In the context of the ESA GSTP programme, TELETEL has built a proof of concept testbed prototype for deterministic on-board Ethernet networks based on Time Sensitive Networking (TSN) technology. Specifically, a representative testbed of the onboard space avionics architecture was setup consisting of four (4) TSN Nodes and two (2) TSN Switches, traffic scenarios have been executed and core functionality has been validated. The successful outcome of this activity paves the way towards the design and development of a fully-fledged TSN test tool/simulator for deterministic on-board Ethernet networks.