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Radar satellites: A new tool forpollution monitoring in coastal watersTerje Wahl a , Åge Skøelv a , Jan P. Pedersen b , Lill‐Gøril Seljelvb , Jørn Harald Andersen c , Odd Arne Follum c , Tom Anderssen d

, Guro Dahle Strøm e , Tom‐Ivar Bern f , Heidi Hovland Espedal g ,

Hilde Hamnes h & Rune Solberg ia Norwegian Defence Research Establishment (NDRE) , P.O. Box25, Kjeller, N‐2007, Norwayb Tromsø Satellite Station (TSS) , Tromsø, Norwayc Norwegian Pollution Control Authority (SFT) , Horten, Norwayd Informasjonskontroll a.s. , Skjetten, Norwaye Norwegian Space Centre (NSC) , Oslo, Norwayf OCEANOR a.s. , Trondheim, Norwayg Nansen Environmental and Remote Sensing Centre (NERSC) ,Bergen, Norwayh Norwegian Institute of Fisheries and Aquaculture Ltd. , Tromsø,Norwayi Norwegian Computing Centre (NR) , Oslo, NorwayPublished online: 30 Sep 2008.

To cite this article: Terje Wahl , Åge Skøelv , Jan P. Pedersen , Lill‐Gøril Seljelv , Jørn HaraldAndersen , Odd Arne Follum , Tom Anderssen , Guro Dahle Strøm , Tom‐Ivar Bern , HeidiHovland Espedal , Hilde Hamnes & Rune Solberg (1996) Radar satellites: A new tool for pollutionmonitoring in coastal waters, Coastal Management, 24:1, 61-71, DOI: 10.1080/08920759609362281

To link to this article: http://dx.doi.org/10.1080/08920759609362281

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Radar Satellites: A New Tool for PollutionMonitoring in Coastal Waters

TERJE WAHLÅGE SKØELV

Norwegian Defence ResearchEstablishment (NDRE)

Kjeller, Norway

JAN P. PEDERSENLILL-GØRIL SELJELV

Tromsø Satellite Station (TSS)Tromsø, Norway

JØRN HARALD ANDERSENODD ARNE FOLLUM

Norwegian Pollution Control Authority(SFT)

Horten, Norway

TOM ANDERSSEN

Informasjonskontroll a.s.Skjetten, Norway

GURO DAHLE STRØMNorwegian Space Centre (NSC)Oslo, Norway

TOM-IVAR BERN

OCEANOR a.s.Trondheim, Norway

HEIDI HOVLAND ESPEDALNansen Environmental and Remote

Sensing Centre (NERSC)Bergen, Norway

HILDE HAMNES

Norwegian Institute of Fisheriesand Aquaculture Ltd.

Tromsø, Norway

RUNE SOLBERG

Norwegian Computing Centre (NR)Oslo, Norway

Received 20 April 1995; accepted 20 September 1995.The Norwegian ERS-1 oil spill detection project was coordinated by Norwegian Space Cen-

tre (NSC) on behalf of a steering committee with representatives from NSC, Norwegian PollutionControl Authority, European Space Agency, Marine Spill Response Corporation, Esso NorgeA.S., and Statoil, all of which have made valuable contributions to the project. Valuable informa-tion from W. G. Bos and A. H. J. M. Pellemans of Directoraat-Generaal Rijkswaterstaat (theNetherlands) and from John C. Scott of Defence Research Agency (United Kingdom) is grate-fully acknowledged.

Tom Anderssen made his contribution to this project while working at Norwegian DefenceResearch Establishment (1993-94) and at Tromse Satellite Station (1994-95). Hilde Hamnes madeher contribution while working at Spacetec a.s.

Address correspondence to Terje Wahl, Norwegian Defence Research Establishment (NDRE),P.O. Box 25, N-2007 Kjeller, Norway.

61

Coastal Management, 24:61-71, 1996Copyright © 1996 Taylor & Francis

0892-0753/96 $12.00 + .00

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62 T. Wahl et al.

Radar satellites are currently being used on a routine basis for near real-time pollu-tion monitoring in Norwegian waters. This article describes the phased introductionof this service, from infrastructure development, basic science, and field experiments,to near real-time demonstrations and knowledge transfer from research laboratoriesto operational entities. The service is initially based on the European Space Agency'sERS satellites. Future radar satellites such as RADARSAT-1 and ENVISAT-1 willoffer increased coverage. At low wind speed (3-4 m/s) even very thin pollutants canbe detected, but there is a risk of false alarms from slicks of natural origin. Emul-sion oil has been observed with ERS-1 at 10 m/s wind speed.

