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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