The Galileo positioning constellation is gearing up since the launch of the first Full Operational Capability (FOC) satellites in 2014 and initial services declared in December 2016. As of 2018, there are 22 satellites in the constellation, which by 2020-21 will ultimately comprise 24 active FOC satellites in circular medium Earth orbit (MEO) at a mean altitude of 23,222 kilometres. The spacecraft are in three groups of eight active satellites plus two on-orbit spares at different inclinations (56°) to cover the entire globe. Each satellite continuously emits its identify, a precise timing signal from its atomic clock and its position in the constellation.
A competition asked European children to name the Galileo satellites. The first two FOC satellites, Milena and Doresa, were orbited in August 2014, but a malfunction on the upper stage of the Soyuz launch vehicle left them in the wrong orbit. They were subsequently used for testing and will join the constellation in 2018.
Adam, Anastasia, Alba, Oriana, Andriana and Liene reached their Galileo orbit in 2015. They were followed by Danielé, Alizée, Antonianna, Lisa, Kimberley and Tijmen in 2016, then Nicole, Sofia, Alexandre and Irina in 2017.
The satellites can be orbited from Kourou in tandem on Soyuz or in groups of four on Ariane 5 ES. The first flight of Ariane 6 (in its 62 configuration) for Galileo will orbit two satellites in 2020.
First-generation Galileo FOC satellites (2014-2021) weigh 730 kilograms and span nearly 15 metres with their solar panels unfolded. They have a design life of at least 12 years. The satellites are assembled in Bremen, Germany, by OHB, while the payload is assembled in the United Kingdom by Surrey Satellite Technologies Ltd (SSTL). Besides the flight systems, the ‘core’ of the satellites is formed by four very-high-precision atomic clocks, which back each other up to ensure reliability and durability, and to avert failures or errors, as a satellite only needs one clock to function.
These clocks are amazingly precise:
- Two Passive Hydrogen Maser (PHM) clocks work by stimulating a cavity filled with supercooled hydrogen at a precise frequency. This kind of system is the most precise operating today among positioning constellations, with an estimated drift of one second every three million years.
- Two Rubidium Atomic Frequency Standard (RAFS) clocks, which use the transition state of rubidium-87 at 6.8 GHz to measure time. Either of these two clocks can take over from a hydrogen maser clock at any time.
Antennas emit signals in the different frequency bands used by Galileo services:
- E1 (1559-1591MHz).
- E5a and E5b (1164-1214MHz)
- E6 (1260-1300MHz)
ARNS: Aeronautical Radio Navigation Services
RNSS: Radio Navigation Satellite Services
Galileo’s long-term future is already secured, as the European Space Agency (ESA) has signed a contract for 12 additional satellites to complete the constellation and manage the transition from the first to the second generation over the next decade (2020-2030).