International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B2. Istanbul 2004
3.6 Thermal Digital Camera
Information from the thermal spectrum can be used for example
for heat-loss detection, for observations during night time, but
also to detect moisture in the ground.
The thermal digital camera will operate in two thermal infrared
bands (SWIR : 3-5 um and LWIR : 8-12 um), with spatial
resolution between 1.1 and 2.2 m (depending on the
wavelength).
Regarding the existing type of sensors, three alternatives are
available : ternary semiconductors such as Hg,Cd, Te (also
referred to as MCT, Mercury Cadmium Telluride), where the
spectral sensitivity is determined by the value of x, multilayer
structures or QWIP (Quantum Well Infrared Photodetector)
sensors, where the spectral sensitivity is determined by the
thickness of the successive layers, and microbolometers that
work by heating from absorption of IR radiation. The former
two require cooling (to 77K), the latter can work uncooled.
However, large optics (f/1) are required for microbolometers, so
they cannot be considered further in this project. Typical sensor
pixel sizes are 25 um for SWIR and 50 jun for LWIR. For a
refractive instrument, ZnSe can be used as optical material,
covering the 3-12 um spectral range. ;
From a design point of view, it is important to be able to
exchange different systems (sensors) For that reason, the
aperture of the thermal camera is chosen to be the same as for
the multispectral camera : 0.13 m. Using a 0.44 m focal length,
this will have a 1.13 m resolution in SWIR and 2.25 m in
LWIR. To cover the same swath as the mutispectral camera, the
SWIR sensor will be a line array of 1 600 pixels, and the LWIR
sensor will have an 800 pixels wide line.
3.7 SAR
The Synthetic Aperture Radar adds an all weather, day-and-
night capability to the sensors suite. Aimed at environmental
and security applications (oil spills, flooding, ...), it will operate
at short wavelength (X-band).
A preliminary study has shown that a 2.5 m ground resolution
over a 4.5 km swath is achievable, with a 3 kHz pulse repetition
frequency. Before development on the SAR instrument start, an
evaluation of the available power for the instruments during
night time will be performed.
3.8 Atmospheric measurements
Although not directly related to remote sensing activities, a
small part of the UAV payload will be occupied by instruments
that can provide in sifu measurements of the stratospheric
environment, such as temperature (required for atmospheric
corrections) and the detection of chemical compounds such as
water vapour, ozone, carbon monoxide, carbon dioxide,
methane or nitrogen oxides, many of which are linked to global
warming issues.
4. GROUND SEGMENT
4.1 Mission planning & execution
Mission planning and execution are critical parts of the data
acquisition process. Both will be implemented with significant
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internal intelligence, so that it will not be required to
interactively define flight strips, except when there is a
requirement to do so (e.g. imaging a coastal zone should be
done so that it takes tidal information into account).
Mission execution will be determined by the autonomous flight
control system, taking priority issues and weather
circumstances into account, so that the UAV does not have to
loiter over a cloud-covered area. The most basic requirement
for all instruments, at lowest priority, will be to produce a
complete coverage of the project area.
Although the UAV will fly autonomously, it will always be
possible to take over the aircraft’s control from the ground
reception station.
4.2 Data reception & archiving
On board data storage will be very limited, so all data will be
downlinked to a ground reception station. Using X-band line-
of-sight communication, an area of about 200 km in radius can
be covered by a single UAV. Since this coincides with the
annual coverage capability of the system, the ground reception
station does not have to move to follow the UAV. Also, there is
no need for satellite or UAV uplink.
If one of more UAV should be controlled from one ground
reception station located at a distance higher than 200 km, an
additional layer of data-relay UAV crafts, flying at higher
altitude (e.g. 30 km) could be used.
After an instantaneous inspection of data integrity, the data will
be archived as they are received. If any anomaly is detected, the
mission planning will be adapted to re-image the concerned
area.
4.3 Data processing
To deliver suitable images and information products to the
public, all raw data have to be corrected. This includes applying
the (spectral and geometric) calibration information, and
correcting for atmospheric influences. This will be implemented
as a processing chain, comparable to the methodology of
satellite data processing.
During the demonstration flights in the summer of 2005,
differential GPS correction will be possible by using the Flepos
GPS network of 39 receivers distributed over the Flemish
region in Belgium (Flepos, 2004) and the Walcors GPS network
of 23 receivers in the Walloon region of Belgium (Walcors,
2004). Other countries that offer similar services in Europe
include The Netherlands, Denmark and Switzerland.
Quality control and assurance will be formally documented and
made available to the users. The data shall comply to the
requirements of most mapping agencies and private users.
Standard products will be made available to the public, up to
level 3 (information products), via established internet portals.
These products include : aerial imagery (visual or thermal),
ortho-images, elevation models, but also information derived
from these, such as crop forecasts, mapping products, etc., ...
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