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Remote
Sensing
25.1 INTRODUCTION
.1
Remote sensing in general is the detection and identification of
phenomena at a distance from the object of interest using human capabilities or
special sensors. Modern remote sensing instruments are normally based on
optical, electronic or, sometimes, chemical techniques. During the last decades,
considerable steps forward have been achieved in the development of new sensors
but also in the improvement of existing sensors and their application.
.2
When dealing with oil or chemicals spilled at sea, it is essential to be
able to “find” the slick and to identify the type of substance and to
estimate the volume. Bearing in mind that slicks are often difficult to see due
to thin layers or absence of colours the application of electronic remote
sensing techniques is of great value. Observers in spotter planes will have a
better chance of finding slicks from the altitude they fly at, but even trained
observers need reasonable light conditions. The conditions required are not
always available. All Contracting Parties have access to remote sensing
facilities and have established an aerial surveillance organisation.
.3
Details of aircraft and sensors installed available to respective
Contracting Parties are listed in the Aerial Surveillance Handbook. This chapter
provides a summary of the different types of sensors including a brief
description of the application.
.4
This annex deals with the remote sensing of surface slicks. It should be
pointed out that the use of ship-based equipment such as side-scan-sonar to
detect sunken objects or optics for the observation of sunken pollutants falls
equally under the heading of this chapter and is just as relevant to the scope
of the Bonn Agreement. However, it is not dealt with here.
25.2
SENSORS - GENERAL REQUIREMENTS
.1
To be of use in dealing with (oil) pollution incidents, remote sensing
instruments have to provide the capability to give a clear and unambiguous
indication of the pollution on the sea surface from a reasonable distance under
normal conditions. In addition it is desirable to have means to identify the
type of pollution and the source the pollution originates from as well as a
means of estimating the volume. In this respect it is mentioned here that for
estimations of oil pollution the observers in Bonn Agreement member states also
make use of the Bonn Agreement Pollution Observation Log (BAPOL). The procedure
to quantify a detected slick is described in the Aerial Surveillance Handbook.
.2
For airborne application, the equipment should fit into the selected type
of aircraft being compatible with the aircraft power supplies. It is recommended
that all sensors are integrated into one operating system and signals are
real-time presented on a display as well as recorded on tape or disc, including
data annotation. The recorded data can thus be analysed in a ground processing
station if required.
.3
Sensors fall into broad categories according to their mode of operation.
Active sensors emit a signal, and measure some feature of the interaction of the
signal and the target - usually by analysing the return echo. Radar systems and
Laser Fluorimetry are examples of active sensors used for pollution detection.
Passive sensors do not emit a signal, but rely instead on emissions from the
target - usually the reflection or transmission of ambient electromagnetic
radiation. Ultra violet and Infra red line scanners as well as passive microwave
radiometers are examples of these types of sensors.
.4
In general, active scanners can operate at any time of day and to some
extent can penetrate clouds. Passive sensors will only be functional when there
is sufficient ambient radiation, and this usually means during daytime.
25.3
SIDE LOOKING AIRBORNE RADAR (SLAR)
.1
The SLAR is an active sensor that measures the roughness of the sea
surface. Microwaves in the region of three centimetres are transmitted in pulses
and the reflection from the surface is used to build up a radar picture on both
sides of the aircraft. Capillary waves on the sea surface will give a strong
echo and unusually smooth areas such as those caused by a pollution affecting
the surface tension resulting in a dampening of the capillary waves, will show
up against the surrounding clear water.
.2
SLAR is the most common device in use at present. Under normal
conditions, between wind forces 1 to 7 Beaufort, the system will cover an area
of up to 25 kilometres on one side of the aircraft. When flying undisturbed at
an altitude along a straight track the image will cover a total area of 50
kilometres (both sides of the aircraft) although there will be a gap directly
under the aircraft corresponding to 1.5 times the altitude. Within the area
covered, the presence of even thin layers of surface pollution can be detected.
The spatial resolution of SLAR lies around 20 metres on average, which means
that two objects at the same distance from the antenna should have a separation
of at least 20 metres to be detected as two objects. For oil detection the
polarisation of the system is Vertical and for ice detection often Horizontal
polarisation is used.
.3
The main disadvantage of the SLAR, that counts for all radar systems, is
that it responds to any phenomena that suppresses capillary waves. For example
certain current patterns, ice and surface slicks associated with biological
activity can all produce false targets. Conclusively it is emphasised that
though SLAR is the primary long range detection sensor the only information
obtained is an indication that “something” is floating at the surface
probably requiring further investigation.