Keywords image analysis, North Sea, pollution monitoring, radar satellites

Surveillance is one important element in the management of coastal waters, both forensuring compliance to pollution regulations and for making decisions on whether ornot to start major cleanup operations against, for example, oil spills. North Sea countrieshave for more than a decade been flying side-looking airborne radar (SLAR) in order todetect oil spills. Once a likely spill has been detected on the radar screen, additionalinstruments (e.g., ultraviolet, infrared, video, photographic) have been used to verify anddocument the case. Results have been exchanged under the Bonn Agreement, and therehave been several joint campaigns to ensure common data interpretation and follow-uproutines. The Norwegian Pollution Control Authority (SFT) is the main institution inNorway for ocean acute pollution monitoring and management. In this article, the pro-cess of introducing radar satellites for pollution monitoring in coastal waters aroundNorway is described.

Radar Satellites

With the launch of the European Space Agency's satellite ERS-1 in July 1991, a newtool became available for pollution monitoring, namely the radar satellite. ERS-1 is car-rying a synthetic aperture radar (SAR). This instrument works on principles similar tothe SLARs in traditional surveillance aircraft, producing a greyscale image where theimage intensity represents the measured strength of the radar echo (backscatter) fromthe ocean surface. The radar wavelength of ERS-1 is 6 cm and the incidence angle is23°. This makes the instrument very sensitive to the presence of short waves (wave-length 5-10 cm) on the ocean surface, a phenomenon known as "Bragg resonance"(Elachi, 1987). Such short water waves are easily dampened by oil slicks; therefore oilslicks will typically appear as dark spots against a wind-roughened sea surface in thesatellite image (Alpers & Huhnerfuss, 1989; Clark 1993; Creamer & Wright, 1992; Htth-nerfuss et al., 1994; Scott & Bagg 1991).

The resolution of a radar in the range direction is independent of the distance to thetarget. Because of the great distance to Earth (800 km), however, a traditional SLARtechnique employed on a satellite would give too coarse resolution in the flight direction(azimuth). Therefore advanced signal processing using frequency information (Doppler-effect) in the radar signal is needed to locate the radar echo more accurately (Elachi,1987, 1988). By storing ERS-1 raw radar echo data for later processing, a "syntheticaperture" of several km length can be obtained although the physical length of the ERS-1 antenna is only 10 m. After heavy SAR-processing on ground, this technique gives animage resolution of 30 m x 30 m for ERS-1. For oil pollution use, the ERS-1 SARimages are usually averaged down to a spatial resolution of 100 m. Table 1 shows themajor ERS-1 parameters.

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Radar Satellites: A New Tool for Pollution Monitoring 63

Table 1ERS-1 and ERS-2 parameters

Altitude 775 kmPeriod 100 minInclination 98.5°Frequency 5.3 GHzResolution 30 mSwath-width 100 kmIncidence angle 20°-26°

Satellite Coverage

The ERS-1 satellite is moving in a nearly circular orbit in a plane that is rotating 360° ina year (sun-synchronous orbit). Because of the rotation of the earth and the fact that theradar is not looking straight down, the resulting coverage pattern of the SAR instrumentis quite complicated. Because sun-synchronous orbits are almost polar, the coveragecapability measured in usable passes per day is dependent on latitude. Using sphericaltrigonometry, it can be shown that the coverage capability at latitude § of a radar satel-lite with a (fixed) radar swath-width W, doing N revolutions per day in an orbit ofinclination /, is approximately

NWnRV(sin if - (sin <|>)2

where R is the radius of the earth. Here it has been assumed that there are no restrictionsdue to power constraints, and that both day and night satellite passes can be utilized. (Inpractice, such restrictions do exist; today's radar satellites cannot operate the SAR in-strument continuously due to power constraints. This is not essential for the geometricaspects discussed here, however.) Note also that the formula must not be used too closeto the poles.

Most radar satellites move in sun-synchronous orbits roughly 800 km above theearth, with typical values N = 14.3 and / = 98.5°. Table 2 shows the resulting coveragecapability at various latitudes for radar swath-widths 100 km, 200 km, and 400 km,

Table 2Satellite coverage (per day) at various latitudes for different swath-widths

Latitude

65° N35° N

0°N

100 km

0.180.090.07

Swath-width

200 km

0.360.180.14

400 km

0.720.360.28

Note. Combined use of RADARSAT-1 and ENVISAT-1 from 1999 on will givecoverage every second day at the equator.

