The GNSS RO mission represents a revolution in atmospheric sounding from space, with precise, accurate, and all-weather global observations useful for weather, climate, and space weather research and operations. The FORMOSAT-3 constellation has been unanimously regarded by the major user community as “the world's most accurate, precise, and stable atmospheric thermometer in space.” The success of the FORMOSAT-3 mission has initiated a new era for the operational GPS RO sounding applications in the world. With the success of FORMOSAT-3, there is a strong interest from both research and operational communities to maintain and enhance the existing FORMOSAT-3 mission. Observing system simulation experiments (OSSE) have been conducted to evaluate the impact of the GNSS RO follow-on mission on the prediction of typhoons in the vicinity of Taiwan. OSSE experiments based on Typhoon Shanshan (2006) indicated that the proposed follow-on configuration provided a significant improvement over the existing FORMOSAT3 for typhoon prediction in terms of track, intensity and rainfall prediction. These preliminary results strongly support the use of the proposed follow-on design for the FORMOSAT-3 Follow-on mission.