32 
namely the visible-short-wave infrared from 0.45 to 2.35 
um, where the instrument works as an imaging 
spectrometer with a spectral resolution of about 10 nm and 
the thermal infrared range from 8 to 12.3 um, divided into 3 
spectral bands with a typical width of 1 um, where the 
instrument works as an imaging radiometer. The spatial 
resolution at nadir is about 50 m over a swath (image size) 
of 50 km. Access to any site on earth could be provided 
within three days (ESA ESAC, 1996). 
In order to support such activities, ESA has identified the 
necessity to initiate appropriate studies and measurement 
campaigns. In this frame, the presented paper discusses a 
study that concentrates on the definition of an airborne 
imaging spectrometer which could represent a precursor of 
the spaceborne instrument and which will therefore be 
named APEX (Airborne PRISM Experiment)(Del Bello, 
1995). 
APEX is a project to develop an airborne PRISM simulator 
which will contribute to the 
* preparation, 
* calibration, 
A 
Protective 
Window 
* validation, 
* simulation, and 
* application development 
of the PRISM mission. In addition APEX will be an 
advanced imaging spectrometer serving as a testbed for 
other imaging spectroscopy applications (Itten, 1997). 
2. THE APEX INSTRUMENT 
APEX will be an instrument with the following uniqueness 
in technical, usage and applications standpoint: 
e  pushbroom imager with approx. 1000 pixels across 
track and a swath width of 2.5 — 5 km 
* spectral wavelength range covering 450 — 2500 nm 
* 200 user selectable and predefined bands, adapted to the 
specific mission and application 
* a spectral sampling interval « 15 nm at a spectral 
sampling width « 1.5 times the sampling interval 
* provides calibrated data and a suite of user oriented 
products up to fully geocoded and calibrated data. 
    
Figure 1. APEX Block Diagram 
2.1 The Imaging Spectrometer Optomechanical 
Subsystem 
The ground imager maps the ground (swath of £14°) on 
the spectrometer slit of 50 mm height and 0.05 mm 
width. High image performance is required in order to 
provide maximum energy throughput at the slit and to 
define the swath width precisely. The ground image has 
to be colour corrected in the total spectral range between 
450 and 2500 nm. Behind the ground imager a beam 
splitter will be placed to separate the spectral range in a 
visual and an IR channel. The use of a beam splitter in 
front of the collimators allows to choose more suitable 
glasses for the colour correction and for improving the 
transmission within the reduced number of spectral 
bands. The two collimators project the light coming from 
the slit towards the dispersive elements of the 
spectrometers. 
Prisms will be selected as dispersing elements. The light 
scattering in a system containing gratings is considered to 
be a risk that cannot be taken. For the visual channel, the 
prism materials CaF2 / ZnS are selected, for the IR 
channel the prism materials are CaF2 and Sapphire. 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXII, Part 7, Budapest, 1998