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London-based Inmarsat, which provides mobile satellite telecommunications, has launched its latest spacecraft.

Alphasat I-XL rode an Ariane 5 to orbit, lifting clear of the Kourou spaceport in French Guiana at 16:54 local time (19:54 GMT) on Thursday.

The satellite is the product of a major public-private partnership involving Inmarsat and the European Space Agency.

The 6.6-tonne Alphasat incorporates a host of new technologies that should benefit both parties.

Thursday's Ariane flight lasted just over half an hour and saw the rocket deploy a second satellite, also - INSAT-3D, a meteorology mission for India.

Alphasat represents the first test of a new heavyweight class of chassis, or bus, that will allow European manufacturers to make telecoms spacecraft that weigh up to 8.8 tonnes with a power output of 22kW.

This has led some to refer to the Alphasat design as the "A380 of space".

For Inmarsat, the most important aspect of the new satellite is the inclusion of an advanced digital signal processor made in Portsmouth in southern England.

This processor, allied to the platform's smart 11m by 13m antenna system, can channel significant bandwidth and power on to specific locations on the ground.

It will ensure Inmarsat's L-band radio-frequency allocation is used in the most efficient way possible. It should also be much more responsive to customers' varying demands, be they big TV companies, shipping concerns, airlines, the armed forces, or any group that uses on-the-go telecommunications in remote areas.

"Agility is key to this kind of satellite," says Rupert Pearce, Inmarsat's CEO.

"Its technology will work into the mid-2020s and beyond, which is a challenging thing for an operator to back because you have to figure out how you can make that technology relevant for so long. That's why the flexibility in Alphasat is so important."

Inmarsat has invested £240m ($370m/280m euros) in the project. Esa has put up about £320m ($490m/340m euros). The French space agency, the other big institutional partner on the venture, has also made a considerable financial outlay.

In addition to its commercial duties, Alphasat has a number of experimental payloads to test.

The most noteworthy of these is a laser-based communications system.

Developed in Germany, this technology will form the basis of Europe's forthcoming orbital data relay system, which will permit gigabit connections between Earth observation satellites and the ground.

Alphasat will validate the laser terminal by downlinking pictures from the EU's Sentinel-1a radar spacecraft when it launches next year.

"We will use the Alphasat terminal with Sentinel-1a to check all the functionality is there. Then we will deploy the full European Data Relay System," says Magali Vaissière, Esa's top telecoms official now based at Harwell in the UK.

Alphasat was built in a collaboration between Europe's two big satellite manufacturers - Astrium and Thales Alenia Space. They will both be able to offer the new big bus in their product lines.

Quite how much demand there will be remains to be seen. When Alphasat was first envisaged, there was a trend towards ever bigger telecommunications satellites. That trend may have stalled with telecoms operators currently showing interest in spacecraft that can be sized to launch on smaller, cheaper rockets.

"This bus will not form a major part of the market; it is really just for the top end, for those who need a lot of processing power and flexibility. But we have a number of possibilities we are discussing right now," says Eric Béranger, who heads up satellite production at Astrium.

Both Alphasat and INSAT-3D must raise their obits in the coming days to their final orbital slots. For Alphasat, this is some 36,000km above the equator at 25 degrees East. The spacecraft must also deploy its antenna, umbrella style. This is a critical event. Without this equipment, Alphasat has no mission.