The inertial navigation system/synthetic aperture radar (INS/SAR) integrated navigation system can provide all-weather navigation information both day and night. Altitude is often used as independent information to assist the navigation system in reducing vertical channel errors. In this study, GPS geometric dilution of precision (GDOP) analysis method is introduced into the INS/SAR integrated navigation. By comparing the effects of the presence or absence of altitude assistance and different distributions of SAR measurement ground points on the observability and navigation accuracy, the analysis results based on the GDOP analysis method are provided. The simulation results show that an altitude-assisted navigation system with SAR measurement ground points on two sides achieves the best observability. In addition, there is a positive correlation between the observability and elevation angle. The flight experiment results verify the effectiveness of the proposed method and are consistent with the simulation results.