DAYS after Air France's ill-fated Airbus A330 plunged mysteriously into the southern Atlantic Ocean four hours after leaving Rio de Janeiro for Paris on May 31st, rescuers were still searching for debris. After such a long delay, all hope of finding any survivors among the 228 people aboard the stricken aeroplane was reluctantly abandoned.

The wreckage is thought to lie up to some 3,700 metres below the waves, possibly in one of the many trenches that riddle the rocky undersea mountain range west of the Mid-Atlantic Ridge. Its exact location may not be known for several weeks.

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The battery-powered sonar pingers on the aeroplane's two “black boxes” will carry on bleeping for a month. Their acoustic signals carry 6,000 metres through seawater and should be detectable from the surface.

But the dense basaltic rock of the ocean floor will hinder the search party's sonar equipment, creating erroneous echoes and shadows that mask the wreckage's presence. All told, it could be years before the French accident-investigation team makes its final report.

One of the hazards of flying over oceans is the lack of radar coverage. Even the latest radar equipment can reach out no more than 550 kilometres (300 nautical miles) from land. Once out of radar range, pilots flying intercontinental routes make scheduled radio contact every half an hour or so with air-traffic control stations, behind or ahead of them, to report their positions. The rest of the time, no one knows exactly where they are.

A number of countries, especially those surrounded by oceans or by vast expanses of rugged wilderness are none too happy with this. America, Australia and Canada have been among the most active proponents of satellite navigation for commercial aircraft.

The Canadian authorities began operation of such a system in January. It uses what is known in aviation circles as ADS-B, short for automatic dependent surveillance-broadcast. The technology combines the precise position of the aircraft, as identified by global-positioning satellites, with data about its flight number, speed, direction and attitude (whether it is climbing, descending or turning). The combined data are then broadcast automatically every second to all suitably-equipped ground stations and other aircraft within a radius of 240km (150 miles).

As a result, ground controllers and pilots can see exactly where all the traffic in the vicinity is at any given instant. That is far more precise than today's air-traffic control systems, in which it can take anything up to half a minute for ground controllers to get a radar fix from a passing aeroplane's echo transponder. An aircraft can thus be eight kilometres farther on from where it appears on an air-traffic controller's radar screen.

Compared with conventional radar, ADS-B lets aeroplanes travel much closer together without sacrificing safety. It offers pilots greater control over their own flight paths, so they can select more direct paths instead of having to stick to today's zigzag routes. Its use can allow landings to be spaced just 45 seconds apart. The technology also provides ground controllers with valuable extra time to resolve potential conflicts before they become serious.

Trials under way by the Federal Aviation Administration in Alaska have amply demonstrated ADS-B's safety benefits. Alaska had the worst aircraft-accident record in the whole of America but fatal accidents fell by 47% after aeroplanes flying in south west Alaska were fitted with ADS-B transponders. The Alaska trials have also shown that ADS-B works equally well at low altitudes and even on the ground, making it ideal for monitoring aeroplanes as they taxi and race down the runway.

The authority is now having ADS-B equipment installed at Louisville in Kentucky, and Philadelphia and Houston. It hopes to give the go-ahead in September 2010 for ADS-B to be introduced across America. The plan is to have the system up and running by 2013, and all aircraft operating in American airspace fitted with ADS-B transponders by no later than 2020.

Although ADS-B would not have prevented the crash into the Atlantic Ocean, it might well have helped locate the debris more quickly. That is because it could allow pilots to radio their precise positions, once a second, to ground stations many thousands of kilometres away. (The range over land is deliberately limited to 240km to avoid overloading people with information.)

Knowing the exact location of the accident would allow rescue craft to be on the scene within hours, pulling any survivors from the water. That too would be a great benefit of the new air-traffic-control technology.