Chemistry Reference
In-Depth Information
(Valenzuela 1978, Wetzel 1990) and as such the detectability of oil spills
and sea slicks remains questionable; the conditions yielding adequate con-
trast levels are not well documented.
In this paper, the capabilities for slick detection at low grazing angles
are examined with the use of a standard marine radar system chosen to rep-
resent the typical performance of this low cost technology. The resolution
and data quality achieved with this kind of instrument are briefly reviewed
and compared with those of traditional satellite and airborne radar systems.
Two examples of the successful detection in the coastal zone of controlled
surfactant slicks with a marine radar system are presented to illustrate the
potential for operational and research applications. Finally, the information
contained in long-term radar monitoring of a specific area is explored.
2 Spatial and temporal resolution
The digital marine radar system used in this study consists of a standard X-
band (9.4 GHz) Racal Decca marine radar attached to an off-the-shelf PC-
based digital capture board. A detailed description of the system and its
operation can be found in Robinson et al. (1999). The radar is fitted with a
horizontally polarised linear radar antenna with a 1 degree horizontal
beamwidth which rotates at a period of 2.4 s. The microwave energy is
emitted as pulses of fixed duration which determine the radar's spatial
resolution in range. The pulse peak power and pulse repetition rate (PRR)
is determined by the pulse length setting, so that on short pulse setting for
example, the radial dimension of the emitted pulse is 12 m with a nominal
peak power of 7.0kW.
On reception, the radar signal is log-amplified and intercepted by the
digital capture board. The capture board includes a facility to integrate a
number of successive returns to produce multi-look backscatter measure-
ments and thus reduce the speckle in the digital images. The number of
“looks” together with the PRR and the antenna rotation speed determine
the images' resolution in azimuth. The integrated multi-look backscatter
signal is then sampled at a rate of 20, 10 or 5 MHz resulting in a pixel
resolution in range respectively equal to 7.5, 15 or 30 meters. The spatial
resolution in the images is generally chosen to coincide with the physical
dimensions of the pulsed antenna footprint. Typical values of the system's
spatial resolution are summarised in Table 1.
The capture board allows pre-determined areas of the marine radar's
view field to be extracted and stored as 8-bit images on the PC. The cover-
age of the digital image is controlled by various capture parameters de-
scribed in detail in Robinson et al. (1999). In the present case, the maxi-