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64 T. Wahletal.

assuming full use of the satellite. The coverage is much better (twice as good) in theNorwegian Sea (65° N) than in the Mediterranean Sea (35° N). This is due to twoeffects, both favoring high latitudes.

1. Circles of constant latitude have smaller circumference at high latitude.2. The satellite pass is more inclined when passing a circle of latitude at high lati-

tude.

For these reasons, it seemed natural to introduce radar satellites for oil spill detection inwaters around Northern Europe.

The limited swath-width (100 km) and the resulting sparse temporal sampling ofERS-1 indicate that the main contribution of radar satellites in pollution monitoring willbe that of early warning and large area coverage. As soon as a probable oil spill isdetected other assets, such as aircraft and ships, are better suited to the task. This is thephilosophy underlying the Norwegian ERS-1 Oil Spill Detection Project.

The Norwegian ERS-1 Oil Spill Detection Project

In the early 1980s, radar satellite applications were declared a strategic part of Norway'sspace policy. Norway entered the ERS-1 program, and later became a full member ofthe European Space Agency (ESA). Following this facilities were established at Troms0Satellite Station (TSS) for fast reception, processing, and distribution of ERS-1 SARdata. The core of this system was the fast SAR processor CESAR, developed at theNorwegian Defence Research Establishment (NDRE) in the late 1980s (Toverud & Ander-sen, 1988) with a processing speed of one 100 km * 100 km ERS-1 SAR image every 8minutes. A later joint study by the Norwegian Ministry of Defence and Ministry ofEnvironment recommended close collaboration between SFT and NDRE on radar satel-lite surveillance.

The Norwegian ERS-1 Oil Spill Detection Project was originally proposed in re-sponse to ESA's Science Announcement of Opportunity for ERS-1 in 1986. It laterachieved the status of an ESA pilot project and received international funding. An inter-national steering committee was established for the project, chaired by Norwegian SpaceCentre (NSC). The project has consisted of the following phases.

Phase 0: 1990-91.Responsible institution: OCEANOR a.s.Contents: Literature survey, prelaunch preparations, planning of field experiment.

Phase 1: 1991.Responsible institution: OCEANOR a.s.Other institutions involved: SFT, NDRE, Esso, Statoil.Contents: A dedicated oil spill experiment (DOSE '91) was carried out at Haltenbanken

(64° N) in August 1991, shortly after the launch of ERS-1. Under the ERS-1 SARswath, 3 x 20 tons of North Sea crude oil were released and studied under various windconditions (Bern et al., 1993).

Results: Clear evidence was obtained for the wind dependence of ERS-1 SAR im-aging of oil slicks.

Phase IB: 1992-93.Responsible institution: NDRE.

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Radar Satellites: A New Tool for Pollution Monitoring 65

Other institutions involved: SFT, OCEANOR a.s., Spacetec a.s., Norwegian Com-puting Center (NR).

Contents: Digital SAR images were distributed to NDRE via datalink immediatelyafter SAR processing at TSS, to test and demonstrate near real-time operations (seeFigure 1). SAR images were usually analyzed at NDRE within 2 hours from the satellitepass. Selected cases were verified by SFT, using a surveillance aircraft. Experimentswere carried out on automatic image analysis and slick detection (Weisteen et al , 1993).Industrialization aspects were studied.

Results: More than 150 ERS-1 images (100 km x 100 km each) were analyzed. Thefeasibility of near real-time operations was clearly demonstrated (Steelv et al., 1994,Wahl et al., 1994a). Some problems were experienced both in strong winds and on calmdays (due to natural slicks).

Pilot Operation Phase: 1993.Responsible institution: NDRE.Other institutions involved: SFT, OCEANOR a.s., Nansen Environmental and Re-

mote Sensing Centre (NERSC), TSS.Contents: Same setting as in Phase IB, but on a larger scale, and carried out in close

cooperation with SFT. The problem of discriminating between true oil slicks and naturalslicks was addressed in detail. Training of TSS operators in SAR image interpretationwas initiated. There was also fruitful cooperation with the Dutch pollution authorities(Rij kswaterstaat).

Results: Two hundred sixty SAR images were analyzed in near real-time, leading tosuccessful detection of various types of pollutants (Wahl et al., 1994b, 1994c.) (See, forexample, Figure 2.) Practical criteria for discriminating between true oil slicks and natu-ral slicks were established (Hovland et al., 1994).

