By Jonathan Amos
Science reporter, BBC News
The two Spirale spacecraft weigh about 120kg each
Europe's Ariane 5 has kicked off its 2009 campaign by launching two commercial telecoms satellites and two experimental French defence spacecraft.
The rocket lifted away from the Kourou spaceport in French Guiana at 1909 local time (2209 GMT).
The Hot Bird 10 and NSS-9 platforms were injected into their orbits after about half an hour; the Spirale pair were released very soon after.
The demonstrators will pave the way for a missile early warning system.
France is keen to develop a technology that only the US and Russia possess at the moment.
The aim is to try to detect and identify a ballistic missile launch at the earliest possible moment after firing.
It is based on the principle of detection of the heat, or infrared, signal given off by a missile's motor during its propulsion phase.
The 120kg Spirale micro-satellites will take up an elliptical orbit around the Earth and map the planet's infrared sources. Their widely separated positions will achieve stereo vision.
Together, the pair will build a reference database against which future missile flights can be checked for possible false alarms.
"Spirale is not an operational system. It will prove the technology and it will prove the ground technology as well because we need to be able to process the images captured by the satellites," explained Pierre Faucoup at EADS Astrium which has led the project.
"We want to characterise all phenomena that could be confused with a missile firing. For example, we wouldn't want the Sun reflecting off a lake or a mountain to be confused with a missile," the company's head of military satellite programmes (France) told BBC News.
Spirale's mission will last some 14 months. If France decides to go ahead with an operational service, it could be in place in about 10 years.
A global missile early warning system would require at least three satellites.
The Spirale spacecraft piggy-backed on the main Ariane mission.
The Herschel telescope will ride into orbit on an Ariane 5
This saw the two primary passengers - Eutelsat's Hot Bird 10 and SES New Skies' NSS-9 - placed in orbit to provide a range of TV and other telecoms services to customers in Europe and the Pacific region.
Next up for Ariane is a major institutional launch. The rocket will loft the European Space Agency's Herschel and Planck telescopes in April.
The launch will represent one of the most valuable missions in the history of European spaceflight.
The combined cost of the telescope programmes is 1.7 billion euros.