Included in the scanner design is a thermal
reference plate which is placed at one end of the
linescan locus so that referencing takes place at the
scan rate. This arrangement enables the selection of
five sensitivity ranges covering the following
temperature ranges: 2°, 4°, 8°, 16° and 32°. Cooling
is carried out using high pressure compressed air
supplied to a Joule-Thompson minicooler. Further
design details can be found in Berry and Runciman,
1981.
4 DATA ACQUISITION
To date, both the Barr and Stroud IR18 and Rank
Pull in Controls thermal scanners have been evaluated
for remote sensing applications. The initial
assessment of the former system was performed in
August 1984 by flying the camera, in a Cessna 172
light aircraft, over several tests sites in the West
of Scotland.
The Cessna 172 is a four seat aeroplane with two
access doors and one luggage door immediately behind
the pilot's door. Several modifications were
necessary to the aircraft to enable the IR18 to be
used for the production of vertical imagery. The main
features of the design which was used are described
in Ekin (1984). Briefly the approach adopted by Ekin
was to design a retractable camera carriage, attached
to the luggage door, which could be extended in
flight by the camera operator. A pair of rails, on
which four wheels with V - shaped groves ran ensured
the smooth movement of the rig into the extended
position. Figure 4 illustrates the IR18 mounted in a
slightly modified version of the rig described.
Figure 4: Barr and Stroud IR18 mounted on retractable
camera rig in a Cessna 172 light aircraft.
The adoption of a light aircraft and an easily
manipulated TVFS system of the type described offers
several advantages to the user. Firstly, by using
widely available light aircraft the operational costs
of the survey can be significantly reduced. Secondly,
once the site of interest has been located and the
camera rig extended, the scanner operator can view,
via a small monitor, the area being imaged by the
instrument. Thirdly, after the survey has been
completed immediate confirmation that the data has
been recorded successfully can be achieved by playing
back the video tape, either in flight, or more
commonly when the aircraft returns to base.
For other applications however, it may be
unacceptable to use the scanner from a light
aircraft, either because of the need for a more
stable platform, or the need to use the camera in
conjunction with several other sensors. More recent
experience of using the IR18, for defence and
commercial applications by ERSAC Ltd. has involved
using the IR18 in a purpose built camera mount in a
twin engined Piper Aztec aircraft. For maximum
flexibility of operations and integration with a
standard aerial camera, a special adapter plate was
designed and deveolped by ERSAC and Barr and Stroud
to enable the IR18 to be mounted and operated through
a standard Wild RC8 aerial camera mount. This
configuration is illustrated in Figure 4. The most
significant advantage of this design is the
capability for full yaw control; this enables higher
precision to be achieved when flying parallel
flightpaths for the acquisition of imagery over large
are a s
Figure 5: Barr and Stroud IR18 in an adapted Wild RC8
aerial camera mount in a Piper Aztec aircraft
5 DATA PROCESSING
The processing of video based thermal data consists
of three distinct phases:
(a) conversion of the analogue video recording to a
digital format,
(b) pre-processing of the digital data to remove
image inconsistencies, and/or to rectify and
geometrically correct the image, and
(c) processing and analysis of the data using
conventional image processing techniques.
The analogue to digital (A to D) conversion of
video tape is carried out using a digital frame store
(DFS) or 'frame grabber'. These instruments are now
widely available at relatively low cost. The
particular model used for these investigations was
the Quantel 3001 DFS controlled by an IDP 3000 image
processing system.
The pre-processing, and processing stages, were
carried out on a GEMS image processing system. One
particularly useful pre-processing technique is that
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