(Pre-) Operational Phase: 1994-Responsible institution: TSS.Other institutions involved: SFT, NDRE, NERSC.Contents: From summer 1994, TSS took over the daily search for oil slicks in ERS-

1 SAR images, increasing significantly the analysis capacity.

ERS-1 Intelsat

LN-SFT

Oil slick SAR processingwith CESAR

Analysis to detectpossible oil slicks

TSS - Troms0 Satellite Station SFT - State Pollution Control AuthorityNDRE - Norwegian Defence Research Establishment

Figure 1. Near real-time oil slick detection chain used during Phase IB.

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66 T. Wahl et al

Figure 2. A 4 km2 slick (fish oil + diesel) imaged by ERS-1 outside western Norway at 10:50Coordinated Universal Time (UTC), May 6, 1993. An alarm was sounded at 12:00 UTC afteranalysis at Norwegian Defence Research Establishment. The case was confirmed 13:25 UTC bysurveillance aircraft that was sent by Norwegian Pollution Control Authority to the area based onthe ERS-1 alarm. (Source: © European Space Agency/Troms0 Satellite Station.)

Results: The world's first routine production chain for near real-time pollution alarmsfrom radar satellites. During the second half of 1994, 1,731 ERS-1 SAR scenes wereanalyzed. Alarms were sent on a routine basis to SFT, which also informed foreignpollution authorities about slicks located outside the Norwegian sector of the North Sea.From January 1995, SFT started taking planned ERS-1 satellite passes into account on aroutine basis when planning aircraft coverage.

Visibility of Pollution

Extensive use of ERS-1 in Norwegian coastal waters has proved the satellite's ability todetect even very thin pollutants in low wind speed (3-4 m/s) and thick emulsion oil at10 m/s. Figure 2 shows an example where a slick made up of a mixture of fish oil anddiesel was verified by aircraft 214 hours after initial detection by ERS-1. Other examplesof pollutants detected during the project are

• crude oil forming emulsion (observed at 10 m/s wind speed),• run-off water from acid-pitch depository on land,• drilling fluid from oil rigs,

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Radar Satellites: A New Tool for Pollution Monitoring 67

• waste from fish production plants and during active fishing,• natural oil seepage from the seafloor,• crude oil spills from oil rigs (see Figure 3).

There are three main problems concerning detection of pollution signatures in SARimages.

1. At zero wind speed, the ocean appears completely dark in the SAR image andno slicks can be found.

2. At low wind speeds (3-4 m/s), ocean slicks of natural origin are frequently ob-served and may cause false alarms unless trained operators or clever pattern rec-ognition algorithms are used.

3. At high winds the pollutant may be washed down into the sea, and no surfaceeffect is detected by the SAR. For example, a 20-ton controlled crude oil spillwent undetected by ERS-1 in 15 m/s wind speed during DOSE '91, and no oilslick was observed by ERS-1 in the stormy conditions during the Braer accidentat Shetland, U.K., in January 1993.

The ERS-1 mission has stimulated research worldwide on SAR detection of oilslicks (Box et al, 1994; Okamoto et al., 1994; Pellemans et al., 1994; Slogett, 1994,Wismann, 1994). Due to the close cooperation between operational entities and R&Dlaboratories in the Norwegian ERS-1 Oil Spill Detection Project, results have been as-similated quickly in the project.

±s. • - v - , . * . " J ? ; , ' « ' . . . ^ . - ' -:.-.'•

V r *.' , ', ... •' ,-I

1.' t ,•

Figure 3. Crude oil spill from production site in the North Sea. This ERS-1 image was acquiredNovember 25, 1994 at 10:50 Coordinated Universal Time (UTC). Norwegian Pollution ControlAuthority was notified 1 hour later after analysis at Tromso Satellite Station (TSS). Responseto the platform operator showed that oil polluted water was leaking into the ocean. (Source:© European Space Agency/TSS.)

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68 T. Wahl et al.

Operationalization

As explained above, the project began in the R&D domain, but with a clear goal ofdemonstrating practical use within the lifetime of ERS-1. During the Pilot OperationPhase, the operators at TSS underwent basic training by NDRE personnel in interpretingSAR signatures of ocean features (e.g., slicks, shears, fronts, internal waves, and foot-prints of atmospheric waves). This was followed up in spring 1994 with more detailedtraining in slick classification, and from June 1994 TSS was able to take over the dailyoperations, carrying out a detection service on contract for SFT/NSC on a 24-hour basis.The initial goals of the project thus have been met. Since ERS-1 is by definition anexperimental satellite, it will be a matter of definition at what time the service offeredby TSS can be called "operational."

