Onboard software is the software running in the actual spacecraft that is in operation. In essence, it runs as an isolated and independent software that controls equipment such as sensors, propulsion, power system, etc. This specific type of software is considered mission critical and has to be qualified before usage and obey strict programming and performance rules, on various hardware architectures.

Usually, onboard software makes use of qualified specialized libraries, such as the mathematical library developed in the¬†“Pre-Qualification of a Mathematical Library for Flight Sofware”¬†project.