PARIS -- A European government agency has concluded that satellite radar imagery can be used as part of an operational system to detect and track oil spills off Europe's coastlines but that work is needed to persuade end users of the technology's reliability and cost-effectiveness.

The European Joint Research Center (JRC) in Ispra, Italy, has analyzed 15,000 images from the European ERS and Envisat satellites and Canada's Radarsat spacecraft over the past four years. The goal has been to determine whether satellite imagery can be integrated into an operational system for government authorities to track the hundreds of oil spills, large and small, that occur each year in Europe's territorial waters.

While only the most spectacular oil-tanker spills make the headlines, the daily illegal emptying of bilge tanks and the occasional leak from offshore oil platforms constitute the bulk of the oil released into European waters each year, officials said.

The radar scenes have been measured against aerial photographs and eyewitness reports to verify that the satellite images were not confusing normal sea-surface conditions - such as areas of calm in an otherwise turbulent sea - for oil spills.

Guido Ferraro, a JRC oil-spill program manager, said one glaring weakness of radar data is its inability to detect oil that is just beneath the sea surface. For example, he said the major spill in December 1999 resulting from the Erika oil-tanker collapse went undetected by radar imagery because it lay just beneath the surface in the stormy Atlantic Ocean.

"Here we had a huge quantity of submerged oil at sea and the satellites didn't see anything," Ferraro said here Feb. 23 during a conference on space services for maritime users, organized by the International Astronautical Federation and the Eurisy space-advocacy group.

Another drawback to radar data is the relatively high number of so-called false positives, or detected oil slicks that do not exist. The higher-resolution Radarsat imagery and, occasionally, images from satellites with high-resolution optical cameras can be used to verify the presence of oil suggested in wide-angle images taken by satellites like ERS and Envisat, he said.

For coastal-surveillance and environmental authorities, the use of satellites will be accepted only if it can be proved as a good investment, Ferraro said. "Prices are still too high for the final users," he said.