International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B6. Istanbul 2004 
4.2.2 Platform calibration 
The platform calibration is essential for the resampling of the 
new large format image composite based on the four PAN 
channels. Due to the fact, that a mechanical part used in high- 
dynamic environments like a photogrammetric flight never can 
be realized as absolutely stable, the DMC camera housing was 
designed to allow for some angular deformation of the 
individual camera heads relative to each other. These 
deformations are different for each airborne environment and 
have to be estimated from the mission data itself. This on-the- 
fly calibration approach is based on tie point measurements 
from the overlapping regions of pan-chromatic imagery. 
Besides that, the precise knowledge of relative positions of the 
individual camera heads, the calibration parameters from single- 
head calibrations as described above and first approximations 
on the relative misorientation between the camera heads are 
necessary input data required for platform calibration. The 
calibration is solved within a bundle adjustment approach, 
where three already mentioned rotation angles plus a focal 
length refinement for three camera heads relatively to one 
reference camera head are estimated As mentioned in Dórstel et 
al (2003) about 30-50 tie points are sufficient to estimate the 
unknown parameters. The typically obtained accuracy is 
reported with 1/12 to 1/6 of a pixel. 
4.3 Leica ADS40 
In contrary to the frame based approach (single or multi-head) 
described so far, multiple linear CCD lines are used in the Leica 
ADS40 system. The ADS sensor development was driven by the 
experiences with digital airborne line scanning systems at DLR, 
namely the WAOSS/WAAC camera systems, originally 
designed for the 1996 Mars mission and adopted for airborne 
use after failure of the mission. First tests with ADS engineering 
models started in 1997, the official product presentation was 
done during the ISPRS 2000 conference in Amsterdam. The 
imaging part of the sensor consists of typically 10 CCD lines 
with different viewing angles and different multi-spectral 
sensitivity. Each individual line provides 12000 pix with 6.5 x 
6.5 um? pixel size. 
During calibration the pixel positions of each individual line are 
determined. The nomenclature for the different CCD lines is 
like follows: pan-chromatic forward (PANF), nadir (PANN) 
and backward (PANB) lines, multi-spectral forward (red REDF, 
green GRNF, blue BLUF) and backward (near-infrared NIRB) 
lines. The viewing angle relative to the nadir looking direction 
is also specified by extending these identifiers with the 
appropriate inclusion of numbers representing the individual 
viewing angle. For example 28 corresponds to the 28.4deg 
angle between nadir and forward looking direction of the PANF 
channels - the resulting identifier is PANF28. The other viewing 
angles are 14.2deg for the backward PAN lines, 16.1deg for the 
RGB forward lines and 2.0deg for the NIR backward looking 
CCD line, resulting in 14, 16, 02 code numbers. Since each 
PAN channel consists of two individual lines, shifted by half a 
pixel (so-called staggered arrays), this two lines are differed by 
using character A for the first and B for the second line. For 
reasons of completeness it should be mentioned, that the ADS is 
available with a slightly different CCD-line configurations in: 
the focal plane also: In this case the nadir looking PAN 
staggered lines and the forward looking RGB lines are 
exchanged, resulting in nadir viewing RGB channels and an 
additional forward looking PAN channel. Such configuration 
might be advantageous, when the main focus of applications is 
laid on the generation of MS ortho-images. 
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4.3.1 Lab calibration 
The lab calibration of the ADS sensor is based on a coded 
vertical goniometer (CVG) available at SwissOptic (a Leica 
Geosystems company). All details on the calibration facilities 
are given in Pacey et al (1999). The CVG was developed from 
a modified electronic vertical goniometer (EVG), where the 
photomultiplier is replaced by a digital CCD frame camera and 
the glass reference plate (with its high-precisely known marks) 
is replaced with a special glass code plate. These coded targets 
are located at the two diagonals and the two horizontal and 
vertical bi-sections of the plate. The spatial distance between 
neighbouring targets is 10mm. The measurement is done 
automatically with high precision. From the measured 
corresponding object angles the calibrated focal length and the 
distortion function are obtained. The CVG is used for the 
calibration of classical RC30 cameras as well as for the ADS 
sensors, although for ADS the calibration procedure has to be 
modified like follows. 
As described in Pacey et al (1999) lens cone and CCD focal 
plate are calibrated separately first. Afterwards both 
components are assembled and calibrated using the CVG. In 
this case the glass code plate cannot be used any longer since 
the CCDs are fixed in the focal plane now. Therefore, a coded 
target is projected in reverse direction on to the CCD-line of the 
tested lens. In order to allow measurements in of-nadir 
directions an additional mirror scanner is mounted on top of the 
goniometer arm. With this modification each individual pixel 
location on the focal plate can be addressed. As written in 
Schuster & Braunecker (2000) it is sufficient to measure pixels 
every 2-5deg within the field of view. The values for 
intermediate pixels are interpolated numerically. 
4.3.0  Self-calibration by bundle adjustment 
Although a complete measurement and process flow was 
established for lab calibration a new approach for ADS 
calibration was introduced recently. This in situ approach is 
exclusively based on self-calibration, which is — as already 
mentioned before — a system driven approach including the 
calibration of all image-relevant system components. In this 
context especially the inertial measurement unit (IMU) has to be 
mentioned, which is essential for operational processing of 
airborne line scanner data. The mandatory relative orientation 
between IMU body frame and ADS photo coordinate system 
can only be determined via self-calibration, which is one 
advantage compared to the lab calibration approach. The 
applied procedure is given in detail in Tempelmann et al (2003) 
and should be recalled in the following. 
The calibration is based on the orientation fixes approach 
proposed by Hofmann, which is implemented in the bundle 
adjustment software. Again the Brown parameter sets are used 
as calibration terms. Beside that, additional three unknowns are 
used to model the before mentioned misalignment angles. 
Although ADS40 comprises line instead of classical frame 
geometry, many of the Brown parameters are directly 
transferable. Some of the parameters (modelling platen flatness) 
are not useful for line scanners and have to be eliminated. 
Nonetheless, some uncompensated effects remain. These 
remaining effects, which are non compensated via the Brown 
parameter set, have to be modelled by additional polynomials. 
In Tempelmann et al (2003) a 6" degree of order polynomial 
performs sufficiently well and is recommended for X and Y 
components of each sensor line. This extended model will be 
available in the updated bundle software, hence additional 
polynomial coefficients are directly estimated in the bundle. 
In order to realize a sufficiently well overall system calibration, 
special requirements for the calibration flight pattern are