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7. ENGINEERING MODEL (EM) AND
TECHNICAL CO-OPERATION
The complexity, cost and difficulty of developing
and manufacturing a novel airborne digital sensor
ruled out “going it alone”. In early 1997, shortly
before LH Systems was formed, Leica Geosystems
reached a technology agreement with Deutsches
Zentrum fur Luft- und Raumfahrt (DLR), the
German Aerospace Centre in Berlin. This provided
for long term co-operation, with joint development
by both parties and assembly by Leica Geosystems.
DLR’s experience in this area is unparalleled.
Amongst a host of intricate and impressive
achievements in both airborne and spaceborne
technology, it made historic progress with sensors
based on the three-line approach, for example the
WAOSS (Wide Angle Optical Stereo Sensor, built
for the unfortunate Mars-96 mission) (Sandau and
Barwald, 1994), WAAC (Wide Angle Airborne
Camera) (Sandau and Eckhardt, 1996) and HRSC
(High Resolution Stereo Camera) (Albertz et al.,
1996). DLR’s expertise complemented well Leica
Geosystems’ abilities in optics, mechanics and
electronics, together with its deep appreciation of
customers’ requirements acquired through decades of
producing aerial film cameras. It was natural that the
agreement between the parties be transferred to LH
Systems quite soon after its formation.
8. IMU AND GPS INTEGRATION
In order to reconstruct high-resolution images from
line scanner data, the orientation data of each line has
to be obtained. The inventors of the three-line
principle proved mathematically that this could be
done by using observations from image matching
techniques only, as provided in modern
aerotriangulation packages. But computation time
required for this indirect method is so large, that
direct observations from attitude and position sensors
are seen as the easiest way to reduce processing time.
Applying only the indirect method is time
consuming; applying only the direct method is
capital intensive. The decision was made to find an
optimal trade-off by including direct measurements
from GPS and IMU sensors of only certain accuracy
into the aerotriangulation techniques. The advantages
of this trade-off are:
• data processing time to rectify line scanner data
is reduced significantly
• price/performance ratio of medium priced IMU
sensors is likely to improve faster over time
The tight integration (Fig. 14) with the focal plane of
a digital line sensor has a large potential for further
reduction of ground control.
Fig. 14. Main components of tight integration of
IMU/GPS and a three line sensor camera.
In 1998 LH Systems and Applanix Corporation from
Canada set up a working group to analyse the
potential and propose solutions to achieve a tight
integration between IMU, GPS and line sensors,
within the scope of the co-operation agreement that
LH Systems has with DLR. As one of the results, the
engineering model of the airborne digital sensor is
now being flown routinely including IMU and GPS
sensors from Applanix Corporation.
9. PRACTICAL CONCLUSIONS
The features of the film and digital approaches are
compared in Table 1. LH Systems has chosen the
three-line scanner approach for the reasons given
above. The engineering model (EM) has flown (see
Fig. 15, Table 2) and work is proceeding towards the
production model, which will have at least 20,000
pixels in each line, faster integration times and
multispectral bands. This is on schedule for launch at
the ISPRS Congress in Amsterdam.
Photogrammetrists will be able to share data with the
remote sensing community and for the first time
create deliverables with both the depth of
information accruing from image understanding of
multispectral images and the geometric fidelity of
photogrammetry. In the standard version of the new
airborne digital sensor the multispectral images will