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Airborne surveillance: The role of remote sensing and visual observation
Oil & Chemical Pollution 3 (1986/87) 18.1-189
Airborne Surveillance: The Role of Remote Sensing and Visual Observation R. C. Schriel Ministry of Transport and Public Works, Rijkswaterstaat, North Sea Directorate, PO Box 5807, 2280 HV, Rijswijk, The Netherlands
ABSTRACT This paper identifies the multiple uses of airborne surveillance in maritime activities and discusses in detail the uses of visual observation in the marine pollution field. This leads to consideration of the advantages of remote sensing;to a brief review of current types of equipment and to a discussion on choice of airborne platforms for the installation of such equipment. Finally operational aspects of surveillance are considered and emphasis is placed on the requirement for documentation of evidence in operational ship discharge incidents.
1 INTRODUCTION As long as the sea is polluted by oil and other substances by, for example, ships, it will be necessary to extend the prevention regulations and to observe, monitor and in urgent cases combat spillages. Aerial reconnaissance is essential for an effective response to pollutants spilled at sea, both to facilitate location of the pollution and to improve the control of clean-up operations. Further aims of aerial observation of pollution at sea are: to monitor the effectiveness of preventive regulations, to investigate the frequency of violations, and to gather evidence against offenders in specific cases. Pollution can be located visually during the daytime u n d e r conditions of good visibility. However at night and in poor weather conditions remote sensing equipment is required. In addition, interpretation of pollutant appearance in terms of a m o u n t and type requires the use of remote sensing equipment. 181 Oil & Chemical Pollution 0269-8579/87/$03.50 © ElsevierApplied Science Publishers Ltd, England, 1987. Printed in Ireland.
182
IL C. Schriel
Aerial surveillance can be useful in a variety of applications in addition to the pollution applications already identified. Some potential areas of application are as follows:
(i) (ii) (iii) (iv) (v) (vi) (vii) (viii) (ix)
Illegal and permitted dumping of chemicals. Incineration of chemicals. Surveillance of sea traffic. Fishery protection. Monitoring of offshore installations. Search and rescue. Ship traffic control. Sea ice mapping. Prevention of smuggling activities.
1.1 General environmental investigations Although the above activities differ from each other, most can be combined in one aerial surveillance flight. Careful consideration of the different activities, definition of search areas and routes, and choice of observation systems for particular purposes can reduce flying time. International agreements covering the environmental protection of the sea and related matters include: the Bonn Agreement, the London and Paris Conventions, the International Convention for the Prevention of Pollution from Ships (1973), the International Maritime Ship Traffic Separation Scheme Regulations, and Fishery Protection agreements. Subject to the agreement as to the area to be surveyed and the flight frequency, the use of remote sensing, with its potential for producing uniform imagery for interpretation against commonly agreed criteria, makes it possible for individual countries to co-operate through international agreements. This could reduce the total operating cost.
