Of the wake-up calls you might want to get while on the Space Shuttle, an equipment failure alarm is probably not high on the list. Unfortunately that's just what happened to the Atlantis crew earlier today, when a General Purpose Computer failed and roused them from their slumbers with its needy alarm. Happily backup systems were quickly slotted into place, and the crew grabbed a little extra shut-eye before being roused a few hours later by the dulcet tones of Sir Paul McCartney.
The fault that caused GPC number 4 to fail is currently unknown, and one of the Atlantis' crews' tasks today will be to troubleshoot the issue. GPC 2 replaced the faulty unit as the systems computer. The GPCs are in fact IBM AP-101 avionics computers, five of which are onboard the shuttle: four operate in sync, providing redundancy, while a fifth acts as an independent backup. Although now several decades old, the complexity in certifying newer, replacement hardware has meant the AP-101 has remained in service with NASA.
As for McCartney's alarm call later on, the ex-Beatles member sand "Good Day Sunshine" (you can hear it in the video below) and gave the astronauts the message "Good morning guys. Wake up! And good luck on this, your last mission. Well done." NASA had granted the crew an extra 30 minutes lie-in after the GPC issues overnight.
Progress on the Atlantis ST-135 mission proceeds well, with roughly 70-percent of the equipment and supplies the shuttle brought up to the International Space Station transferred. Later this day, the combined crews will talk to US President Obama.