International Archives of Photogrammetry and Remote Sensing. Vol. XXXII Part 7C2, UNISPACE III, Vienna, 1999
87
I5PH5
UNISPACE III - ISPRS/EARSeL Workshop on
“Remote Sensing for the Detection, Monitoring
and Mitigation of Natural Disasters”
2:30-5:30 pm, 22 July 1999, VIC Room B
Vienna, Austria
ies may be carried out with these sensors and with excellent re
sults as recently demonstrated with the VEGETATION instru
ment on SPOT 4, for instance.
Visual data may be used for monitoring oil spills on the ocean
based on the difference in colour and reflection properties of oil
and water. However, the sensitivity is relatively poor and due to
the light and cloud dependence this application has not been fully
explored. Similarly, it has been demonstrated that infrared data
may be used (from aircraft) for this purpose based on the con
trast between surface temperatures. However, ambiguities stem
ming from either cooling due to evaporation or heating of the oil
by the sun make interpretation difficult. This complementary
techniques is best exploited during night hours.
It is concluded that visual and infrared techniques are useful on
their own for monitoring several natural disasters with forest
fires being a good example of exploiting infrared data, even at 1
000 m resolution obtained with NOAA AVHRR. The daily cov
erage offered by this system is an asset. A special feature of the
visual techniques is the compilation of detailed digital DEM’s
with satellite systems that include beam-swinging like that of the
SPOT satellites.
ACTIVE MICROWAVE SYSTEMS
Active microwave systems are based on the principle of pulsed
radar carried on a moving platform - aircraft or satellite - where
an area is scanned by the movement of the radar beam. Two
systems have proved useful: the real-aperture radar (RAR) where
the spatial resolution in the flight direction (the azimuth direc
tion) is determined by the physical dimensions of the antenna and
the observation geometry (platform altitude and angle of inci
dence), and the synthetic-aperture radar (SAR) where a fine reso
lution is obtained by processing the return signals from targets
recorded as the antenna beam observes them in passing - largely
independent of the geometry. In both cases, the spatial resolution
in the across track direction - the range direction - is obtained by
radiating short pulses w'ith the pulse length determining the reso
lution.
Being microwave systems operating a moderate high frequencies
(short wavelengths) they both have the advantage of being inde
pendent of light and weather conditions. Since several natural
disasters are associated with heavy' clouds the latter is of major
advantage although heavy rain that occur in the radar beam may
reduce the sensitivity of the systems.
Real-aperture radar
A side-looking radar is a pulsed radar that maps the ground in a
swath detennined by the antenna, the wavelength and the alti
tude. Thus, with a ty pical antenna of 2 m by 0.1 m on an aircraft
at 2 000 m a swath of 1 200 m will result when operating at X-
band (3 cm wavelength). In the along-track direction (azimuth)
the spatial resolution is a linear function of the distance to the
target (range) and becomes in our example 42 in at mid-swath
where the angle of incidence is 45°. With a pulse length of 0.1 p,
sec, for example, the resolution at ground in the across-track
direction becomes 21 m.
With the azimuth resolution being a function of the range it is
clear that this relatively simple system is only useful from air
craft and is therefore often referred to as a SLAR (Side-Looking
Airborne Radar). It does not require any complicated signal
processing so that data may be displayed in the aircraft in real
time and eventually transmitted to a ground station for real-time
use. Literature gives many examples of this, especially in connec
tion with monitoring of sea ice for ship navigation purposes and
ice-dynamics studies.
In the context of natural disasters SLAR is used for real-time
detection of icebergs to improve safety of shipping and oil rig
operations, for instance.
Another application is detection of oil spills on the ocean and
monitoring of their drift. I shall discuss this application in a later
section but it is worth mentioning that a Norwegian surveillance
system uses SLAR to confirm oil spills that liave been detected
by means of satellite radar in the Norwegian, the North Sea and
Skagerak. For this purpose the spatial resolution offered is quite
suitable.
A SLAR is a low'-cost system in comparison with a SAR system,
and potential users should seriously consider a SLAR as a solu
tion. The inherent spatial resolution may be acceptable in great
many cases.
A SLAR does require an aircraft and the operation of an aircraft
is not weather independent even as of today, as already men
tioned. Also, the operation requires alternative airport or landing
strips which may present a problem in remote areas.
Aircraft operation for regular monitoring is costly and when the
costs are compared with that of purchasing equivalent satellite
data, the satellite becomes an attractive alternative. However,
presently the user does not contribute fund to the implementation
and operation of the satellite system. In future commercial sys
tems these fund are embedded in the price for data, but still it
might be attractive since these implementation and operation
costs may be shared by many users.
Synthetic Aperture Radar
A Synthetic Aperture Radar (SAR) is a side-looking pulsed radar
system installed on a moving platform - aircraft or satellite. In
the along-track direction the resolution is effected by forming a