2 AERIAL SURVEILLANCE BY VISUAL OBSERVATION Visual observation is only possible during day time, between sunrise and sunset, and under favourable weather conditions. An accurate assessment of the quantity of any oil observed visually at sea is virtually impossible due to the difficulty of assessing the thickness and the area covered by the floating oil. However, an order-of-magnitude estimate can be obtained by considering the following factors. The spread of spilt oil by gravity is quite rapid and most liquid oils will soon reach an equilibrium thickness of about O-1 mm characterised by a black or dark brown appearance. For thinner layers the colouration roughly indicates thickness, as shown in
Airborne surveillance: Remote sensing and visual observation
183
Table 1. Having estimated thickness in this way it remains only to estimate the surface areas of the various thicknesses of oil pollution observed to provide a rough estimate of the a m o u n t of pollutant. TABLE 1 Appearance of Oil Film on Water
Appearance of film
Code Film thickness Quantity (~m) (l km -2)
Barely visible under most favourable light conditions
a
0"05
50
Visible as a silvery sheen on the water surface First traces of colour observable Bright bands of colour Colours begin to turn dull Much darker colours
b c d e f
ff 10 0"15 0-30 1"00 2"00
100 150 300 1000 2000
In order to avoid distorted views it is necessary to look perpendicularly on the oil w h e n assessing its thickness from its colour. Photographs will sometimes permit more accurate assessments of quantities to be m a d e particularly if some linear scale reference can be included. Thinking in terms of the M A R P O L regulations concerning the limits for the discharge of oil or oily mixtures from vessels and oil rigs it is feasible in day time to observe the slick colour and widths which provide a basis on which to judge whether a discharge is a violation or not. The relevant M A R P O L regulations are as follows. Machinery space bilge water can be discharged within 12 miles of the shore provided the oil concentration does not exceed 15 ppm; outside 12 miles from land such discharges can be made provided the concentration does not exceed 100 ppm. Tanker ballast water can only be discharged more than 50 miles from land and the oil discharge rate must not exceed 60 litres per nautical mile. Using the appearance/thickness relationship of Table 1 a n d the above discharge limits, m a x i m u m permitted slick widths can be calculated, see Table 2. During daylight the quantity of oil which is permitted to be discharged can be determined from Table 2 in terms of observed slick widths. The quantity/pathwidth relationship mentioned in Table 2 underestimates the quantity actually discharged. Due to weathering (evaporation, natural dispersion etc.) part of the d i s c h a r g e d oil disappears whilst the colour relationship mentioned above, gives actual quantities at the m o m e n t of observation.
184
R. C Schriel
TABLE 2 Path Width Evidence for Different Discharges Location
More than 50 miles from coast Between 12 and 50 miles from coast Less than 12 miles from coast
Maximum pathwidth (m) colour appearance
Type of discharge
Ballast water Bilge water All discharges from vessels All discharges from vessels
a
b
c
650 4 4
375 2 2
160 1 1
0-6
0"3
0"16
d
e
f
80 0"5 0"5
30 0"2 0"2
15 0-1 0"1
0"10 0"04 0'02
Thus Tables 1 a n d 2 are suggested as a basis for providing evidence of violation. For example if a ship discharges a slick of colour appearance listed in Table 1 and the width is estimated, the total quantity of oil involved can be deduced and related through assumed oil/water separator discharge capacity to a concentration ofoil in the discharge in parts per million. In this way, a relationship between slick width and concentration limits in parts per million can be determined leading to the possibility of conviction on the basis of observed slick widths if the oil within the observed layer is just observable, if it is silvery sheen, or, with even greater assurance of violation, if the oil is clearly visible, say brown or black. Again if the slick width exceeds the predicted values, further assurance is obtained. A similar approach can be adopted to the tanker regulations which are expressed in terms of litres per mile rather than parts per million. This approach could be extended to night operations based on the width of slicks observed by remote sensing equipment. The observed path width provides a realistic basig for the j u d g e m e n t of whether a discharge is a violation or not. General acceptance of this type of evidence of violation is needed for an o p t i m u m control of regulations for the discharge of oil at sea. For practical reasons the following is suggested as a basis for establishing a violation. Within the territorial waters any observable oil spill is a violation. Outside territorial waters a slick width of more than one metre a n d the appearance of first traces of colour is a violation. Whilst in the area more than 50 miles from the nearest land, for tanker ballast discharges a slick width of 160 metres is a violation. To be more accurate there is a need for knowledge of the total volume of water discharged from platforms and vessels in order to calculate more precisely the total a m o u n t of oil going into the water a n d so define more accurately the limiting slick widths.
Airborne surveillance." Remote sensing and visual observation
185
3 AERIAL SURVEILLANCE BY REMOTE SENSING For detection of spills by remote sensing equipment the following sensors have been considered: (i) (ii) (iii) (iv) (v)
Side-looking airborne radar (SLAR). Infra-red/ultraviolet line scan (IR/UVLS). Microwave radiometry (MWR). Laser fluorosensor (LF). Day and night ship identification.
