The European Space Agency’s Galileo In-Orbit Validation Element (GIOVE)-B spacecraft has had its Navigation Signal Generation Unit (NGSU) modified to produce signals similar to those agreed between the European Union and US government and its launch has been delayed from March to April due to seasonal eclipse phenomena.

In July 2007 the EU and US administrations announced that they had agreed a compatible signal design for the EU’s Galileo satellite navigation system and the US military’s Navstar Global Positioning System (GPS). This meant the two systems would be interoperable and users would have greater signal availability and coverage around the world.

Work on the signal design began with a June 2004 co-operation agreement that created the joint EU/US compatibility and interoperability working group that would develop it. The signal will be implemented on Galileo’s Open Service and Navstar’s GPS III spacecraft. GPS III satellites will provide increased anti-jamming power and security and accuracy, be backward compatibility, and transmit its Galileo-compatible civil signal, L1C.

While Galileo will be operational by 2013, the US GPS III service will not be available worldwide until the end of the next decade although the launch of its first block IIIA satellites should begin in 2013. This is because of US department of defense plans for the deployment of GPS III to progressively replace the operational GPS IIF, IIR and IIR-M satellites.

“The delay [from the earlier launch date of December 2006] has allowed us to change the NGSU. Launching from mid-March to April is not the best time because of the eclipse [season],” says ESA.

In March and September satellites such as GPS and Galileo’s, operating in medum Earth orbits, find themselves passing through the shadow of the Earth. This is not a good time to deploy a spacecraft because it means no power from the solar panels and causes spacecraft thermal management problems due to the lack of heat from the Sun.

The NSGU change followed the delay of GIOVE-B’s launch from December 2006 to December last year, which subsequently slipped to March due to difficulties with Samara Space Center Soyuz rocket supply and competing Arianespace launch scheduling conflicts.