positioning problems. We do not, as yet, know
whether this can be done in practice, but it looks
promising given the new technologies of avionics and
GPS.
Miniature avionics units are readily available to the
model aircraft hobbyist. These units are available
within a range of sophistication depending on price.
At one end of the scale is the autopilot with a height
lock which automatically engages should line of sight
communications be interrupted. The model will
cruise to a preset altitude and will circle slowly until
retrieved or until it runs out of fuel. This type of
system can be purchased locally for around
AUD$2,500.
At the sophisticated end of the scale, surveillance
RPVs (remotely piloted vehicles) carry an integrated
System comprising an avionics unit for RPV control
and stability in three dimensions, a GPS (Global
Positioning System) unit for navigation and position
fixing and a telemetry unit for real-time links to a
ground station. Using equipment such as this, the
photo mission takes on the precision and flight
planning characteristics of a full-scale operation.
Way points can be pre-set and RPV movements
monitored from the ground station on a video display.
Changes can be made in real time to any pre-set
parameters. A full system of this type costs
approximately | AUD$40,000. With this latter
equipment it would seem that stability and RPV
positioning problems have been largely solved
enabling us to concentrate on image capturing.
Our experiment relied entirely on digital
photogrammetry, not as a solution to the platform
stability problem, but in an attempt to circumvent the
platform stability problem. The positioning problem
was approached indirectly as well. Rather than
trying to control the RPV's position precisely we
decided to continuously record images and then
select appropriate frames subsequently. This meant
a normal 25 frames per second video camera could
be used. This approach meant that our equipment
requirements for the photo operation amounted to
just three items: An Aerial Platform, a Video Camera
and a Pilot. We were very fortunate to enlist the
services of Jim Oliver as a provider of an RPV and
also a pilot, and Joe Van der Maat of the AudioVisual
Section of QUT for the video camera. Without the
excellent cooperation of these two people this
experiment would not have left the ground.
The RPV used was in fact a model aircraft, that is a
quarter-scale Cessna L-19/0-1, commonly called a
"Bird Dog", with a wing span of 108 inches, a wing
area of 1450sq inches. Jim's willingness to cut a
hole in the bottom of his model and to modify the
160
cabin area to accommodate the camera is due to his
generosity and inquisitive nature and we thank him
immensely. The model was powered by a 38cc
petrol engine (for reliability), the camera was a hand-
held CamCorder.
4. PROBLEMS ENCOUNTERED ON FLIGHT
As anticipated wind-gusts were the major factor
influencing the stability and positioning of the aircraft.
Notwithstanding the early morning take-off time,
there was sufficient air turbulence to cause yawing
rotation, tip and tilt of an extreme nature with respect
to normal aerial photography. Another major image
defect was due to vibration from the petrol engine.
The camera had been heavily padded and
suspended in foam and bubble-wrap for two
reasons: one, was to eliminate vibration; and
secondly, to protect it in the event of a mishap. The
specially procured engine mounts were also meant to
eliminate vibration. Video cameras seem to be
especially sensitive to vibration and in this case the
situation was exacerbated by the vertical attitude of
the camera.
Solutions to the positioning and stability of the RVP
already exist in the form of miniature avionics units
discussed previously. Vibration problems might be
eliminated if the lens was remote from the recording
tape, such as a CCD (charged coupled device)
Camera linked by telemetry to a ground recorder.
Depending on funding these hardware systems will
be trialled in future experiments.
5. DESCRIPTION OF THE TECHNIQUE
The technique is based on using digital imagery at all
stages of the mapping process. Normally, film is
used either for data acquisition or for the production
of the final map. Using digital techniques leaves
control of the process with the user.
The first step is to acquire video imagery using a
remotely controlled platform, which in our case was
a remote controlled model aeroplane. The video
camera was mounted in the aeroplane and directed
vertically downwards. The camera was switched on
before the flight and after the flight the camera was
used in play-back mode to check that adequate
coverage had been obtained.
The second step is to select a pair of images from
the tape which have suitable stereoscopic overlap
and to process these images through a digital
photogrammetric system to produce an orthophoto.
The images are acquired from the tape and
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B4. Vienna 1996
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