3.1 SLAR
The SLAR has unique day/night all weather application and provides a long range capability covering large areas and detecting small targets. A suitable SLAR has to be designed exclusively for maritime surveillance. For oil slick detection it is preferable to install a SLAR system with vertical antenna polarisation. Its main applications are:
(i)
Surveillance of sea traffic. (ii) Oil spill detection. (iii) Fishery protection. (iv) Search and rescue. (v) Sea ice mapping. (vi) Smuggling activities. (vii) Combatting assistance. 3.2 Infra-red/ultraviolet line scanner
For detailed investigation, the IR/UVLS system can be applied. This gives information at close range to obtain high resolution imagery on the spreading of the oil on the sea surface and provides an indication of the oil thickness. The relative thickness indication can be obtained from a comparison of the differences between IR and UV imagery. With information from the IR sensor which detects the thicker layers, clean up operations can be directed for maximum efficiency. Other applications are: inspection of suspected illegal discharges ofoil and chemicals, assessment of quantity of pollution of ship wakes and other environmental investigations. 3.3 Microwave radiometer
This sensor is currently under development and is being considered as a means of providing a thickness profile along a fight track, which
186
R. C Schriel
combined with the relative thickness information of the IR/UVLS might be used for volume calculations. So far, however, layer thickness measurements by this means have not altogether proved as satisfactory as first thought possible.
3.4 Laser fluorosensor This sensor is also under development and is intended to provide a coarse type classification of the oil. The identification will have to be verified by in situ measurements carded out by survey vessels equipped with thickness measuring and sampling facilities.
3.5 Identification systems For ship identification, during day time and under good visibility conditions, standard photographic techniques and television systems are available. It is a deficiency that no all-weather identification system is available to identify sources of pollution in terms of ships' names, nationality etc. Systems that may be helpful during night time and good visibility are: low light level camera, thermal imaging systems, searchlights and flare signals.
3.6 Summary Currently available operational remote sensing systems are: (i)
A side looking airborne radar for long distance detection and observation under nearly all weather conditions. (ii) An infrared line scanner and ultraviolet light scanner for short distance detection investigation under good weather conditions. (iii) Ship identification systems by photographic techniques and television systems, only during daytime and good weather conditions. These remote sensing systems do have their shortcomings but they can be used optimally to obtain useful results. Future developments of remote sensing systems on pollution of the sea should be directed towards enhancement of the capability to detect oil spills at long distance/range; the identification of the source of the pollution; calculation of the amount of pollution through thickness measurements in all ranges and the identification of ships' names. The technical requirements of these remote sensing systems are; light weight/low volume; swath width of each sensor close to 45 deg. and a high
Airborne surveillance." Remote sensing and visual observation
187
resolution. The system should have all weather capability (including light rain) and layer thickness measurements should be unambiguous. The integration of sensor displays is essential. A workable system for the identification of ships during poor weather and at night is essential.
4 AIRBORNE PLATFORMS The aircraft chosen for aerial observation should feature a good all round visbility and carry suitable navigation and communication aids. Over near shore waters, and assisting in combatting activities the flexibility of helicopters and the use of airships may provide an advantage, for instance when surveying an intricate coastline with cliffs, coves and islands. The endurance of helicopters is between 2 and 4 hours whereas airships can operate for about 20--40 hours. On the other hand, over the open sea, the requirement for changes in flying speed and altitude as well as the ability to hover on the spot for a certain period are less essential. Instead speed and pay-load range are more important and these requirements are more readily met by fixed-wing aircraft. Airships can, however, because of their extended endurance be situated above an accident area. In general, the requirement to carry remote sensing equipment for extensive surveys over remote sea areas calls for a twin or multi-engined aircraft to provide the extra margin of safety together with a pay-load range performance for at least 5-6 survey flight hours. Specifications for aerial observation craft are available at the Rijkswaterstaat North Sea Directorate in Holland.