25.4
SYNTHETIC APERTURE RADAR (SAR)
.1
With respect to the subject, detection of surface pollution, the SAR is
similar to the SLAR. From a technical point of view there are some important
differences. Where the SLAR uses a fixed antenna length, the SAR system can
define the antenna length by sampling echoes over a period of time. The
mechanical part of the antenna is very small. The advantage of the SAR is its
improved spatial resolution that remains the same over the entire area covered.
For special applications multi-polarised SAR can be delivered. Improved
resolution is strongly related with the cost involved. Resolution down to one
metre is possible, but at relatively high costs.
.2
At this stage of development SAR is used in satellites and in special
projects such as terrain height mapping. Operational use of SAR in aircraft with
the objective of detecting oil is not yet common. As developments continue and
bearing in mind the likelihood of lower costs, it might be worthwhile
considering a SAR, especially in cases where multi tasking is applicable to the
surveillance system.
25.5
ULTRAVIOLET LINE SCANNER OR CAMERA (UV)
Surface
pollution, especially oil, is a good reflector of the ultraviolet component of
sunlight. An ultraviolet scanner or camera is a passive device detecting
reflected ultraviolet with a wavelength of about 0.3 micrometers. The sensor is
mounted vertically in the belly of the aircraft and can build up a continuous
image of an entire slick, even the extremely thin areas, as the aircraft passes
over the slick. It cannot distinguish between types of pollution or different
layer thickness.
25.6
INFRARED LINE SCANNER (IR-LS)
.1
The IR-ls is very similar in operation to the UV-ls and the two are very
often combined in a UV-IR line scanner. The sensor detects infrared radiation
with a wavelength in the band of 8-12 micrometers emitted from the oil. These
layers of oil radiate more slowly than the surrounding clear sea and shows up as
variations in grey levels (or in defined colours). Thicker layers (greater than
about 0.5 millimetres) will absorb sunlight more rapidly than the surrounding
sea and show white on the display.
.2
The InfraRed sensor provides the capability within limits to obtain
information on the relative layer thickness of oil slicks on the water surface.
The sensor does not penetrate the water. It is not as sensitive to oil as the UV
and so comparison of the outputs from the two sensors, especially when presented
real time parallel to each other on the display, will show the thicker parts of
the slick. This information is essential when combating activities are executed,
as the combating vessels should concentrate on these thicker parts. It is
obvious that other temperature-related effects, such as cooling water
discharges, can mislead the IR sensor.
25.7
MICROWAVE RADIOMETER (MWR)
The
passive sensor MWR is rather similar to the UV/IR-ls. It detects microwave
radiation with wavelengths between 0.3 and 3 centimetres. Oil appears always to
be at higher temperatures than seawater in the microwave region and the
temperature depends on the thickness of the oil layer. The relationship is not a
simple one, but by careful selection of operating wavelengths and careful
analysis of the results the system provides the capability of a relatively
accurate account of the volume of oil in the slick. A minimum layer thickness of
0.1 millimetre of oil is required to make proper use of the system. Recognizing
that operational discharges according the MARPOL regulations or even much higher
will not result in layer thickness over 0.1 mm.
25.8
LASER FLUORESENSOR (LFS)
This
is an active sensor emitting an intense beam of coherent light, generated by a
laser, to the sea surface immediately below the aircraft. The receiving
apparatus is designed not to respond to the direct reflection of the beam, but
to detect and to analyse the fluorescence of the pollution resulting from the
laser strike. Currently laser is being operationally tested in
25.9
THERMAL IMAGER
Related
to video cameras, but designed to operate in the infra-red region, imagers will
generally not give such precise description of the surface slick as an IR-ls.
However, they have the advantage of providing a real-time image of the entire
slick, unlike a line scanner that builds the image up line by line as the
aircraft passes overhead.
25.10
LOW-LIGHT LEVEL TELEVISION CAMERA (LLLTV)
The
LLLTV can be filtered to operate in the ultraviolet region and so provide an
ultraviolet analogue to the thermal imager. When used in the visible region,
LLLTV can provide the possibility of imaging ship’s names or other identifying
features in near darkness.
25.11
IDENTIFICATION CAMERA (IC)
Detection
of discharging ships during hours of darkness is possible by the applications
provided by the SLAR or SAR. Identification of the ship is a necessity with
respect to gathering evidence. The identification camera is a sensor consisting
of an Intensified CCD chip in a camera. The combination of the camera and an
infrared flash provides the tool. A snapshot taken can be processed by frame
grabbing software resulting in the ship’s name. Using the sensor is preferable
to pointing a strong light at the ship since this is considered to be an act of
hostility.
25.12
PHOTOGRAPHIC CAMERA (PHOTO)
Conventional
photography provides a valuable, simple and readily understood record of the
scene of an incident or operational discharge. When vertically mounted in the
aircraft the camera contributes to the evidence to an official statement.
