Blog Editor’s Note: A great article by long-time RNTF member Guy Buesnel. As you read through the article, you will see that clock failures have not been unique to Galileo. Problems like these are good reminders that no system is perfect and all will fail at some point.
Galileo has been making headlines once again, and this time not for the right reasons.
Galileo has been making headlines once again, and this time not for the right reasons. It was reported on January 18th, 2017 that several of the atomic clocks responsible for the satellites’ ability to calculate precise time have failed.
The importance of precise timing
Timing is everything in GNSS – very precise time is required to calculate an accurate value of the delay in receiving signals that have been transmitted from a given satellite. This allows users to determine their position on Earth accurately. Also, many applications today take advantage of the very precise timing that GNSS can provide via the atomic clocks in use on the satellites.
The atomic clocks
Each Galileo satellite is equipped with four clocks. Two are Rubidium Atomic Frequency Standard (RAFS) clocks like those found in GPS and GLONASS satellites. The other two are the more accurate (and much more complex) Passive Hydrogen Maser (PHM) clocks that offer the Galileo constellation increased timing accuracy. Whilst only one clock in working order is required for each satellite, a minimum of two is required to provide redundancy.
A PHM clock uses the properties of the hydrogen atom to serve as a frequency reference. It is a complex and high cost device but has a significantly higher precision than the Rubidium clock. Typically, PHM clocks are expected to have a 20-year lifetime.
An RAFS clock uses the transition of Rubidium 87 atoms as a frequency reference. RAFS clocks are less costly and more compact than PHM clocks, and have an expected lifetime of 12 years or more.
The failures
The six PHM clocks that failed are almost exclusively on the In-Orbit Validation satellites. According to a statement from the European Space Agency (ESA), the failure was “related to the fact that when some healthy [hydrogen maser] clocks are turned off for long periods, they do not restart due to a change in clock characteristics.”
The ESA has since been able to remotely restart one of the failed PHM clocks, leaving only five PHM clocks offline.
Meanwhile four RAFS clocks have failed – all of them on Full Operational Capability satellites. The ESA also stated the rubidium-based clock failures “all seem to have a consistent signature, linked to probable short circuits, and possibly a particular test procedure performed on the ground.”
India has had the same experience with RAFS clocks – it was announced that 3 clocks (one primary and two backups) on board satellite IRNSS 1a had failed.
The impact of satellite clock failures
Whilst a total of nine clocks have failed, so far, no more than two have failed in a single Galileo satellite. Provided each satellite has at least one clock remaining, they can continue to function as normal. For now, then, these clock failures won’t have a direct impact on the performance or stability of Galileo.
However, the impact to the IRNSS programme is much more severe – the failure of all three RAFS clocks mean that the satellite is totally unusable and will have to be replaced. India has already plans to do this later in 2017.
That said, the clock failures highlight a concern that all members of the GNSS community should share: failure can happen at any stage of a GNSS system – from the satellite level, right down to the device or chipset firmware layer.
