Full text: Remote sensing for resources development and environmental management (Volume 1)

Symposium on Remote Sensing for Resources Development and Environmental Management / Enschede / August 1986 
CAESAR: CCD Airborne Experimental Scanner 
for Applications in Remote Sensing 
N.J.J.Bunnik & H.Pouwels 
National Aerospace Laboratory NLR, Amsterdam, Netherlands 
C.Smorenburg & A.L.G.van Valkenburg 
TNO Institute of Applied Physics, TPD, Delft, Netherlands 
ABSTRACT: In the framework of the Dutch national remote sensing programme, an airborne multispectral scanner 
based on the use of CCD detectors has been developed and tested. The objectives of the project were to obtain 
technical experience with the electro-optical properties of commercially available CCD's and to provide the 
user community with a versatile airborne system to be used in experimental remote sensing programmes. 
Starting from user requirements for land and sea observation a trade-off has been made between the complexity 
of the optical system and its related development cost. Because of the existing constraints with respect to 
the sensor size, a modular design consisting of 4 three-channel CCD cameras has been chosen. By means of ex 
changeable filters different configurations for land and sea observation are available. Besides narrow band 
spectral filters for land observation, broadband correlation filters have been developed as an option. 
A forward-looking camera can be used for the acquisition of bidirectional multispectral data. The sensor 
signals are digitized in 12 bit words in order to cover the full dynamic range with high radiometric resolu 
tion. Several selections can be made for along track integration time, swath width and radiometric resolu 
tion. Auxiliary data on flight velocity and aircraft attitude are registered together with digital image data 
on high density digital tape with a maximum data rate of about 8M bits per sec. Preflight relative and abso 
lute calibrations for each detector element have been performed. By means of a dark current correction, cali 
bration and aircraft attitude data, system geometric and radiometric corrections are applied prior to digital 
analysis. Results of test flights executed over land and sea sites are presented in order to demonstrate the 
performance and flexibility of CAESAR. 
In the framework of the National Remote Sensing 
Programme of the Netherlands, a CCD Airborne Ex 
perimental Scanner for Application in Remote 
Sensing, called CAESAR has been built. The project 
has been executed by the National Aerospace Labo 
ratory NLR. The TNO Institute for Applied Physics 
TPD has been subcontracted for the development of 
the sensor part of the system. The project was 
financed by the Ministry of Education and Sciences. 
The definition phase has been started in October 
1981. The final tests were completed in 1986. 
The objectives of the project were to increase 
technical knowledge of and practical experience 
with the application of linear CCD detectors in 
remote sensing for land and sea observation. Subse 
quently an advanced airborne system would become 
available for research activities in the framework 
of the national programme. Experience could be 
brought together from TPD in the field of advanced 
optics and solid state detectors, from NLR in the 
field of flight operations, airborne data handling 
and processing and knowledge on remote sensing from 
a number of research institutes with user interest. 
The application of linear detector arrays to 
multispectral imaging offers the following ad 
vantages in comparison with the classical mechani 
cal scanning techniques: 
- substantial increase of the integration time by 
which the spatial resolution can be increased, 
- improved image quality in terms of geometry, 
- absence of mechanical moving parts. 
The resulting disadvantages are related to the 
increased requirements for the image quality in the 
focal plane of the main optics and the required 
radiometric calibration of all individual detector 
elements. Moreover, for most commercially available 
CCD detectors manufactured from silicium semicon 
ductors, the wavelength range is limited between 
400 and 1100 nm. 
CCD detectors are used in the German Modular Opto- 
electronical Multispectral Scanner (MOMS) and the 
HRV instruments of the French SPOT satellite. The 
Visible and User Infrared High resolution scanner 
(VNIR) of the Japanese MOS-1 satellite also will 
make use of CCD detectors. The advantages of an 
airborne system are related with compact dimensio 
ning and flexibility with respect to the possibi 
lity to change spectral filters and the configu 
ration for experimental purposes. 
In the development of an instrument like the 
CAESAR scanner a number of subsequent steps can be 
distinguished. Within a set of technical con 
straints the user requirements have been defined. 
In the definition phase trade-off studies were per 
formed for several configurations. The user re 
quirements are elaborated and evaluated with refe 
rence to technical possibilities and cost con 
straints. Based on the accepted design specifica 
tions the design and development phases are exe 
cuted, comprising selection and testing of pur 
chased components, manufacturing and testing of 
subsystems, integration and testing, calibration, 
execution of flight and acceptance tests. In paral 
lel system software is developed and tested and 
system documentation is produced. 
The CAESAR scanner should be configured both for 
land observation (in particular vegetation) and for 
observation of (sea)water colour. The technical 
constraints were that only one CCD detector would 
be available for each spectral channel, that only 
commercially available CCD's would be used and that 
standard objectives would be applied as imaging 
optics. User requirements are in general dealing 
with the selection of the position, width and 
number of spectral bands, the dynamic range and 
radiometric resolution, interband image registra 
tion and correction for geometric and radiometric 
errors, swath width, spectral resolution and view 
direction and finally the illumination conditions 
during the execution of flights.

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