By Maria Sheahan

FRANKFURT (Reuters) - Europe launched four more Galileo satellites on Thursday, moving a step closer to having its own navigation system and marking the first time it has sent up so many satellites at once.

The satellites, which blasted off from Europe's spaceport in French Guiana aboard an Ariane 5 rocket at 10.06 AM local time (1306 GMT), will be part of the European Union's alternative to the U.S. Global Positioning System, or GPS.

The launch brings the number of Galileo satellites in orbit to 18, of a planned total of 30, according to the European Space Agency (ESA). The satellites weigh about 700 kgs (1,543 pounds), and are equipped with antennae and sensors and powered by two 5-square-metre (53.8 square foot) solar wings.

Since the EU decided to go ahead with Galileo 16 years ago, the program has suffered some setbacks, including delays, financing problems, two satellites being put into the wrong orbit and questions about whether Europe really needs a rival system to GPS.

The EU aims to use Galileo to tap into the global market for satellite navigation services, which it estimates will be worth 250 billion euros ($267 billion) by 2022. Also, it says that around 6 to 7 percent of the $16-trillion EU economy depends on the availability of global navigation satellite signals.

Galileo is to start offering an initial service in the coming weeks. Once all its satellites are in orbit, which is expected in 2020, the system will allow users to determine their position more accurately than GPS alone and will help in search and rescue missions.

Russia and China have also launched their own global positioning systems to underpin their defense industries and civilian commerce.

The four Galileo satellites launched on Thursday reached their intended orbit of around 24,000 kms (14,900 miles) above Earth around four hours after launch, said Arianespace, which was responsible for the launch.

This was the first time a European Ariane 5 rocket was used to launch Galileo satellites instead of a Russian Soyuz. Two further Ariane 5 flights are planned for Galileo during the next two years.

(Editing by Mark Potter and Alexandra Hudson)