fentISS is a Spanish SME who focuses its activity and expertise in embedded real-time system software for mixed-criticality systems on-board satellites. The company has developed the type 1 hypervisor XtratuM which is qualified for space (ECSS level B) over SPARC-Leon processors and ARM-Cortex R4/R5 and A9. As of today, XtratuM is flying in forty two satellites, forty from the OneWeb constellation (launched on February 2019 and February 2020) and two French satellites, ANGELS and Eyesat, launched on 18 December 2019. fentISS has also developed real-time operating systems to run over XtratuM in particular LithOS which provides an ARINC-653 standard interface (currently running in ANGELS and Eyesat). Main areas of fentISS expertise are real-time software architectures, virtualization, mixed criticality, schedulability analysis for hard real-time systems, security and fault tolerance.