The launch date for the four in-orbit validation satellites being built as part of Europe’s Galileo satellite navigation system has slipped again, with the first two spacecraft unlikely to be in operation before June 2011 following modifications to the satellite payloads, European government and industry officials said.

The four In-Orbit Validation (IOV) satellites are under construction by a consortium led by Astrium Satellites and Thales Alenia Space. They were intended to be in orbit long before the initial launch of the operational constellation, whose first 14 satellites are under construction by a team led by OHB Technology of Bremen, Germany.

OHB is under contract to deliver the first of its Galileo satellites by late 2012, just 18 months after the first two IOV spacecraft are launched if current schedules do not slip.

The four IOV satellites have already suffered multiple delays and cost overruns. The latest issues that forced a further slippage of several months relate to the removal of Chinese-built search-and-rescue payloads that had been placed on the four nearly complete IOV spacecraft, and to the satellites’ security units, built by Thales Alenia Space.

The Chinese search-and-rescue payloads were removed following a decision by the European Commission that non-European payloads would not be allowed on the Galileo spacecraft. The decision was made earlier this year after the Chinese hardware had been built, delivered and integrated onto the satellites.

This hardware is now being removed. Rene Oosterlinck, director of navigation programs at the 18-nation European Space Agency (ESA), which is overseeing development of the Galileo system, confirmed June 11 that replacing the Chinese hardware was responsible for part of the latest delay.

“It is not as simple as you may think to replace this hardware with dummies,” Oosterlinck said. “The mock-ups not only have to have the same size and weight as the Chinese payloads, they have to have the same thermal characteristics, otherwise this could not be viewed as a validation of the Galileo satellite design.”

Oosterlinck said the delays could have been worse. He said that long before the Chinese gear was deemed unsuitable for Galileo, ESA had begun development of the dummy payloads — not in anticipation of the restrictions eventually imposed by the European Commission, but as a backup in case the Chinese payloads were late in arriving.

“We had the dummies ready, but when it became clear that the Chinese were going to be on time, we did not carry development through to qualification for flight. That has to be done now,” Oosterlinck said.

For the security units, he said, a relatively minor work-around has been proposed by Astrium Satellites and Thales Alenia Space and has been accepted following presentations to ESA on June 8.

“This is a very complex element,” Oosterlinck said. “It requires redundancies inside the unit itself and this, more than the search-and-rescue payloads, has caused the delay. But I am confident that we are on a schedule to permit the launch of the first two IOV satellites in May or June.”

The four IOV satellites are scheduled for launch two at a time on European versions of Russia’s Soyuz rocket, whose first flight from Europe’s French Guiana spaceport has suffered its own set of delays and is now scheduled for late this year.