A real-time operating system ensures that tasks executed at consistent times, with minimal jitter.

Conventional operating system run task when they can be scheduled. They cannot guarantee that a task will be executed at a specific time, or if it will ever be executed.

On a satellite, some tasks must be executed, interrupting whatever task is currently executing (e.g. commanding the radio receiver to read telecommands from the ground at the start of a pass), and other need to be executed at consistent intervals ( e.g. attitude-control).

A Real-Time Operating System provides both of these. A priority system ensures that resources are available to high-priority tasks when needed, and that periodic tasks are appropriately scheduled.

ESA missions commonly use the RTEMS RTOS.