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APEX - AIRBORNE PRISM EXPERIMENT:
AN AIRBORNE IMAGING SPECTROMETER SERVING AS A PRECURSOR INSTRUMENT
OF THE FUTURE ESA LAND SURFACE PROCESSES INTERACTIONS MISSION
Michael E. Schaepman and Klaus I. Itten
Remote Sensing Laboratories
Dept. of Geography
University of Zurich
Winterthurerstrasse 190
CH - 8057 Zurich
Switzerland
mail: apex geo.unizh.ch
Commission VII, Working Group 1
KEY WORDS: airborne, simulation, imaging spectroscopy, hyperspectral imaging, calibration
ABSTRACT
The European Space Agency (ESA) has identified the necessity to initiate a study that concentrates on the definition of an
airborne imaging spectrometer which could represent a precursor to the spaceborne PRISM (Process Research by an
Imaging Space Mission). The study included the definition of an Airborne PRISM Experiment (APEX) that will contribute
to the preparation, calibration, validation, simulation, and application development for the PRISM mission. The APEX
instrument is defined as an airborne pushbroom imager with 1000 pixels across track and 200 user selectable or 300 fixed
spectral bands covering the wavelength range from 450 — 2500 nm. The complete APEX system will include an aircraft,
navigation data, laboratory and in-flight calibration as well as a data archiving and processing facility.
The definition of the specifications of the APEX instrument is based on a sensor model taking into account various
parameters of the expected operation range of the instrument. The approach used defines the radiometric properties of
expected scene radiances within the operating range of the APEX instrument and derives subsequently the radiometric
resolution requirements.
KURZFASSUNG
Die Europäische Raumfahrtbehórde ESA hat eine Studie initiiert, welche ein Vorláuferinstrument von PRISM definiert.
Diese Studie beinhaltet die Definition des flugzeuggetragenen PRISM Experimentes (APEX), welche im wesentlichen zur
Vorbereitung, Kalibrierung, Validierung, Simulation und Anwendungsentwicklung der PRISM Mission beitragen soll. Das
APEX Instrument ist ein Zeilenscanner mit 1000 Bildelementen pro Zeile und 200 wáhlbaren oder 300 fixen spektralen
Bändern im Wellenlängenbereich von 400 - 2500 nm. Das komplette APEX System beinhaltet ein Flugzeug,
Navigationsdaten, Labor- und Flugkalibration, sowie ein Prozessierungs- und Archivierungssystem.
Die radiometrischen Spezifikationen von APEX basieren auf einem Sensormodell, welches die erwarteten
Strahlungsintensitáten im Einsatzgebiet von APEX berücksichtigt und in die Modellierung miteinbezieht. Daraus wrd dann
auch das (radiometrische) Auflósungsvermógen von APEX hergeleitet.
interactions on a global scale by extrapolating through
time and space using process models.
1. INTRODUCTION
After the successful operation of ERS-1 and ERS-2, the
European Space Agency has set the scene for an The space segment would be characterised by:
environmental mission by preparing the satellite * a single satellite in a near-polar orbit capable of
ENVISAT-1. enabling the payload to access all land areas with the
required geometric and temporal characteristics;
In formulating European Earth Observation missions * a hyperspectral imager with the required spatial and
beyond this, ESA is considering the development and spectral resolution covering the optical range of the
launch of a high resolution Land Surface Processes and electromagnetic spectrum with good radiometric
Interactions Mission (Readings, 1996) and intends to accuracy and with the capability to perform directional
develop a system to investigate land surface processes and measurements providing the required accessibility and
their interaction with the atmosphere (Del Bello, 1996). revisit characteristics.
This includes the implementation of data acquisition from
an advanced high resolution spectro-directional space A preliminary design at system level of satellite
sensor, and by supporting the development of the tools and
techniques of data interpretation to:
increase the understanding of biophysical processes and
land/atmosphere interactions at the local scale; and
* exploiting the increase in knowledge of (small scale)
processes and to advance the understanding of these
instrumentation fulfilling this goal started under the name
Process Research by an Imaging Space Mission (PRISM)
(Posselt, 1996). This instrument will combine the main
features of a visible/short wave infrared imaging
spectrometer and of a thermal infrared multi-channel
imager. PRISM would cover two main wavelength regions,
International Archives of Photogrammetry and Remote Sensing. Vol. XXXII, Part 7, Budapest, 1998
31
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