The system include space segment, TTAC, ground control, mission control. The space segment includes different domains of hardware and software which maintain the SV orbit (flight control), sending telmetry down to earth, receiving commands/data/software update, various sensors and their control software. That doesn't even include the mission system which takes photos. TTAC has to track the SV and communicate with the SV. It also maintains the data link between ground control and the SV... This list goes on and on. The spec for the whole system is over thousands of pages and includes all kinds of hardware/software. You can simply say what the "OS" is.