5 OPERATIONAL ASPECTS
5.1 Planning To execute aerial observation of oil at sea a flight route plan should be prepared in advance taking into consideration ship traffic routes, offshore activities, pipe-line lay-out and areas of specific interest. If no information about ship-traffic is available an investigation may have to be carded out first. In order to follow the flight route plan it will be necessary to carry suitable navigational aids. It is preferable to install a ship-type navigation system of high accuracy. The flight route should be drawn on a chart of an appropriate scale, taking into account any available information which may reduce the search area as much as possible. It is advisable to lay down a grid on the
188
R. C Schriel
chart with a distance between the grid lines of about 10 km for surveillance with visual observation and about 40 km for surveillance with remote sensing. It is convenient to operate at a flying altitude of 1000 ft (300 m) since in good weather conditions this provides visibility o v e r 8 km. Using this chart, the aircraft flies on a base line between the gridlines investigating each observed oil spill regardless of size and recording them in terms of position, dimensions, colour, and relevant information on shipping. Such information can be used for statistical evaluation after accumulation of sufficient data over a certain period. Oil slicks of sufficient size to be combattable are reported directly and the aircraft assumes a control/assistance role in the subsequent clean-up operations. After drawing a surveillance grid a decision has to be made on the frequency of observation flights within the established flight plans. The frequency of surveillance flights depends on the importance of the area to be surveyed in terms of ship traffic density and offshore activities. Other factors are, the total dimensions of the area to be surveyed, the surveillance method, and aircraft speed and endurance. With a typical fixed-wing aircraft with a speed of 160 km h -1 it is possible to survey 2400 sq km h -1 by visual observation and 15 000-30 000 sq km h -1 by remote sensing; the 15 000 sq km being for oil and 30 000 sq km for fishing vessels (the latter provide a larger target). One approach is to fly over each selected area every day of the week 2--4 times (pm and am by visual observation and night-pm-am-evening by remote sensing) over a year. This frequency provides a great deal of information for statistical survey purposes. 5.2 Identification and documentation
For identification of oil slicks and for evidence of violations of oil spill discharges by ships, cameras are indispensable. It is recommended that cameras should be carried on all missions including exercises as well as actual surveillance flights. Cameras are useful for recording the appearance of oil slicks as noted above but in the requirements for the provision of evidence relating to illegal ship discharges and culprit identification they are indispensable. In the latter circumstances when the target is located the photographic run illustrated in Fig. 1 provides an opportunity to obtain the required shots. These photographic shots are taken while the aircraft flies at an altitude of 300-500 ft above the sea, commencing when crossing the sailing course of the vessel at right angles and at a distance of about 3 to 5 ship-lengths in front of the vessel. Then a run parallel to the sailing course is made at 300 ft altitude above the sea.
Airborne surveillance." Remote sensing and visual observation
189
DETECTION PHOTO OF SHIP'S WAKE ~ : : : : ' ; ' L E CLIMB 500 PT
IR/UV FILE DUMP
SHIP'SWAKE
~
DUMP
DESCEND TO 500 FT IR/UV POLAROID
PHOTO OF SHIP'S WAKE
PHOTO N A M E ~ PHOTO NAMI ~PHOTO
) PHOTO DETAIL''~1~"
DESCEND TO 200 FT
~ ' ~
OILSLICK
Fig. 1. Procedures used for investigating oil discharges. At the e n d of this r u n the aircraft turns a n d flies astern of the vessel at right angles to its course at a distance of about 1.5 to 5 ship-lengths. After a turn a second parallel r u n is made. Preparations are subsequently m a d e for an overhead imaging r u n by short range remote sensing. At night time a n d in p o o r weather conditions, an alternative to the standard p h o t o g r a p h i c camera system m a y be to use a thermal observation system. Data collected with the remote sensing sensors have to be presented in real-time o n - b o a r d the aircraft for verification purposes a n d for documentation. All relevant data have to be stored o n a data cartridge for e x a m i n a t i o n after the surveillance flight. A n alternative is to provide a data link between the shore stations or seaborne e q u i p m e n t a n d the aircraft. It is advisable to use a data registration form to register all relevant observed i n f o r m a t i o n during the surveillance flight a n d to store t h e m in a c o m p u t e r for evaluation. Such a form w o u l d ideally cover location, time, dimensions, oil type, source of pollution, colour, detection systems in which the pollution was positively recorded, a n d other relevant remarks.