Image Analysis: Man Versus Machine

At an early stage in the Norwegian ERS-1 Oil Spill Detection Project, it was believed thatautomatic detection schemes would be mandatory for operational use of radar satellites inpollution monitoring. Experience has shown otherwise. The capability of the human eye(and brain) in detecting and discriminating between slicks in SAR images is far beyondthe reach of today's computers. As long as coverage is limited by concentrating on certainhigh-priority geographical areas, manual interpretation is perfectly feasible. Good soft-ware tools for contrast manipulation, positioning, distance measurement, and digitizationare useful to the operators, however. Automatic initial screening of images should beconsidered for wide area coverage in order to reduce the number of images that requiremanual interpretation. Because of the possible presence of natural ocean slicks, automaticrecognition of true oil slicks will remain a research topic for some time to come.

The results obtained thus far through manual image analysis mean that once a groundstation is able to receive, process, and display SAR images, a basic slick detection ser-vice can be put in place without heavy investments after some operator training. Theauthors believe that much of the experience gained from the operations at TSS shouldbe valuable for use in other coastal areas.

Future Satellites

ERS-1 was joined by the similar ERS-2 in April 1995. This should ensure continuedoperations with well-proved technology until 1998/99. Later in 1995 Canada will launchRADARSAT-1, which will have the ability to steer its radar beam and cover a muchwider area (up to 500 km) by using the new scanSAR technique (Elachi, 1988). Theradiometric sensitivity of the RADARSAT-1 SAR instrument, however, is such that itcannot be expected to detect oil slicks at the larger incidence angles. NonethelessRADARSAT-1 will nicely complement ERS-2. In 1999, ERS-2 will be replaced byESA's next generation satellite ENVISAT-1. This satellite seems to be a promising toolfor slick detection, combining a wide swath-width (405 km) with good slick sensitivity.Norway is planning active use of both RADARSAT-1 and ENVISAT-1, and TSS willbe upgraded to handle data from both of these satellites. Basic parameters for RADAR-SAT-1 and ENVISAT-1 are given in Table 3.

Digital satellite images are usually voluminous, but ERS-1 low resolution (100 m)SAR images are sufficient for slick detection and can be easily transferred on standardIntegrated Services Digital Network communication lines. RADARSAT-1 and ENVISAT-

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Radar Satellites: A New Tool for Pollution Monitoring 69

Table 3RADARSAT-1 and ENVISAT-1 parameters

AltitudePeriodInclinationFrequencyPolarizationResolutionSwath-widthIncidence angle

RADARSAT-1

800 km101 min98.6°5.3 GHzHH10-100 m45-500 km20°-49°

ENVISAT-1

790 km101 min98.6°5.3 GHzHH/VV25-150 m60-405 km15°-45°

1 will have wider swaths, but a practical trade-off between the desired pixel size anddistribution time for oil slick detection also can be worked out in these cases.

There will be a significant coverage and detection potential offered by radar satel-lites in the coming years, even at lower latitudes. It seems that the cost aspects will bemore critical to their use for environmental monitoring than the technological aspects.

Conclusions

Radar satellites offer a new tool for detection of slicks caused by pollution. The major ad-vantages of satellites are wide coverage and easy access to remote areas, a well-controlledimaging geometry, and (for ERS low resolution images and RADARSAT scan-SARimages) competitive image prices per km2 covered. Satellites cannot replace other plat-forms for close range inspection, repeated coverage, or evidence in court, however. Thepresence of natural slicks in coastal waters also may complicate the image interpretation.

The radar satellites discussed in this article are moving in almost polar orbits. Thereforecoverage is best at high latitudes. This is an important reason why Norway has beenactively engaged in this field. A near real-time chain is now working in Norway forpractical use of radar satellites in pollution monitoring. The project has been character-ized by a close interplay between R&D institutions and operational entities throughoutthe process.

Satellites know no borders. A single receiving station, such as TSS, is in principlecapable of receiving and analyzing ERS-1 SAR images from the coasts of Belgium,Denmark, Estonia, Finland, Germany, Iceland, Ireland, Latvia, Lithuania, the Nether-lands, Norway, Poland, Sweden, the United Kingdom, the European coasts of Russia,and parts of France and Greenland. Thus in principle a limited number of receiving andanalysis facilities will be capable of handling nearly all of the world's coastal waters. Inthe coming decade several radar satellites capable of detecting oil slicks will be in op-eration. It is recommended that pollution authorities worldwide consider the use of radarsatellites as an additional source of environmental information from their coastal waters.

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