PRISMA will demonstrate Guidance, Navigation, and Control strategies for advanced autonomous formation flying. The Swedish Space Corporation (SSC) is the prime contractor for the project which is funded by the Swedish National Space Board (SNSB). The mission consists of two spacecraft: MAIN and TARGET. The MAIN satellite has full orbit control capability while TARGET is attitude controlled only. PRISMA will perform a series of GNC related formation flying experiments. SSC is responsible for three main sets of experiments: Autonomous Formation Flying, Proximity Operations and Final Approach/Recede Manoeuvres, and Autonomous Rendezvous. Many formation flying scenarios, including experiments on PRISMA, require the use of orbits that are not naturally safe. This includes trajectories that, if nominal orbit control were lost, could result in collision or formation evaporation - secular drift that could eventually cause the loss of relative navigation. This paper will focus on the relative orbit safety concept within PRISMA, presenting the detection and recovery algorithms developed.