino southern oscillation (ENSO) and the Amazon vegetation greenness based on the improved moderate resolution imaging spectroradiometer (MODIS) normalized difference vegetation index (NDVI) data for 2000–2012. The opposing findings from previous MODIS measurements are attributed to normalizing the MODIS reflectance to a common view and sun geometry and the use of a less conservative cloud mask. These are important developments, particularly to study the intraannual dynamics of Amazon vegetation greenness. However, we argue that, among the leading global ocean-atmosphere oscillations, ENSO may not be the most prominent indicator of Amazon vegetation productivity.

Operational status of apex and characteristics of the apex open science data set

The APEX system is fully operational since 2011 and ready to provide well-calibrated data as standard processing output of the APEX PAF and the VITO CDPC to the user community. We sincerely hope that the freely available APEX Open Science Data Set will assist the community in gaining insights into the huge potential of APEX and help in assessing its applicability to specific domains of spectroscopy research.

Spectral stability monitoring of an imaging spectrometer by means of onboard sources

The In-Flight Characterization (IFC) facility of the Airborne Prism Experiment (APEX) is presumably the first onboard characterization unit implemented in an airborne imaging spectrometer. This study is meant to test methodologies for the retrieval of temporal relative center wavelength drifts based on IFC data. A rare Earth material filter with a set of well-known absorption features is imaged through the IFC on APEX and recorded at several time positions. The shift of the center wavelengths covered by a spectral feature is estimated by means of curve matching algorithms. Two algorithms are evaluated: in the former the shift is determined by using the correlation coefficient as merit function to determine changes of the feature shape and position, while the latter evaluates the distance between centers of gravity. These methods have demonstrated an uncertainty in the order of 6–9 % of a pixel. A test case has been designed in which the APEX system was exposed to a temperature profile with a thermal excursion of 26°C, reproducing flight conditions. Results show the spectral stability of the APEX imaging spectrometer.