THE STRUCTURE OF THE APEX (AIRBORNE PRISM EXPERIMENT) PROCESSING AND

ARCHIVING FACILITY

Hueni, A.1, Bieseman J.

2, Dell’Endice, F.

1, Alberti, E.

1, Meuleman K.

2 and Schaepman M.

1

1 Remote Sensing Laboratories, Department of Geography, University of Zurich, Switzerland

2 VITO-TAP, Mol, Belgium

ABSTRACT

APEX is a Swiss-Belgian project for the realization of an

airborne imaging spectrometer within the framework of the

ESA Prodex Programme. The project’s emphasis is on

delivering products characterized by high level accuracy to

the user community. This objective relies on the concept and

the actuation of two fundamental phases: (a) instrument

calibration and (b) data processing. An accurate instrument

calibration procedure is required in order to achieve a proper

knowledge of the instrument behavior. The calibration

information is structured into calibration cubes. These

calibration cubes are then integrated into the specialized

processing for data calibration to convert the raw system

data into physical (spectral, radiometric, spatial) units.

Higher-level products can be ordered and configured by the

end users via according web interfaces. The dedicated

APEX Processing and Archiving Facility (PAF) is hosted

and operated by VITO.

Index Terms— Processing, archiving, calibration,

imaging spectrometer

1. INTRODUCTION

The airborne imaging spectrometer APEX (Airborne Prism

Experiment) is a dispersive pushbroom system designed to

contribute to the understanding of processes associated with

air, water and land at local and regional scale in support of

global applications and to support the preparation,

simulation, calibration and validation of future spaceborne

imaging spectrometers [1].

APEX has been designed to deliver imaging spectroscopy

data characterized by high spectral, radiometric and

geometric accuracy. The targeted data accuracy relies upon

three main factors: (a) provision of a highly precise

instrument by the industrial partners, (b) detailed instrument

calibration of the instrument characteristics by means of the

Calibration Home Base (CHB) at DLR, Oberpfaffenhofen,

Germany and (c) data calibration and higher level product

generation at the APEX Processing and Archiving Facility

(PAF) at VITO in Mol, Belgium [2].

The APEX system is currently undergoing an extensive

phase of assessing its system performance, following the

final acceptance of the principal investigators and ESA.

In this paper, we describe the main aspects of the APEX

PAF with a focus on product generation.

2. DATA ARCHIVING

Flight scenes in their raw format and calibration data are the

foundation of the data chain. Any higher-level product can

be reprocessed based on the raw input and its archiving is

thus compulsory.

The APEX archiving system is based on iSCSI SAN

(Storage Area Network) storage. All data are managed by

entries in a processing and products database, implemented

as a relational schema in a MySQL database [3].

3. CALIBRATION PROCESS

The calibration of a measuring instrument or system is the

fundamental tool to ensure the reliability of measurements,

in terms of accuracy and traceability. The instrument and

data calibration concept and process determine the

performance characteristics of the overall system [4].

The accuracy of the APEX data processing results relies on

two fundamental steps: (a) the instrument calibration

concept, its application by means of adequate measurement

procedures applied at the CHB, the consequent

determination of the calibration coefficients, and (b) the

integration of this information into the processing chain

(Fig. 1).

The traceability is provided by the calibration laboratory.

The CHB is a purpose built facility for calibration and

characterization of imaging spectrometers, whose equipment

and setup delivers inputs to the instrument with certified

high degree accuracy and repeatability, for the acquisition of

spectral, radiometric and geometric calibration data. The

CHB high quality standard combines with the

automatization possibility to minimize the duration the

measurement processes [5-7].

978-1-4244-4687-2/09/$25.00 ©2009 IEEE

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Fig. 1: Instrument and Data Calibration chain

CHB measurement data are acquired and processed by the

CTM (Calibration Test Master) software, to provide the so

called calibration cubes. These cubes are matrixes in ENVI

format where every layer corresponds to a specific

instrument parameter, characterizing the system response at

the detector pixel level (Fig. 2).

Fig. 2. Structure of a Calibration cube

4. PROCESSING LEVELS & PRODUCTS

Data pass through several, well-defined processing stages

during the processing from raw instrument data to end user

products. Data at these distinct stages are referred to as

Level-<N> data, where N is the number of the stage. Within

the APEX project, levels are defined as follows [10]:

Level name Description

level-0 Raw data as produced by the instrument (digital

numbers)

level-1 Radiometrically, (spectrally) and geometrically

calibrated, uniform data (radiances)

level-2 Surface reflectance data: corrected for atmospheric and

topographic influences

level-3 Application oriented products

Level-0/1 processing is effecting the data calibration. The

radiometric calibration transforms the sensor raw output

information (DN) to at-sensor radiance (mW/m2/sr/nm).

