207
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.
INTRODUCTION
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.
USER REQUIREMENTS
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.
