APEX status pt.1: instrument development and performance

ESA APEX (Airborne Prism EXperiment) is a project for the realisation of an airborne dispersive pushbroom imaging spectrometer, a dedicated data Processing and Archiving Facility (PAF, hosted at VITO) and a Calibration Home Base (CHB, hosted at DLR) for instrument calibration operation. It has been developed by a joint Swiss-Belgian consortium. The APEX instrument is facing its finalisation phase undergoing intense experimental activities in view of its validation and performance assessment. Environmental tests were executed to simulate flight environment conditions. The first APEX airborne campaign has been held in June 2009 covering a variety of water targets over Switzerland and Belgium. Extensive pre- and postflight characterisation and calibration campaigns were accomplished. Instrument data evaluation, performance analysis and optimisation of the data processing schemes adopted have followed. This paper outlines the activities performed and presents the first products achieved

Improving the performance of hyperspectral pushbroom imaging spectrometers for specific science applications

Hyperspectral imaging spectrometers offer the unique chance of recording image data of a broad range of targets in the reflected solar energy spectrum. These instruments are designed upon certain requirements such as signal-to-noise ratio (SNR), spectral resolution and bandwidth or noise equivalent delta radiance. These parameters are determined by investigating one or several typical targets (e.g. vegetation, limnology, soil, atmosphere) that the instrument will sense during its operational life by means of specific instrument models. Depending on the specific application, users can demand hyperspectral image data that might cover a portion or the whole sensor spectral range and, more importantly, may have requirements different from the ones the instrument was designed for originally. Therefore, in order to meet the user requests the spectrometer settings should be modifiable. Many instruments are potentially programmable from the electric point of view, in a way that the sensor setting parameters could be changed, e.g. exposure time, on-chip averaging, the so-called binning, amplifier gains. By tuning these parameters the sensor performances can be modified according to the user needs. The Airborne Prism Experiment (APEX)1, a hyperspectral imaging spectrometer developed by a Swiss-Belgium consortium on behalf of the European Space Agency (ESA) and under the scientific supervision of the Remote Sensing Laboratories (RSL), has been designed upon certain requirements (e.g. radiance levels, SNR) but, nevertheless, the electric settings can be changed by means of a mission control file in order to fulfil user requests that differ from the default scenario. Namely, the APEX instrument allows changes of exposure time, on-chip binning and frame period. We designed and implemented a software utility that optimizes the instrument parameters based on the possible range of hardware settings and the user application requirements. This utility is based on the detector electrical and optical description, which is modelled in terms of signal and noise by using the SNR equation2. In order to develop such a model the instrument optical characteristics, i.e. transmission, must be known. The utility can be regarded as an APEX sensor simulator but it can be easily adapted to any other hyperspectral imaging apparatus. Users (i.e. sensor manufacturers, operators, scientists) can formalize their requirements and feed them into the model. E.g. a scientist is aiming at estimating the amount of leaf chlorophyll content within a vegetation target with a required minimum of detectable differences. Therefore he has to identify the needed values of SNR, spectral resolution and sampling interval as an input for the simulator. The utility evaluates all the possible solutions in terms of exposure time and on-chip binning in order to determine the one that matches the scientist needs the best. A broad variety of error deviations are reported in order to help the users in interpreting the simulation results, estimate the error and accuracy budgets accordingly. Depending on the input requirements the discordance between the users needs and the results can be significant. In such a case the utility performs a further step by analyzing post-processing strategies, as for instance off-chip binning, in a way that the requirements can be someway be met. The presented utility has a twofold advantage: (1) it allows manufacturers and sensor operators to offer an instrument that is adaptable to needs of the end-users community and (2) it lets users, mainly scientists, understand what can be achieved with a given hyperspectral instrument. The weakness of the utility relies on the lack of information about the optical and electrical parameters, which might be caused by the confidential nature of technical details, namely in private companies. We firmly believe that this utility can (a) optimize the programming of hyperspectral imaging spectrometers to gather more accurate image data and (b) let users exploit the broad range of applications that can be investigated with the available large spectral range

