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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