Coordinated Data Access at ESA-ESRIN

To return data to Earth is satellites' main reason for existing, and as they grow more complex the amount of data they create is ever growing. Low-Earth orbit missions in particular are often out of touch with ground stations for long periods of time, so results need to be processed and temporarily archived in advance of a downlink, with data compression techniques being applied to increase storage capacity.

The data handling system is basically a computer, but one made of rather different components compared to its terrestrial counterparts. Off-the-shelf computers would not last long in orbit. Space is awash in radiation that swiftly degrades the functioning of unprotected microprocessors.

As cosmic rays or other high-energy particles pass through a spacecraft they can disrupt data systems by randomly flipping memory bits. These 'single-event upsets' are temporary in nature but reduce computer reliability. A more serious radiation hazard still is a 'single event burnout' – when a charged particle causes a voltage surge that permanently burns out part of a chip.

For secure running of data systems in orbit, specially radiation-hardened microprocessors are vital. These chips incorporate radiation shielding and are designed to continuously monitor their software for corrupt data to correct – a facility known as 'error correcting memory'. Like much other space hardware, they employ the principle of redundancy: 'triple modular redundancy' involves multiple CPUs carrying out parallel calculations then comparing results to screen out likely errors.