On December 5 at 00:19 UTC, an Arianespace Soyuz 2.1b/Fregat launched two more satellites for Europe’s Galileo Navigation Satellite System from the Guiana Space Center in Kourou, French Guiana.

Galileo is the European Union’s version of the U.S. military’s Global Positioning System (GPS), built to ensure Europe has a backup in case of problems with GPS. Galileo’s signals are compatible with GPS, and many consumer or commercial devices can use signals from both GPS and Galileo in addition to other global and regional satnav systems. 

These satellites, 27 and 28, are the first two Batch 3 satellites in the constellation. The final constellation will consist of thirty satellites: 24 active satellites and six on-orbit spares. Spares are important because several clocks have failed on Galileo satellites. In 2017, a total of nine rubidium and hydrogen clocks failed across five of the operational satellites, though no satellite lost more than one of each type. The same model of rubidium atomic clock was used on India’s IRNSS 1A satellite; it lost all three of its clocks. 

The first test satellite for Galileo was GIOVE-A or Galileo In-Orbit Verification Element-A. It was launched in late 2005 and only recently decommissioned in November 2021. It was a rush job to secure their frequency allocation on orbit before it expired in 2006. SpaceX’s Tintin satellites served a similar purpose for the Starlink constellation.

The operational satellites like Galileo 27 and 28 contain the usual complement of two rubidium clocks and two hydrogen maser clocks, which are highly accurate frequency standards typical of a navigation satellite. 

A navigation satellite works by sending signals to receivers that contain the precise location of the satellite in orbit. It uses the same signal to tell the receivers the exact time it sent that information, which it gets from those highly accurate clocks. The receiver then takes the signals from at least three satellites and does a bunch of math to determine the user’s location. The more satellites the receiver can get signals from at one time, the more precisely the receiver’s location can be determined.

Just under four hours after launch, the two German-made satellites were separated from the Fregat upper stage after two burns of that stage. The operational orbit for the Galileo satellites is just over 23,000 kilometers at an inclination of 57 degrees.  Each satellite is taller than the average Christmas tree and weighs about twenty-three times as much: 732 kilograms.

More Information

PDF: VS26 press kit (Arianespace)

Galileo clock anomalies under investigation (ESA)

Galileo prototype GIOVE-A switched off after 16 years in orbit (ESA)

Galileo Overview (Spaceflight 101)

Launch video