Airborne Surveillance: The Role of Remote Sensing and Visual Observation R. C. Schriel Ministry of Transport and Public Works, Rijkswaterstaat, North Sea Directorate, PO Box 5807, 2280 HV, Rijswijk, The Netherlands
ABSTRACT This paper identifies the multiple uses of airborne surveillance in maritime activities and discusses in detail the uses of visual observation in the marine pollution field. This leads to consideration of the advantages of remote sensing;to a brief review of current types of equipment and to a discussion on choice of airborne platforms for the installation of such equipment. Finally operational aspects of surveillance are considered and emphasis is placed on the requirement for documentation of evidence in operational ship discharge incidents.
1 INTRODUCTION As long as the sea is polluted by oil and other substances by, for example, ships, it will be necessary to extend the prevention regulations and to observe, monitor and in urgent cases combat spillages. Aerial reconnaissance is essential for an effective response to pollutants spilled at sea, both to facilitate location of the pollution and to improve the control of clean-up operations. Further aims of aerial observation of pollution at sea are: to monitor the effectiveness of preventive regulations, to investigate the frequency of violations, and to gather evidence against offenders in specific cases. Pollution can be located visually during the daytime u n d e r conditions of good visibility. However at night and in poor weather conditions remote sensing equipment is required. In addition, interpretation of pollutant appearance in terms of a m o u n t and type requires the use of remote sensing equipment. 181 Oil & Chemical Pollution 0269-8579/87/$03.50 © ElsevierApplied Science Publishers Ltd, England, 1987. Printed in Ireland.
182
IL C. Schriel
Aerial surveillance can be useful in a variety of applications in addition to the pollution applications already identified. Some potential areas of application are as follows:
(i) (ii) (iii) (iv) (v) (vi) (vii) (viii) (ix)
Illegal and permitted dumping of chemicals. Incineration of chemicals. Surveillance of sea traffic. Fishery protection. Monitoring of offshore installations. Search and rescue. Ship traffic control. Sea ice mapping. Prevention of smuggling activities.
1.1 General environmental investigations Although the above activities differ from each other, most can be combined in one aerial surveillance flight. Careful consideration of the different activities, definition of search areas and routes, and choice of observation systems for particular purposes can reduce flying time. International agreements covering the environmental protection of the sea and related matters include: the Bonn Agreement, the London and Paris Conventions, the International Convention for the Prevention of Pollution from Ships (1973), the International Maritime Ship Traffic Separation Scheme Regulations, and Fishery Protection agreements. Subject to the agreement as to the area to be surveyed and the flight frequency, the use of remote sensing, with its potential for producing uniform imagery for interpretation against commonly agreed criteria, makes it possible for individual countries to co-operate through international agreements. This could reduce the total operating cost.
2 AERIAL SURVEILLANCE BY VISUAL OBSERVATION Visual observation is only possible during day time, between sunrise and sunset, and under favourable weather conditions. An accurate assessment of the quantity of any oil observed visually at sea is virtually impossible due to the difficulty of assessing the thickness and the area covered by the floating oil. However, an order-of-magnitude estimate can be obtained by considering the following factors. The spread of spilt oil by gravity is quite rapid and most liquid oils will soon reach an equilibrium thickness of about O-1 mm characterised by a black or dark brown appearance. For thinner layers the colouration roughly indicates thickness, as shown in
Airborne surveillance: Remote sensing and visual observation
183
Table 1. Having estimated thickness in this way it remains only to estimate the surface areas of the various thicknesses of oil pollution observed to provide a rough estimate of the a m o u n t of pollutant. TABLE 1 Appearance of Oil Film on Water
Appearance of film
Code Film thickness Quantity (~m) (l km -2)
Barely visible under most favourable light conditions
a
0"05
50
Visible as a silvery sheen on the water surface First traces of colour observable Bright bands of colour Colours begin to turn dull Much darker colours
