Brief update from Jean-Michel Bois, Head of the ESA Operations Team at ATV-CC, on recent technical issues affecting the ATV-4 computers.
After a first occurrence last week (involving DPU 2), a second ATV Data Processing Unit (i.e. computer) – DPU 3 – was automatically deactivated by the on-board software on Sunday, 28 July.
It must be recalled that ATV has three computers, which work in parallel, all computers being synchronised and performing a comparison between them when specific actions are required.
A single computer can manage the ATV; mainly for safety reasons, and in particular during rendezvous or for fuel transfer from the ATV tanks to ISS during attached phase, the agreement of two computers among the three is required; this configuration is called ‘Triplex Mode’. A degraded configuration can be also authorized in certain circumstances with only two computers (agreement two among two; called Duplex Mode).
ATV Albert Einstein had already had its DPU No. 2 isolated (i.e. taken out of service) last Tuesday night (23 July) after an internal incoherency detection by the Fault Management Layer – basically a piece of software that manages the comparison/voting mechanism between the three units. Since then, ATV was running in Duplex Mode and could continue providing services to the ISS.
On Sunday, 28 July, in the afternoon Toulouse time, DPU 3 was also isolated for a similar reason, with the result that ATV was being managed only a single computer, DPU 1. No risk was generated by this controlled configuration, and there was no impact on the ISS or crew safety.
Nevertheless, this second failure put in question planned ATV activities, including the refuelling scheduled for 1 August.
All the teams at ATV-CC – from ESA, CNES and industry – worked intensively throughout Sunday, Sunday night and Monday to prepare the necessary recovery operations. The objective was to safely restart the three computers as soon as possible. As it is now the summer leave period, several key personnel were recalled from vacation to support their colleagues in this computer reintegration activity.
By the morning of Monday, 29 July, the procedures were ready to be implemented.
As this sensitive activity was rather exceptional and requires solid coordination between teams, a short internal training was conducted at the end of the morning.
The reintegration of the two deactivated computers was conducted on Monday afternoon with success.
ATV is now back to its nominal configuration with three active computers and is ready to continue its mission.
The next step will be the pressurisation of the fuel tanks on 31 July followed by the fuel transfer on 1 August; these will be followed by air and oxygen transfers early in August and support to the ISS to welcome the arrival of Japan’s HTV vehicle.