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International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B5. Istanbul 2004
orientation between them (Figure 2). Moreover, the system
remains modular and, depending on the needs, units can be easily
removed. Following are the main three operational modes:
1. Camera * GPS: 4.5 kg
2. Camera + GPS/IMU: 6kg
3. Camera + GPS/IMU + Lidar: 12 kg
2.1 Imagery
The digital camera is composed of the Hasselblad Biogon SWCE
903 camera with a focal length of 38mm that is attached to a
digital back (Kodak ProBack Plus). The size of the CCD chip is
4072x4072 pixels with 9um pixel size. A home made electronic
device controls the synchronization of the CCD with the shutter
aperture by generating a pulse that is time-registered in the GPS
receiver. The choice of the lens was based on its low distortion, a
comparison of MTF curves and field tests. Images are stored on
two internal 4GB Flashcards that allow taking up to 800 shots.
2.2 Airborne Laser Scanner (ALS)
The Lidar scanner unit is either the LMS-Q140i-60 or the newer
model LMS-Q240-60. Both models are manufactured by Riegl
and differ mainly in the maximum measurement range and
interfaces. The laser wavelength of 900um fits well measurements
of natural targets and above all the snow covered surfaces. The
maximum range is around 500m and the range resolution is
25mm. The scanner performs up to 80 scan lines per second at 10
kHz data rate. The rotating mirror induces a swath of 60° that
corresponds well to the field of view of the digital camera that is
55°. Data are synchronized thanks to PPS pulse of the GPS
receiver and a standard PC governs their storage through Ethernet
or ECP ports.
LN-200 IMU
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Figure 3: Data flow and synchronization for the IMU/GPS units.
2.3 Navigation devices for sensor’s georeferencing
As with the use of sensors, the system remains modular in
accommodation of navigation devices, namely the GPS receivers
and the IMU. The system currently employs a Javad Legacy GD
GPS receiver on board of the helicopter and additional GPS
receivers on the ground. Positive experience has also been made
with the Leica’s SR530 receivers. In its most basic setup (CCD +
GPS) the GPS data collection rate is set to 5 Hz, which is
sufficient sampling rate for the dynamic of a helicopter. This rate
Is reduced to 1 Hz or less when IMU is employed.
The IMU is tactical-grade strapdown inertial system (LN-200 Al)
with 400Hz measurement rate. The IMU data are synchronized
through small custom interface (Viret, 2003) and sent via Ethernet
link to a standard portable PC as schematically depicted in Figure
2. Again, modularity was in the design priorities here, and thus
the GPS receivers as well as the PC are easily interchangeable in
case of hardware failure. The recorded IMU data are used in a
post-mission integration with the differential carrier phase GPS
data via a Kalman Filter employing 25 to 30 states.
Figure 4: The system in action with Alouette III helicopter. The
holder for supporting the system weight during transition flights is
located under operator's right leg.
2.4 Helicopter mount
The helicopter mount (Figure 4) is independent from its carrier,
which has several advantages. First, the installation time is as
short as a few minutes. Second, changing carriers does not require
re-calibration of the sensors. Third, its flexible handling allows
maintaining optimal geometry of the sensors in steep and flat
terrains for the benefit of higher mapping accuracy. Finally, most
of the rotor-induced vibrations are dampened when the operator
holds the system and activates the imaging sensors (Skaloud and
Vallet, 2001). An additional simple holder can be added to the
exterior of the helicopter to support the system during approach
flights.
2.5 Flight management
The system is designed to map smaller areas at large-scale, which
permits a simple but efficient flight management concept. The
system operator conducts photograph overlap and shots timing
whereas the navigator/pilot steers along the flight line. The flight-
line navigation uses the display of a rugged PC running
PenMap™ software accepting the NMEA/GGA message sent by
the GPS receiver. Receivers like Javad GD benefit of
WAAS/EGNOS capability, which guarantees sufficient
navigation accuracy in speed and position for manual aiming of
the camera and laser sensors.