Improving quality of imaging spectroscopy data

Imaging spectroscopy is moving into quantitative analysis of ecosystem parameters, which require high data quality. Thus, imaging spectrometers shall provide users with very accurate and low uncertainty measurements such that truthful products and reliable policies can be generated. However, the quality of imaging spectroscopy data, which can be interpreted as the distance between the measurement and the true value, depends on a series of disturbance factors that can be divided into instrument factors, environmental factors, and data processing factors. Those factors lead to data non-uniformities and inconsistencies that, if not properly identified, quantified, and corrected for, can compromise the quality of the scientific findings. This thesis investigates various techniques aimed to ensure the consistency of imaging spectroscopy data, namely in the spectral domain, throughout the data acquisition and specific data processing schemes, as for instance the calibration to radiances, with particular emphasis on instrument factors. The impact of data inconsistencies and non-uniformities on the quality of imaging spectroscopy data is first estimated. A scene-based technique for the characterization of keystone non-uniformity is then proposed. Moreover, a laboratory approach is established as the most reliable technique for the achievement of high accuracy calibration and characterization of imaging spectrometers. Last, an algorithm that identifies optimal sensor acquisition parameters for the retrieval of specific products in spectral regions of interest is presented. It has been concluded that laboratory calibration and characterization procedures offer a higher degree of fidelity with respect to scene-based methodologies when non-uniformities and calibration parameters have to be determined and implemented into correction schemes. A critical discussion of the main findings analyze advantages and drawbacks of the proposed techniques and suggests further improvements as well as future perspectives for the continuation of this work

Calibration algorithms for an imaging spectrometer

This paper presents a software calibration/characterization utility aimed to automatically perform the laboratory calibration of an imaging spectrometer. Quantitative remote sensing algorithms requires well-documented instrument optical performances along with characterization of nonuniformities as, for instance, smile and keystone. Automatic calibration data acquisition and processing facilitate the understanding of the instrument properties and allow the implementation of specific correction schemes. The concept of calibration cube is also introduced as a promptly accessible data structure for the retrieval of optical properties in any detector position. A case study along with all its relevant results is also introduced, based on the Airborne Prism Experiment (APEX) imaging spectrometer. Recommendations and suggestions are also given for customized implementations of this tool

An algorithm for tracking APEX spectral stability by means of the In-Flight Characterization facility (IFC)

During their life span, imaging spectrometers are likely to be affected by deviations in spectral performances. Such fluctuations are mainly due to vibrations and temperature/pressure changes at the moment of launch or aging of the instrument. Prior to taking the spectrometer to the laboratory for a time- consuming re-characterization and re-calibration, it is good practice to monitor its spectral performance in- flight. For the Airborne Prism Experiment (APEX) spectrometer, this can be achieved by means of an onboard In-Flight Characterization (IFC) facility. IFC data are acquired at closed shutter with a stable input signal coming from a 75 W Quartz Tungsten Halogen (QTH) lamp. A filter wheel is interposed in the optical path leading to the detector; the spectral filters mounted on the wheel are characterized by a number of narrow spectral features. In this paper the development and tuning process of an algorithm to be used for the spectral stability monitoring of APEX is presented. The study is based on simulated IFC data and aims at identifying a spectrum-matching technique to be included in the final algorithm. In this context four spectrum-matching methods are tested in a varying range of simulated measurement conditions. We found that the methods employing the correlation coefficient and the RMSD as merit functions are more suitable and robust approaches for the estimation of the wavelength shift

Automatic calibration and correction scheme for APEX (Airborne Prism Experiment)

Hyperspectral sensors provide a large amount of both spatial and spectral information. Calibration plays an important role in the efficient use of such a rich data source. However, calibration is extremely time consuming if undertaken with traditional strategies. Recent studies demonstrated that various non-uniformities, and detector imperfections drastically affect the hyperspectral data quality if not known and corrected for. The APEX (Airborne Prism Experiment) spectrometer adopts an automatic calibration and characterization strategy with the ultimate goal of providing scientific products of very high accuracy. This strategy relies on the control test master (CTM), an advanced software/hardware equipment able to control independently the instrumentation, and to process online or offline the large amount of data acquired to characterize such a sophisticated instrument. Those data, once processed by the master processor, will generate several coefficients that in turn will feed the processing and archiving facility (PAF), a software module that calibrates the acquired scenes, and corrects for artefacts and non-uniformities

Supporting facilities of the airborne imaging spectrometer APEX

The facilities to support the ESA’s airborne APEX hyperspectral mission simulator are described. These facilities include calibration tools, such as specific processing in a dedicated Processing and Archiving Facility (PAF), operational calibration and characterization using the Calibration HomeBase (CHB), the In-Flight Characterization facility (IFC) and the Calibration Test Master (CTM). Further on, a preview on major applications and the corresponding development efforts to provide scientific data products up to level 2/3 to the user are outlined. Products dedicated for the retrieval of limnology, vegetation, atmospheric parameters, as well as general classification routines and rapid mapping tasks are currently under development and prepared for dissemination by the APEX Science Center (ASC) and the APEX Operations Center (AOC)

Imaging spectrometers

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