b c d e f
ff 10 0"15 0-30 1"00 2"00
100 150 300 1000 2000
In order to avoid distorted views it is necessary to look perpendicularly on the oil w h e n assessing its thickness from its colour. Photographs will sometimes permit more accurate assessments of quantities to be m a d e particularly if some linear scale reference can be included. Thinking in terms of the M A R P O L regulations concerning the limits for the discharge of oil or oily mixtures from vessels and oil rigs it is feasible in day time to observe the slick colour and widths which provide a basis on which to judge whether a discharge is a violation or not. The relevant M A R P O L regulations are as follows. Machinery space bilge water can be discharged within 12 miles of the shore provided the oil concentration does not exceed 15 ppm; outside 12 miles from land such discharges can be made provided the concentration does not exceed 100 ppm. Tanker ballast water can only be discharged more than 50 miles from land and the oil discharge rate must not exceed 60 litres per nautical mile. Using the appearance/thickness relationship of Table 1 a n d the above discharge limits, m a x i m u m permitted slick widths can be calculated, see Table 2. During daylight the quantity of oil which is permitted to be discharged can be determined from Table 2 in terms of observed slick widths. The quantity/pathwidth relationship mentioned in Table 2 underestimates the quantity actually discharged. Due to weathering (evaporation, natural dispersion etc.) part of the d i s c h a r g e d oil disappears whilst the colour relationship mentioned above, gives actual quantities at the m o m e n t of observation.
184
R. C Schriel
TABLE 2 Path Width Evidence for Different Discharges Location
More than 50 miles from coast Between 12 and 50 miles from coast Less than 12 miles from coast
Maximum pathwidth (m) colour appearance
Type of discharge
Ballast water Bilge water All discharges from vessels All discharges from vessels
a
b
c
650 4 4
375 2 2
160 1 1
0-6
0"3
0"16
d
e
f
80 0"5 0"5
30 0"2 0"2
15 0-1 0"1
0"10 0"04 0'02
Thus Tables 1 a n d 2 are suggested as a basis for providing evidence of violation. For example if a ship discharges a slick of colour appearance listed in Table 1 and the width is estimated, the total quantity of oil involved can be deduced and related through assumed oil/water separator discharge capacity to a concentration ofoil in the discharge in parts per million. In this way, a relationship between slick width and concentration limits in parts per million can be determined leading to the possibility of conviction on the basis of observed slick widths if the oil within the observed layer is just observable, if it is silvery sheen, or, with even greater assurance of violation, if the oil is clearly visible, say brown or black. Again if the slick width exceeds the predicted values, further assurance is obtained. A similar approach can be adopted to the tanker regulations which are expressed in terms of litres per mile rather than parts per million. This approach could be extended to night operations based on the width of slicks observed by remote sensing equipment. The observed path width provides a realistic basig for the j u d g e m e n t of whether a discharge is a violation or not. General acceptance of this type of evidence of violation is needed for an o p t i m u m control of regulations for the discharge of oil at sea. For practical reasons the following is suggested as a basis for establishing a violation. Within the territorial waters any observable oil spill is a violation. Outside territorial waters a slick width of more than one metre a n d the appearance of first traces of colour is a violation. Whilst in the area more than 50 miles from the nearest land, for tanker ballast discharges a slick width of 160 metres is a violation. To be more accurate there is a need for knowledge of the total volume of water discharged from platforms and vessels in order to calculate more precisely the total a m o u n t of oil going into the water a n d so define more accurately the limiting slick widths.
Airborne surveillance." Remote sensing and visual observation
185
3 AERIAL SURVEILLANCE BY REMOTE SENSING For detection of spills by remote sensing equipment the following sensors have been considered: (i) (ii) (iii) (iv) (v)
Side-looking airborne radar (SLAR). Infra-red/ultraviolet line scan (IR/UVLS). Microwave radiometry (MWR). Laser fluorosensor (LF). Day and night ship identification.
