HF sky-wave radars rely on signals refracted in the ionosphere to provide a means of over the horizon target detection. Several sky-wave over-the-horizon radar (OTHR) systems have been successfully implemented and the detection of targets located up to approximately 3500 km from the radar location has been demonstrated. The Jindalee Operational Radar Network (JORN) is an over the horizon HF sky-wave radar system currently being developed for the Commonwealth of Australia. The prime contractor is Telecom Australia and GEC-Marconi have been subcontracted to design the radar. The GEC-Marconi Research Centre have been developing some of the critical components of the final radar system. One of the most critical components in any HF sky-wave radar system is a model of the ionosphere. A model of the ionosphere is necessary so that geographic target positions can be inferred from measured radar returns using raytracing techniques. This process is known as coordinate registration (CR). This paper describes an ionospheric model developed for use in the JORN and outlines how this model fits into the overall CR process.