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Figure 2. Integration of the equipment (IMU, VIS-Camera and
Camera-Laptop) in a helicopter
The influence of image compression was partly investigated for
the projects up to now. This topic remains to be considered if
data transfer rate limits image acquisition. So for the project
LUMOS an image compression with fixed factor of 2 was
applied without significant influence of the quality of image
processing.
Parameters describing pixel positions on the focal plane (so-
called interior orientation) are necessary for georeferencing of
the image data. They were determined during a calibration
process in optical laboratories at DLR.
3.2.2 Infrared camera
Cameras working in the infrared range of the electromagnetic
spectrum have the advantage to be applicable even at night. In
most cases the spectral texture in the infrared allows an easier
image data interpretation as with visible cameras. The main
disadvantages of infrared cameras are the small number of
pixels (so the swath or the ground resolution has to be reduced)
and high costs. Table 2 shows the parameters of the used
thermal IR camera.
Parameter Value
Detector MCT, cooled at 77?KIR 18
Type MK III, Barr & Stroud ltd.
768 x 500
15,28? x 10,20?
Number of pixels
Field of view
Radiometric dynamics 8 Bit
Spectrum 8 — 14 um
Frame rate 28 Hz
Ground sampling 0.5 m
distance, flight height
3500ft
Swath width 380 m
Table 2. Parameters of a typical IR camera configuration
3.3 Inertial measurement unit
Onboard or real-time georeferencing requires the exact
knowledge of all six parameters of the so-called exterior
orientation (three translations x, y, z, three rotations 0,0,'
of the camera for each snapshot. Depending on the desired
accuracy of data products, these parameters have to be
determined with an accuracy in the range of one ground pixel
distance and one instantaneous field of view (field of view for
one pixel).
A typical technical solution for direct and precise
measurements of the exterior orientation parameters of the
camera during imaging is to use an integrated GPS/Inertial
system, which combines efficiently inertial sensors technology
and GPS technology. DLR owns such a system (POS-AV 410
of Applanix Corp.), which fulfils the required specifications
(Lithopoulos 1999, Scholten et al 2001). The system consists of
an Inertial Measurement Unit (IMU) LN200 and a control unit
with integrated GPS receiver. The IMU sensor head was
mounted close to the camera. The IMU realizes measurements
of accelerations and angular velocities and records movements
of the camera/IMU. The time synchronized IMU and GPS data
are processed in a control unit within an inertial navigation
algorithm. The system provides real time output of position
and orientation with a rate up to 200 Hz. In combination with a
differential GPS correction, an absolute accuracy for position
of 0.5 to 2 meters and for attitude of 0.015 to 0.05 deg can be
obtained.
Image and attitude data have to be recorded synchronously.
Therefore, Applanix' trigger pulses were monitored and used
for commanding the image acquisition process.
3.4 Data Transmission
Onboard captured images and related orientation data are sent
to the ground station via radio transmission. The data
transmission channel consists of the transmitter and antenna
onboard and receiving station on ground. The data transmission
rate is the most limiting factor for the specification of the real
time airborne monitoring system. It defines the maximum
image acquisition rate. The transmission rate values were 2
and 4 Mbps for LUMOS and Eye in the Sky configurations
respectively.
3.5 Ground Station
Image data received on the ground station are transmitted to
the traffic computer for image processing. Within the LUMOS
project a special network of three ground stations with a
distance of about 25 km was developed and installed to ensure
the coverage of whole Berlin-City with surrounding area.
Directional radio link lead the data from each station to the
server, where the best of the three data packages is selected
and provided to the image processing. For the project “Eye in
the Sky” another technical realisation was applied. It is based
on a digital transmission system with a mobile GPS antenna
placed on the roof of the building for tracking.
4. DATA PROCESSING
4.1 Direct georeferencing
The real time orientation data stored in a control PC describe
the actual position of the camera. This position is given by
longitude, latitude and ellipsoid height with respect to the
geodetic datum WGS84 and the rotation angles of IMU
measurement axes given by roll, pitch, and heading with
respect to the local tangential coordinate system.
The misalignment between the IMU and camera axes (bore
sight angles) has to be estimated offline once per system
installation in the airborne platform using the traditional aero
triangulation method.
During the measurement flights, real time orientation data,
misalignment angles and camera parameters (interior
orientation) define a transformation from image space to object
space and vice versa. Assuming a medium terrain height, the
position of the vehicles can be estimated. Consequently, for
each pixel of interest a (x,y)-tuple can be determined and each