3.1 SLAR
The SLAR has unique day/night all weather application and provides a long range capability covering large areas and detecting small targets. A suitable SLAR has to be designed exclusively for maritime surveillance. For oil slick detection it is preferable to install a SLAR system with vertical antenna polarisation. Its main applications are:
(i)
Surveillance of sea traffic. (ii) Oil spill detection. (iii) Fishery protection. (iv) Search and rescue. (v) Sea ice mapping. (vi) Smuggling activities. (vii) Combatting assistance. 3.2 Infra-red/ultraviolet line scanner
For detailed investigation, the IR/UVLS system can be applied. This gives information at close range to obtain high resolution imagery on the spreading of the oil on the sea surface and provides an indication of the oil thickness. The relative thickness indication can be obtained from a comparison of the differences between IR and UV imagery. With information from the IR sensor which detects the thicker layers, clean up operations can be directed for maximum efficiency. Other applications are: inspection of suspected illegal discharges ofoil and chemicals, assessment of quantity of pollution of ship wakes and other environmental investigations. 3.3 Microwave radiometer
This sensor is currently under development and is being considered as a means of providing a thickness profile along a fight track, which
186
R. C Schriel
combined with the relative thickness information of the IR/UVLS might be used for volume calculations. So far, however, layer thickness measurements by this means have not altogether proved as satisfactory as first thought possible.
3.4 Laser fluorosensor This sensor is also under development and is intended to provide a coarse type classification of the oil. The identification will have to be verified by in situ measurements carded out by survey vessels equipped with thickness measuring and sampling facilities.
3.5 Identification systems For ship identification, during day time and under good visibility conditions, standard photographic techniques and television systems are available. It is a deficiency that no all-weather identification system is available to identify sources of pollution in terms of ships' names, nationality etc. Systems that may be helpful during night time and good visibility are: low light level camera, thermal imaging systems, searchlights and flare signals.
3.6 Summary Currently available operational remote sensing systems are: (i)
A side looking airborne radar for long distance detection and observation under nearly all weather conditions. (ii) An infrared line scanner and ultraviolet light scanner for short distance detection investigation under good weather conditions. (iii) Ship identification systems by photographic techniques and television systems, only during daytime and good weather conditions. These remote sensing systems do have their shortcomings but they can be used optimally to obtain useful results. Future developments of remote sensing systems on pollution of the sea should be directed towards enhancement of the capability to detect oil spills at long distance/range; the identification of the source of the pollution; calculation of the amount of pollution through thickness measurements in all ranges and the identification of ships' names. The technical requirements of these remote sensing systems are; light weight/low volume; swath width of each sensor close to 45 deg. and a high
Airborne surveillance." Remote sensing and visual observation
187
resolution. The system should have all weather capability (including light rain) and layer thickness measurements should be unambiguous. The integration of sensor displays is essential. A workable system for the identification of ships during poor weather and at night is essential.
4 AIRBORNE PLATFORMS The aircraft chosen for aerial observation should feature a good all round visbility and carry suitable navigation and communication aids. Over near shore waters, and assisting in combatting activities the flexibility of helicopters and the use of airships may provide an advantage, for instance when surveying an intricate coastline with cliffs, coves and islands. The endurance of helicopters is between 2 and 4 hours whereas airships can operate for about 20--40 hours. On the other hand, over the open sea, the requirement for changes in flying speed and altitude as well as the ability to hover on the spot for a certain period are less essential. Instead speed and pay-load range are more important and these requirements are more readily met by fixed-wing aircraft. Airships can, however, because of their extended endurance be situated above an accident area. In general, the requirement to carry remote sensing equipment for extensive surveys over remote sea areas calls for a twin or multi-engined aircraft to provide the extra margin of safety together with a pay-load range performance for at least 5-6 survey flight hours. Specifications for aerial observation craft are available at the Rijkswaterstaat North Sea Directorate in Holland.
