and gradiometry and stable platforms. All these applications
have a requirement for precise attitude measurements.
Figure 3 shows a typical POS/AV application for an airborne
survey instrument. The position of the ground point is
determined from the position of the aircraft, the pointing
angle or line-of-sight (LOS) angle of the survey instrument
and a digital terrain model (DTM) with respect to local level.
Accurate determination of individual points requires accurate
measurements of both position and orientation. The
integrated approach of POS generates both quantities.
Imaging Scanners: These include digital multi-spectral
scanners such as the casi and the Daedalus and other types of
scanners such as thermal infrared. In this application the
image formed by the forward motion of the aircraft is
distorted by pitch, roll, and platform crab angle. These
parameters are measured by POS/AV and recorded on tape. In
post-processing POS’ attitude and position measurements are
combined to correct for sensor motion artifacts. Following
correction, and by using a DTM, image pixels can be
accurately geocoded and finally the individual strips
mosaiced. To perform these operations geocorrection
software such as GEOCOR from CCRS must be used. Up until
recently airborne scanners have been used primarily for
remote sensing applications. By incorporating attitude data
from POS/AV a scanner can now be used as mapping
instrument as well.
recording attitude in real-time POS not only provides the
required precision pitch/roll measurements but also provides
ready to use data. Further, and by being able to perform in-air
alignment POS eliminates the need for time-consuming
ground alignment procedures prior to take-off.
Film Cameras: This is an area where POS/AV has the
potential to offer a breakthrough in airborne survey and
mapping. By providing precise arcsec-level
pitch/roll/heading measurements, POS/AV can reduce and
potentially even eliminate the need for ground control
points and aerotriangulation. Tests conducted using Leica
RCIO and RC30 cameras indicate that such accuracies are
possible. For this level of accuracy solid mounting of the
IMU on the sensor is very important.
The small size of the IMUs used in POS permits mounting of
the IMU right on the sensor. Precise time alignment of the
POS data with the shutter exposure pulsc is also key in
obtaining good quality results. POS' event marker input is
used to time stamp the shutter pulse down to sub-usec
accuracy. This is particularly important for this level of
accuracy since time can differ between GPS receivers by up to
a few msec.
Digital Frame Cameras: The resolution of digital frame
cameras is still a long way from that of film, but these
cameras are finding an increasing number of applications.
GPS = aircraft hull
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Figure 4. POS/AV with External GPS Receiver
Scanning Lasers: This is a new generation of airborne
sensors opening a new class of applications and making old
survey and mapping applications casier and faster. An
airborne scanning laser flies at altitudes of up to 1km and
scan swaths typically 400 wide. With footprints smaller than
a foot and scanning rates of a several KHz a fairly dense grid
of ranges can be collected within the scanned swath. One fly-
over can provide DTMs of the area with unprecedented
density and accuracy. In order to take full advantage of the
capabilities offered by this type of instrument, however,
precise measurements of the sensor’s attitude must be
performed. Because of the difficulty in providing ground
control points for this type of sensor, an attitude measuring
system is an integral part of the sensor. By measuring and
470
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B3. Vienna 1996
POS is being used to provide attitude measurements for such
cameras that permit the rapid mosaicing of individual frames
to cover large areas. With POS flying over relatively flat
terrain, it is possible to mosaic hundreds of image frames
within a few hours and form large area maps for fast delivery
to the end customer.
Synthetic Aperture Radar: POS can provide high precision
position and attitude measurements over the span of the
synthetic aperture (typically of the order of 10-20sec). Sub-
millimeter ^ position and sub-arcsec relative attitude
measurements are required for SAR and have been achieved
with POS.
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