Oblique photography in general satisfies the public and the Courts as part of
the evidence rather than the more complex imagery from the other sensors. It is
recommended that cameras are an integrated part of the remote sensing system and
that on the photographs data-annotation is printed.
25.13
VIDEO CAMERA (VC)
Much
the same applies to video recordings as to photography. The advantage of video
is that it provides a more instant record and of course a moving picture. After
landing the crew can immediately present an overview of the situation at sea,
provided required equipment is available.
25.14
FURTHER DEVELOPMENTS AND IMPROVEMENTS
.1
Sensor manufacturers presumably will continue, in some cases at the
request of the user, to develop new sensors or improve the existing ones.
Proposals are expected in near future, in particular on the difficulties
encountered by the operational users concerning the discrimination between
substance discharged and capabilities to estimate volumes.
.2
Worth mentioning is the application of spectral imaging scanners. Remote
sensing for the purpose of the detection of oil slicks, in some countries, is
slowly shifting towards earth observation in the broadest sense. The objective
is to make more efficient use of the available means (aircraft) and also to fill
gaps in the existing sensor package.
.3
In general it is recommended to closely follow the market and study the
new sensors or improvements. Digital photo cameras, improved navigation (dGPS),
airborne AIS and others can be very useful tools for the Bonn Agreement members.
25.15
SYSTEMS
.1
As already stated, sensor operation can be most effective when handled
through one integrated sensor system. A one-man operating system provides the
capability to switch on/off the sensors and to route the data to storage and
presentation. The operator selects all sensors required and, depending on the
data presentation needed to identify the pollution, combines the data from
different sensors. Navigational data obtained from the aircraft system is used
as input into the operating system and superimposed on the sensor data.
.2
Data handling, for presentation and storage, is important so that the raw
data can be processed in a ground processing station after landing. Storage on
retractable hard disc, floppy disc, or tape are possibilities. Images as
presented on the display to the operator can also be stored on video tape for
quick presentation to authorities.
.3
In addition, as a result of data handling in a digitised form it is
possible to transmit the data directly to a ground station. Some systems allow
for the direct transmission of imagery from an aircraft using either fast but
short-range VHF or slower but long-range HF radio. Recognising that when a ship
is caught “red-handed” and is bound for a port in the coastal state the
advantage of a down link system can be that images or photos are directly sent
to the Port State Control authorities.
25.16
PLATFORMS
.1
World wide, most experience with remote sensing has been obtained using
small fixed-wing aircraft. Selecting a type of aircraft for remote sensing
operations depends on a list of aspects based on the objectives to be met once
having the tool: the size and weight of the instruments to be installed, the
area to be covered and the endurance. Selection of the sensor package also
depends on the tasks to be fulfilled. Search-and-Rescue normally requires a
homing device; border patrol may be difficult without a 360 radar. The standard
package for pollution patrol flights consists of SLAR, UV/IR-ls, photo-cameras
and can be extended with a MWR and/or LSF. If operation during darkness is an
option an Identification camera is useful.
.2
A number of different types of aircraft are in use by the Bonn Agreement
Contracting Parties and can be visited during Bonn Agreement exercises; the
aircraft are described in the Aerial Surveillance Handbook.
.3.
Attempts have been made to use special sensors, such as cameras and
thermal imagers, on board vessels. Mounted on the masts sometimes images can be
obtained. However, in general it is found that the platforms are not stable and
even when mounted in high masts still too low for good use.
.4
In the event of an actual combat operation captive balloons, lifted from
a vessels deck, are useful tools. Mounted on a platform hanging under the
balloon, a video camera and preferably an IR-camera provides details on the oil
slick to be combated directly to the master of the vessel. The imagery assists
the master to manoeuvre his ship towards and into the oil slick (thicker parts).
25.17 SATELLITES
.1
The detection of oil and other harmful substance discharges by means of
remote sensing systems in aircraft has been described in previous paragraphs.
Relatively new is remote sensing by means of satellites. The synthetic aperture
radar (SAR) on board the satellites, as installed in the ENVISAT, the ERS-2 and
the Radarsat, proved in various international test programmes to be able to
detect water surface phenomena even as small as 200 m˛, from an altitude of 900
km. The Low Resolution SAR images (100 metre) are considered to be comparable to
SLAR with regard to detectability.
.2
Although the satellite SAR does not discriminate the type of pollution,
it provides an indication of a possible pollution as well as a clear indication
of the location and the dimensions. It is reiterated that the satellite cannot
(yet) identify the pollution nor the possible polluter and in that respect has
the same qualification as the airborne SLAR or SAR. The detected spot has to be
verified. Other disadvantages compared to airborne surveillance are the
inflexibility of the system as a result of fixed orbit and the repeating cycle.