5 OPERATIONAL ASPECTS
5.1 Planning To execute aerial observation of oil at sea a flight route plan should be prepared in advance taking into consideration ship traffic routes, offshore activities, pipe-line lay-out and areas of specific interest. If no information about ship-traffic is available an investigation may have to be carded out first. In order to follow the flight route plan it will be necessary to carry suitable navigational aids. It is preferable to install a ship-type navigation system of high accuracy. The flight route should be drawn on a chart of an appropriate scale, taking into account any available information which may reduce the search area as much as possible. It is advisable to lay down a grid on the
188
R. C Schriel
chart with a distance between the grid lines of about 10 km for surveillance with visual observation and about 40 km for surveillance with remote sensing. It is convenient to operate at a flying altitude of 1000 ft (300 m) since in good weather conditions this provides visibility o v e r 8 km. Using this chart, the aircraft flies on a base line between the gridlines investigating each observed oil spill regardless of size and recording them in terms of position, dimensions, colour, and relevant information on shipping. Such information can be used for statistical evaluation after accumulation of sufficient data over a certain period. Oil slicks of sufficient size to be combattable are reported directly and the aircraft assumes a control/assistance role in the subsequent clean-up operations. After drawing a surveillance grid a decision has to be made on the frequency of observation flights within the established flight plans. The frequency of surveillance flights depends on the importance of the area to be surveyed in terms of ship traffic density and offshore activities. Other factors are, the total dimensions of the area to be surveyed, the surveillance method, and aircraft speed and endurance. With a typical fixed-wing aircraft with a speed of 160 km h -1 it is possible to survey 2400 sq km h -1 by visual observation and 15 000-30 000 sq km h -1 by remote sensing; the 15 000 sq km being for oil and 30 000 sq km for fishing vessels (the latter provide a larger target). One approach is to fly over each selected area every day of the week 2--4 times (pm and am by visual observation and night-pm-am-evening by remote sensing) over a year. This frequency provides a great deal of information for statistical survey purposes. 5.2 Identification and documentation
For identification of oil slicks and for evidence of violations of oil spill discharges by ships, cameras are indispensable. It is recommended that cameras should be carried on all missions including exercises as well as actual surveillance flights. Cameras are useful for recording the appearance of oil slicks as noted above but in the requirements for the provision of evidence relating to illegal ship discharges and culprit identification they are indispensable. In the latter circumstances when the target is located the photographic run illustrated in Fig. 1 provides an opportunity to obtain the required shots. These photographic shots are taken while the aircraft flies at an altitude of 300-500 ft above the sea, commencing when crossing the sailing course of the vessel at right angles and at a distance of about 3 to 5 ship-lengths in front of the vessel. Then a run parallel to the sailing course is made at 300 ft altitude above the sea.
Airborne surveillance." Remote sensing and visual observation
189
DETECTION PHOTO OF SHIP'S WAKE ~ : : : : ' ; ' L E CLIMB 500 PT
IR/UV FILE DUMP
SHIP'SWAKE
~
DUMP
DESCEND TO 500 FT IR/UV POLAROID
PHOTO OF SHIP'S WAKE
PHOTO N A M E ~ PHOTO NAMI ~PHOTO
) PHOTO DETAIL''~1~"
DESCEND TO 200 FT
~ ' ~
OILSLICK
Fig. 1. Procedures used for investigating oil discharges. At the e n d of this r u n the aircraft turns a n d flies astern of the vessel at right angles to its course at a distance of about 1.5 to 5 ship-lengths. After a turn a second parallel r u n is made. Preparations are subsequently m a d e for an overhead imaging r u n by short range remote sensing. At night time a n d in p o o r weather conditions, an alternative to the standard p h o t o g r a p h i c camera system m a y be to use a thermal observation system. Data collected with the remote sensing sensors have to be presented in real-time o n - b o a r d the aircraft for verification purposes a n d for documentation. All relevant data have to be stored o n a data cartridge for e x a m i n a t i o n after the surveillance flight. A n alternative is to provide a data link between the shore stations or seaborne e q u i p m e n t a n d the aircraft. It is advisable to use a data registration form to register all relevant observed i n f o r m a t i o n during the surveillance flight a n d to store t h e m in a c o m p u t e r for evaluation. Such a form w o u l d ideally cover location, time, dimensions, oil type, source of pollution, colour, detection systems in which the pollution was positively recorded, a n d other relevant remarks.