The initial specification requires a SNR of 2 100 for the 
average of all channels for each detector. In this model 
approach, the average SNR is listed in Table 1. 
  
  
Detector | SNR(Max) | SNR(Val) | SNR(Min) 
VIS 974 226 30 
SWIR 1322 414 19 
  
  
  
  
36 
Tab. 1. Average SNR for each Detector and Level 
5. DATA HANDLING 
5.1 Processing and Archiving 
The data acquired will be processed with a given number 
of intermediate steps. Each of these processing steps will 
produce an output that is either given in a defined level 
(i.e. 0 to 3) that will be distributed to the customer with 
some restrictions or with undefined levels for internal use 
and archiving of raw data and intermediate processing 
steps only. 
After downloading the data from the aircraft it will be 
transferred to the processing facility (Level 0). The first 
analysis of the data incorporates the generation of 
quicklooks. The data will then be reformatted to the 
scenes level (Level 1). After the predefined calibration is 
applied to the data, the sensor specific calibrated data are 
archived as Level 1A. The final Level 1D includes the 
spectral and geometric calibration. Level 2 data products 
include parametric geocoding and atmospheric 
correction. The final processing step (Level 3) will 
depend on user defined processing requests and require 
application specific analysis methods. 
5.2 Operationalization 
After a successful development phase of the APEX 
system, the experiment will be made available for an 
operational phase for a wide remote sensing community 
interested in hyperspectral imaging and beyond. During 
the realization phase of the APEX, assessments of the 
following points must be undertaken: 
definition of a concept for a reference mission 
definition of the costs associated for such a mission 
identification of potential customers 
planning of the APEX utilization phase 
To a customer, the APEX will consist of the remote 
sensing project team. This team will be responsible for 
marketing the services and applications provided by 
APEX, assures the technical performance of the 
instrument, manages all the missions, and provides the 
customer with all the data according to the requested 
processing level. The aircraft operator is responsible for: 
items such as flight planning, clearances, flight . 
execution, etc. The flight crew will consist of the aircraft 
pilot and an operator associated with the correct 
recording of the requested scenes and the proper 
operation of the instrument during data acquisition. 
5.3 Exploitation 
After the successful development, laboratory and flight 
testing of APEX, the system will be made available for 
an operational phase of approximately 5 years. For this 
phase the system is owned by ESA and will be operated 
by a joint Swiss-Belgian team. It is anticipated to offer 
the system for international campaigns and projects, to 
national and local interested research and applications 
groups. In first priority the system will be deployed in 
ESA flight campaigns. Secondly it is envisaged to offer it 
as well for EU campaigns and other international 
endeavours. ESA member states may apply for flight 
coverages, and since one important task will be the 
applications development for PRISM, call for individual 
proposals will be issued and evaluated against available 
funding. 
Since many research issues in the understanding of 
imaging spectrometry and the appropriate processing of 
the data are still to be dealt with, research institutions are 
encouraged to answer to corresponding Announcements 
of Opportunities. 
National bodies such as environmental protection 
agencies may place orders for flight testing APEX in 
their operations and applications developement. 
Data shall be made available under special agreements, 
taking account of the established data and pricing 
policies of ESA (with the exception of purely 
commercial operations) and the need for covering 
operational costs. 
6. CONCLUSIONS AND OUTLOOK 
With this ESA/ESTEC initiated pre phase A study for the 
definition of an airborne imaging spectrometer it has 
been shown that the development of a well calibrated and 
operational imaging spectrometer within Europe can be 
realized. The presented concept provides an integral view 
of the experiment, including the calibration and data 
handling. The operationality of the experiment relies not 
only on the sensors performance in terms of SNR and 
other measures but also on the access to the data and the 
availability of the instrument to the remote sensing 
community. 
Given that APEX is realized, not only PRISM data can 
be calibrated but also parts of other (space and airborne) 
imaging spectrometers and multispectral scanners can be 
validated such as the ENVISAT-1 MERIS instrument, 
the NASA JPL's AVIRIS, the EU's DAIS, and other 
operational instruments. 
7. ACKNOWLEDGEMENTS 
This work has been carried out under the ESA ESTEC 
study no. 11848/96/NL/CN "Definition of an Airborne 
Imaging Spectrometer". The support of the Agency in 
general and Dr. U. Del Bello and Dr. R. Meynart of ESA 
ESTEC in particular is greatly acknowledged. 
8. REFERENCES 
Berk, A., Bernstein, L.S., and Robertson, D.C., 1989. 
MODTRAN: A Moderate Resolution Model for 
LOWTRAN7. Report GL-TR-89-0122, Geophysics 
Lab., Bedford, USA. 
ESA ESAC, 1996. The evaluation of the nine candidate 
earth explorer missions — the report of the earth science 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXII, Part 7, Budapest, 1998