RTEMS

OAR Corporation built it in 1988 for the US Army Missile Command, and the acronym has been quietly rebranded as the customers became less martial: Missile Systems, then Military Systems, and today Multiprocessor Systems. The letters never changed.

RTEMS is a single address space with no processes: the application is linked together with the kernel into one binary, much like FreeRTOS, but with a far larger API — POSIX threads, file systems, a network stack and first-class Ada support. Open source, deterministic scheduling and a long qualification record have made it a common choice for satellites and probes at both ESA and NASA, where nobody wants to be the first to fly something new.