GPS World, February 2017

SYSTEM SYSTEMS OF Clocks Fail on 5 Galileo Satellites All Have Backup T he European Space Agency ESA issued a statement on Jan 19 addressing Galileo clock failures GPS World Innovation editor Richard Langley provided the following summary of the satellites and clocks involved based on information received to date 5 satellites affected 3 IOVs 2 FOCs Total of 10 failures 1 fixed so 9 continuing failures 5 masers on IOV satellites 2 masers on FOC satellites but 1 of these fixed 3 rubidiums on FOC satellites No satellite currently has fewer than 2 working clocks The following ESA press release provides additional details on the failures and actions being taken to address the problem Press Release Text As first reported November 2016 anomalies have been noted in the atomic clocks serving Europes Galileo satellites Anomalies have occurred on five out of 18 Galileo satellites in orbit although all satellites continue to operate well and the provision of Galileo Initial Services has not been affected Highly accurate timing is core to satellite navigation Each Galileo carries four atomic clocks to ensure strong quadruple redundancy of the timing subsystem two Rubidium Atomic Frequency Standard RAFS clocks and two Passive Hydrogen Maser PHM clocks The current Galileo constellation consists of 18 satellites in orbit adding up to a total of 36 RAFS clocks and 36 PHM clocks RAFS Clocks In recent months a total of three RAFS clocks unexpectedly failed on Galileo satellites all on Full Operational Capability FOC satellites the latest Galileo model These failures all seem to have a consistent signature linked to probable short circuits and possibly a particular test procedure performed on the ground with investigations continuing to identify a root cause No RAFS clock fai lures have occurred aboard the four Galileo In Orbit Validation IOV satellites the original Galileo model In addition the RAFS clock on ESAs very first test navigation satellite GIOVE A launched in 2005 has been checked and was reactivated successfully Continuing investigations on the ground have identified potential weaknesses in the RAFS clock design but no root cause has yet been established PHM Clocks In the past two years there have been five PHM clock failures on the IOV satellites and one PHM failure on the FOC satellites Thes e fa i lures a re bet ter understood linked to two apparent causes One is a low margin on a particular parameter that leads on some units to a failure The second is related to the fact that when some healthy PHM clocks are turned off for long periods they do not restart because of a change in clock characteristics in orbit To date two PHM clocks have failed owing to the first mechanism and four to the second Corrective Actions For the remaining 33 RAFS clocks in orbit the risk of failure is believed to be lower owing to different testing procedures on the ground before launch In addition new operational measures have been put in place to 12 GPS WORLD WWW GPSWORLD COM FEBRUARY 2017 further mitigate the risk All these measures have no effect on Galileos overall performance While investigations by ESA and its industrial partners are continuing there is consensus that some refurbishment is required on the remaining RAFS clocks still to be launched on the eight Galileo satellites being constructed or tested and awaiting launch For the remaining 30 PHM clocks working in orbit operational procedures are being studied to significantly reduce the risk of future failure These measures are being validated ahead of their planned introduction in a few weeks Looking Forward Overall three out of four IOV satellites have experienced clock anomalies and two out of 14 FOC satellites As ESA Director General Jan Woerner commented during his Jan 18 press briefing no individual Galileo satellite has experienced more than two clock failures so the robust quadruple redundancy designed into the system means all 18 members of the constellation remain operational This includes one satellite that supports only the Open Service for mass market applications and two satellites in elliptical orbits that are nevertheless expected to be reintegrated into the full constellation for use from these orbits Similarly Galileos Initial Services which began on Dec 16 have been unaffected by these anomalies The impact of RAFS and PHM clock refurbishment on Galileos launch schedule is under study but ESA is confident that the clock issues will be resolved and remains committed to launch the next four Galileo FOC satellites before the end of this year