S утро.
Fig. 4 CAESAR sensor system.
Fig. 5 CAESAR raw image acquired during the
third test flight, April 23, 1985.
Area :•Ouderkerk a/d Amstel,
Netherlands
Channels : 520, 685 and 785 nm
Height : 2000 ft
Pixels : 0.15 x 0.50 m
tions of cable shieldings have been carried out prior
to the final acceptance tests.
RADIOMETRIC CALIBRATION AND PREPROCESSING
Relative radiometric calibration is required for
quantitative comparison of data corresponding with
a single multispectral image and with temporal
series of multispectral images. In case of absolute
radiometric calibration, the transfer function is
known between the spectral radiance at the entrance
pupil of the instrument and the measured detector
signal. Within the accuracy limits which can be
achieved, an absolutely calibrated scanner can be
used for multispectral radiance measurements. Abso
lute calibration is only required if the user wants
to apply radiometric corrections by means of exter
nal data (from other instruments or models) and for
the retrieval of object parameters from physically
defined radiance values.
The CAESAR system has been calibrated relatively
and absolutely. For a calibration of the sensor
part, a dedicated facility has been built. For each
detector element and each spectral band the response
function is measured, including the measurement of
the darkcurrent. By means of a calibrated detector
the radiance at the entrance pupil of the CAESAR
modules is determined, providing input data for the
absolute calibration.
The same calibration facility has been used for
geometric calibration. Test targets have been
applied. An equally important part of the radio-
metric calibration is the determination of the
transfer functions for all channels between the
analog input signal and the resulting digital words
after analog to digital conversion. By the NLR for
this purpose an electrical calibration facility has
been designed and built.
During the data preprocessing system corrections
are applied for the removal of radiometric and geo
metric errors. Aircraft velocity and attitude data
are measured by an inertial navigation platform
during flight. These data, together with position
data are registered together with CAESAR sensor
data and synchronisation data provided by an inter
nal clock.
After data conversion from high density tape to
computer compatible tape, selected CAESAR data are
preprocessed on a mainframe computer. All radio-
metric calibration data stored in look-up tables
are applied to radiometric correction of each
single pixel value. Geometric correction software
developed earlier for preprocessing of airborne
side-looking radar data, has been modified in order
to comply with the different scanning geometry of
the down- and forward-looking channels of CAESAR.
Geometric distortion are to a large extent removed
by means of algorithms based on a motion model
using flight altitude and navigation platform data.
After the required image resampling data from
different spectral channels and different dates of
overflight can be registered. Precision registra
tion of images acquired over land can be performed
as a next step by means of ground control points.
In such a way CAESAR data can be used as overlays
for existing map projections.
CONCLUDING REMARKS
During 1986 a performance demonstration programme
will be executed in The Netherlands in close co
operation with users. The aim of this programme is
a first validation of the capabilities of CAESAR
for land and water observation. It will be investi
gated for which applications high resolution CAESAR
data can be used in combination with available high
resolution satellite data in a multistage approach.
LAN]
Sherry C
Depártame
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