This step corrects for the individual sensitivity of each pixel,

effectively correcting for eventual artifacts (such as striping

or vignetting). Pixel specific dark current is removed from

the raw data as well. The calibration process is a frame-

based operation, where a frame corresponds to spectra of

one across track line. Radiometrically calibrated data are

referred to as level-1a.

The next step (level-1b) is the data geometric calibration,

that removes artifacts caused by real optical systems, such

as spectral misregistration (also referred to as smile) and

spatial misregistration (frown). The correction process

involves some interpolation routine, which will invariable

compromise the radiometric accuracy of the data [8].

Consequently, some users may want to base their research

analyses on non-resampled data.

Depending on the severity of the smile, a correction by

simple interpolation may lead to new artifacts during

atmospheric correction due to atmospheric absorption

features coinciding with the corrected spectral band.

Therefore, the smile correction should be moved to the

Level-1/2 processing, where knowledge about the

atmospheric transmission will allow a more realistic

interpolation. An according feature has been added to the

latest version of the ATCOR software package [9].

Assuming that a spectral uniformity is reached, i.e. the

spectral misregistrations have been corrected, the spectral

calibration is reduced to the process of assigning the correct

wavelength to each spectral band.

The higher level processing workflow for spectral data is

usually a sequential procedure from raw imagery to rectified

and calibrated imagery, further to surface reflectance data

and finally to products. Within the APEX PAF, an

optimized workflow is foreseen which tries to avoid

redundancies by organizing level-2/3 in a product-oriented

modular system (see [10]).

Fig. 3 gives an overview of the processing flow after level-1

processing up to final data product maps. The geometric

processing is split in two parts. Before starting with the

level-2 processing all pixels are indexed with their

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geographic location and the DEM-related parameters are

resampled to the raw image geometry. The rectification step

is done only on the final data products (i.e. level-3B) to

avoid resampling artefacts and processing overhead.

Spectral reference data are a crucial input to this processing

chain and are used for both level-2 and level-3 processing

steps. Spectral databases are well suited to supply such

reference spectra by selections based on metadata queries

[11].

Implementation of the above approach is supported by the

flexible design of the processing and archiving software and

hardware framework available at VITO. The sequence of

processing modules is defined via the workflow manager, a

Java software that allows processing in a cluster of

machines according to the Master/Worker and Task/Data

Decomposition patterns [2, 12].

Fig. 3: Level-2/3 processing scheme of APEX

The flexibility allows the inclusion of new algorithms in a

rapid manner as long as a configuration via XML file is

possible. XML configuration can be achieved for virtually

any algorithm by using a wrapper object that translates

XML parameters to internal algorithm calling syntax.

APEX higher level products may be distinguished into

standard, validated products and research products.

Scientists whishing to contribute to the APEX PAF by

developing new algorithms are welcome to do so in

collaboration with the APEX Science Center (ASC) at the

Remote Sensing Laboratories (Fig. 4). Finalized, validated

algorithms are handed over to the APEX Operations Center

(AOC) at VITO for integration into the APEX PAF and

creation according user interfaces for the parameterization

of the module by the end users via the PAF web interface.

Fig. 4 Structure and organization of the APEX Science

and Operation Centers

5. TESTING ACTIVITIES

The APEX processing structure and organization requires

testing activities to verify, validate and finalize each single

step. The APEX PAF is a dedicated infrastructure and its

fine-tuning relies on knowledge acquired during the APEX

instrument development phase and subsequent performance

tests.

Detailed characterization and calibration activities have

been carried out at the CHB, leading to the creation of the

first APEX calibration cubes. The calibration information

could be transferred to the PAF in order to test the Level-0/1

data calibration algorithms. Other higher level processes and

products are also undergoing the validation process.

The performance assessment of image scenes is based on

test flight data. Fig. 5 shows a radiometrically calibrated

scene derived from the first APEX test flight in October

2008.

Fig. 5 APEX Scene, radiometrically calibrated

(Wittenberge, Oct. 2008)

6. DATA DISSEMINATION

Users are foreseen to interact with the APEX PAF via

web page interfaces. These allow the search and selection of

level-1 data, the specification of processing parameters for

higher-level processing and the monitoring of product

orders. The generic web interface allows the user to select

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the processing options, e.g. bands for atmospheric correction

algorithms, dependant on the sensor type (see Fig. 6).

Fig. 6 Specification of sensor specific atmospheric

processing parameters in the web interface

Processed data are transferred to a new FTP account and

an email is sent to the user, specifying the download point

and access details.

7. CONCLUSION

The APEX Science Center at the Remote Sensing

Laboratories in Zurich, Switzerland, is interested in

collaborating with researchers who would like to test their

imaging spectroscopy algorithms on APEX data. Scientists

are also invited to contribute working algorithms to be

operationalized at the APEX Operation Center at VITO,

Mol, Belgium.

8. REFERENCES

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