Situations can arise when a satellite cannot rely on external signals for navigation and must use onboard instruments to determine its position, velocity, and attitude in orbit. A spacecraft terrain relative navigation system is presented relying on measurements obtained from a synthetic aperture radar that are fused with inertial measurement unit data in a multiplicative extended Kalman filter. The method of processing the SAR images to retrieve information for the navigation filter is shown, including autofocusing and image geolocation steps. Monte Carlo simulation results are presented in which actual filter performance is compared to predicted filter performance. Specifically, two test cases, with varied initial attitude uncertainty, demonstrate that the SAR based terrain relative navigation system produces consistent state estimates and successfully bounds, or in some instances, significantly reduces the navigation uncertainty of the spacecraft throughout its trajectory by up to 95%
