In what has been described as the greatest change in radio-at-sea communications in over 90 years, a revolutionary new maritime distress system to immediately alert shore-based search and rescue teams went on line over the weekend.

Instead of trying to contact passing ships, the crew of a vessel in trouble will only have to hit one button to raise the alarm. A message giving the ship's name and position will be flashed automatically to a land-based station that then will take over responsibility for coordinating assistance and rescue operations.The International Maritime Organization's Global Maritime Distress and Safety System will be phased in gradually over a seven-year period. All vessels of over 300 tons built after Feb. 1, 1995, must comply with the new communication requirements that will apply to every ship, regardless of age, by Feb. 1, 1999.

The IMO, the U.N. agency responsible for maritime safety and the prevention of marine pollution, described the new system as "the biggest change to radio communications at sea since the introduction of radio at the beginning of the century."

Instead of relying on a combination of radiotelephony and Morse radiotelegraphy, which have a maximum range of around 150 nautical miles and often suffer from poor reception, the new global distress system will take account of technological advances such as satellite communications. Messages can be sent ship-to-shore, shore-to-ship, or ship-to-ship between locations virtually anywhere in the world.

Incidents of ships just disappearing without trace because the crew did not have time to send an SOS message will be a thing of the past.

Even if a vessel goes down very suddenly, an emergency position-indicating radio beacon that is activated as soon as it gets wet would float free and then transmit a continuous signal giving the ship's identity and position to within four miles.

The new distress system has been designed in conjunction with the London- based International Maritime Satellite Organization, or Inmarsat, a cooperative of 64 countries that was set up in 1979 to develop satellite communications for the shipping community. The organization now also provides mobile satellite communications for aeronautical and land-based industries as well.

While vessels in coastal waters still will be able to use terrestrial radio systems, oceangoing vessels will have to use SafetyNet, Inmarsat's maritime communications service. Weather reports and other maritime safety information also can be transmitted over SafetyNet. Distress messages would have priority over all other traffic.

Inmarsat has launched its own satellites and also leases space from other operators that together cover virtually all the world's navigable waters. Users have a choice of satellite communication facilities, with the most sophisticated Inmarsat-A system providing voice, fax, telex and electronic mail services for maritime users worldwide and a cheaper Inmarsat-C version for the transmission of text or data messages.

Some 13,000 vessels have been fitted with Inmarsat-A terminals, while around 1,800 carry Inmarsat-C equipment.

A potential growth area for maritime satellite service is customs clearance before a ship has reached port. A few trials have been conducted over the past couple of years after a Soviet ship was the first ever to clear customs this way, although such procedures are still comparatively rare, Mr. Thacker said.

But other commercial documentation such as bills of lading, ship manifests and containership bay plans are being sent increasingly by satellite in the form of electronic data interchange messages.

Another area that Inmarsat thinks could benefit the shipping industry tremendously is the ability for a vessel's engine or hull to be continuously monitored by shore-based experts who could alert the crew if anything unusual is detected.

A ship's owner also could receive regular position reports and keep in contact with the ship even without the crew knowing.