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International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B1. Istanbul 2004
with much less jitter caused by an uncompensated platform
movement. As shown in Figure 2, four CCD line sensor
packages are placed parallel to each other onto the focal plane
of the camera system. Three packages serve as forward, nadir
and backward looking sensors. Each of those packages consists
of 3 lines, generating R (Red), G (Green) and B (Blue) images
to be combined into a color image. In addition, there is another
CCD line sensor package for a near infra-red (NIR) image
between the backward looking and the nadir looking packages.
Each line sensor can produce a high-resolution, two-
dimensional image in during the helicopter flight, generating 10
images in total and simultaneously, which are overlapping
10095 with each other. The position and attitude data of the TLS
camera for each time instance are acquired with GPS (Global
Positioning System) and IMU (Inertial Measurement Unit). An
antenna for the GPS is mounted on top of the helicopter, while a
set of fiber-optic gyroscopes is incorporated next to the TLS
camera into the stabilizer. In principle, such a spatial data
acquisition system does not need aerial triangulation with
ground control points, but the processing may be based on
direct georeferencing. STARLABO Corporation acquired the
patent rights for the system in Japan, the USA, Europe,
Australia, and Canada (Murai, 1993).
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Figure 2. Principle of STARIMAGER
The advantages of STARIMAGER can be described as follows
(Tsuno, 2002a; Tsuno, 2002b):
1) Photographic developing and scanning processes are
unnecessary and there is no image deformation such as
damage or deterioration of a film. Moreover, since the
dynamic range of an output image is linearly proportional to
the luminance of the image, it is possible to recover easily
even objects that lie in heavy shadow areas.
2) Due to the use of a GPS/IMU the number of ground control
points can be reduced and direct georeferencing becomes
possible, depending on the required accuracy.
Due to the high-performance stabilizer, an acquired original
image has no jitter and no blur. Therefore these images are
suitable for real-time processing in emergency cases, where
the burden for post-processing must be reduced (see Figure
33.
The camera can be mounted on the platform in an oblique
mode, together with the stabilizer. This kind of imagery is
useful for special applications, as for instance in texture
mapping on the vertical walls of buildings.
A helicopter allows low altitude and low speed operations,
leading to very high-resolution images. Since the system is
less influenced by bad visibility and weather conditions,
UG
—_
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—
images can be produced more frequently and with greater
flexibility than in conventional aerial photography. And, it
can take pictures of the linear-shaped objects efficiently by
following it over at a low speed.
With the TLS principle, the nadir image produces almost a
parallel projection in the direction perpendicular to the flight
direction, and thus more suitable for ortho-image generation.
The system basically does not need mosaic processing in the
flight direction and it can extract the spatial data of linear-
shaped objects seamlessly, such as roads, railways, rivers,
etc. The system produces images with continuous three-fold
overlap, leading to less field survey burden after the data
acquisition. Furthermore, a corresponding point search in
image matching is easier and the measurement accuracy
improves due to image triplet matching (see Shibasaki,
Murai 1987 and Morita et al., 2001). It produces multi-
spectral data with three RGB channels and one infra-red
channel, having better interpretation potential than just
monochromatic images.
6
—
A TOES V
without stabilizer
Figure 3. The Stabilizer's effect on image quality
with stabilizer
2.2 System configuration
The focal distance of the TLS camera lens system is 60 mm, the
stereo angles are 17, 23 and 40 degrees, between forward and
nadir, nadir and backward, and forward and backward,
respectively (see Table 1 for the STARIMAGER SI-250 system
specifications). Each line sensor consists of 14 400 CCD pixels
with 5 m spacing, and acquires 500 line images in 1 second (2
msec acquisition interval), recorded with a controlling and
recording device installed in the helicopter cabin. In addition, as
shown in Figure 4, a GPS antenna acquires the camera position
signal at 5 Hz and the IMU on the top of the TLS camera
acquires the camera attitude signal at 500 Hz. A Trimble
MS750 serves as Rover GPS and collects L1/L2 kinematic data
at 5 Hz and another Trimble MS750 serves as Base GPS on the
ground.
The stabilizer has a vibration-absorbing spring and 5 gimbals,
absorbing the fluctuations of the helicopter, and keeps the
optical axis direction of the TLS camera stable within a single
pixel of the line sensor. The TLS camera and stabilizer are
controlled by the electronic devices in the helicopter cabin,
which record images, position and attitude data (see Figure 5).
