This paper presents the design and flight test results of a tightly coupled, differential GPS/INS navigation system for precise tracking of medium-range ballistic missiles on test ranges. It describes a 27-state extended Kalman filter that blends data from a space-stabilized inertial platform with DGPS pseudorange and pseudorange-rate measurements to produce real-time estimates of navigation and sensor errors. Navigation system performance results from simulation are presented, and the estimated degradation of reentry navigation accuracy due to the loss of DGPS updates over the course of the trajectory are detailed. System performance was validated with a 130 km flight test on a ballistic missile with a Pershing II reentry vehicle; results of this flight test are presented.