This research is about the availability of using real-time kinematic (RTK) satellite navigation for unmanned aerial vehicle (UAV) shipboard landings. Combining the use of UAVs with various sizes of ships can enhance the efficiency of intelligence gathering and surveillance and also improves the long-range strike capability of the air force. However, limited shipboard landing area as well as interference due to wind disturbance and wave motion make the shipboard landing of UAVs extremely difficult. In order to successfully land aircraft in such a challenging environment, this project requires a high-precision navigation system for UAVs. As a result, the RTK technique is applied in this work. RTK is a satellite positioning enhancement technique that uses differential carrier-phase measurements. The objective of this research is to conduct statistical analysis of RTK positioning availability using a series of experiments. In addition, a software-in-the-loop (SIL) system is developed for UAV shipboard landing that simulates the entire landing process, including the UAV flight path, UAV attitude, wave motions, and the RTK navigational results. This SIL system includes three parts. First, the UAV used in the SIL system is the SP.X-6 aircraft which was developed by the Remotely Piloted Vehicle & Micro Satellite Research Laboratory at National Cheng Kung University for a cross-sea flight project. Second, for the wave motion simulation, the chosen spectrum is the I.T.T.C. two-parameter, which is used as the power spectrum of the sea waves to be simulated, and all the harmonic waves are synthesized on the Longuet-Higgins Model in time domain. Finally, the RTK positioning results are used in the landing process, including two modes: namely, an RTK operational mode and an RTK failure mode. The actual RTK positioning results are derived for experiments under various configurations: static, dynamic, and a limited number of commonly viewed satellites. In summary, the availability analysis results of this paper will provide suggestions on RTK design for UAV shipboard landings.