On the other hand, satellite recordings are independent of weather conditions
that are limiting aircraft (like fog or freezing rain). Also the width of the
radar coverage path is an advantage; 100 kilometres in case of the ERS-2, up to
500 kilometres of Radarsat.
.3
Satellite data, if received in near real time (minimum within 1 hours
after the satellite pass), is useful as an early warning system in case of
combatable spills. The use of near real-time satellite data requires a user
community with the capability to verify possible surface pollution (oil slicks)
by an aircraft. The combined use of satellite and aerial surveillance may
provide a cost-effective solution for countries with certain geographical and
climatological conditions.
.4
In order to take advantage of the availability of satellite SAR images it
is recommended to prepare an inventory of the orbits of the satellite and the
area covered. The covered area can then be incorporated into the aircraft
routing. Furthermore the acquisition schedule of the satellite can be used to
adjust the flight program of a remote sensing aircraft or even reduce the number
of flights by having the aircraft on stand-by if the satellite covers the area
of interest. On receipt of the imagery obtained from satellite the aircraft may
be diverted to check possible pollution or, on occasions when no pollution has
been detected the aircraft may focus on areas not covered by satellite.
.5
It is emphasised that satellite SAR can easily provide an overview of
possible floating pollution over relatively large sea areas. An early warning
system requires follow-up by airborne surveillance at least to verify by human
eye the existence of the detected slick. In many studies a general conclusion is
that satellite SAR contributes valuable information but will not replace aerial
surveillance.
.6
To follow the latest developments on satellite surveillance, the EC has
established a European Group of Experts on Satellite Monitoring and Assessment
of Sea-Based Oil Pollution (EGEMP). The
25.18 MAJOR
POLLUTION INCIDENT
.1
When dealing with an oil spillage, the initial function of the remote
sensing aircraft will be to build up a picture of the extent of the pollution,
and to identify the areas of most concern. The aircraft should run across the
affected area using SLAR/SAR at an altitude that provides the best overall image
of the slick(s).
.2
The preliminary investigation can then be supplemented by scanning the
larger or more threatening parts of the slick(s) using close range sensors, such
as infrared, ultraviolet, microwave radiometry and laser. Photographs or video
should be taken whenever possible, including some of the casualty causing the
pollution. Monitoring the spreading and weathering of the slicks should be
continued at regular intervals.
.3.
Another role of the remote sensing aircraft is to direct and guide
recovery vessels or spraying aircraft. This will require extended periods in the
area identifying relatively thicker parts or more threatening patches of oil.
.4
It is particularly important during an incident that the crew of the
reconnaissance aircraft reports to the control centre at regular intervals, both
to relay the current situation and to check for a change in instructions - the
first stages of an incident are always particularly fluid. Regular returns to
base will be necessary to provide the hard-copy imagery for the on-scene and
overall commanders, unless direct down-link facilities are available to transmit
imagery from the aircraft to surface vessels and offices.
25.19
ROUTINE PATROLS
.1
The primary objective in routine patrolling is to detect combatable oil
slicks at an early stage, to encounter ships and platforms in the act of
discharging oil illegally, and to gather sufficient evidence for a prosecution.
Contracting Parties have agreed a co-operative approach to aerial surveillance,
and this is set out in Chapter 4 of this manual.
.2
Prior planning of the pattern of surveillance is important. Baseline
information from earlier surveillance or from ad-hoc observations will indicate
those areas in which most effort should be concentrated. Statistical techniques
can be used to relate surveillance intensity to the probability of intercepting
an illegal discharge - this will indicate the level of effort necessary and
allow conclusions to be drawn about the incidence of MARPOL contravention.
.3
During a mission the crew will maintain the BONN AGREEMENT POLLUTION
OBSERVATION LOG, noting all relevant information on mystery slicks and actual
polluters observed. A separate form will be used for reporting polluting vessels
according IMO regulations.
.4
Possible offenders should be imaged and photographed using the techniques
set out in sections 25.10-25.13. It is important that the photographs and
imagery show that the vessel is the only possible source of the oil. The
vessel's name should be photographed, if possible in a way that identifies it
unambiguously as the offender, and recorded in the log. Communication should be
established to invite the person on the bridge to provide information on last
port of call and destination as well as to explain the discharge observed.
.5
On return to base, if not directly from the air, the evidence from the
offence should be treated as evidence to court and all precautions required by
the law of the land should be applied in securing it and transferring it to the
competent authorities. For each routine mission, the logs should be taken for
interpretation and statistical analysis and the results recorded in a database
for use in periodic reports and future planning.
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