GPS-aided navigation and unaided navigation on the joint direct attack munition

The Joint Direct Attack Munition (JDAM) provides a low-cost accuracy enhancement to existing MK-83, MK-84, and BLU-109 weapons. Low-cost has been achieved through the use of commercial standards and high-volume production techniques. Accuracy improvements are achieved with a GPS-aided Inertial Navigation System (INS), an adaptive optimal guidance algorithm, an autopilot featuring a robust servo structure, and a tail actuator subsystem. Flight testing has demonstrated an impact uncertainty considerably less than the 13 meter Circular Error Probable (CEP) requirement in CPS-aided missions. This paper focuses on the JDAM navigator and Kalman filter algorithms that support both GPS-aided and unaided missions. The GPS-aided navigator consists of a tightly-coupled GPS/INS integration that supports both conventional and relative GPS operation. During GPS-aided operation, the JDAM Kalman filter is configured to estimate the basic kinematics states, calibrate critical inertial sensors, estimate GPS receiver clock errors, and estimate line-of-sight biases between the weapon and each NAVSTAR satellite tracked during the mission. JDAM also includes an unaided navigation mode that is used if GPS is denied. Unaided navigation relies on a transfer alignment algorithm to align the weapon's navigator to the carrier aircraft's navigator and to calibrate critical inertial sensors before release. Transfer alignment enhancements include senescence error estimation and an adaptive Kalman filter tuning algorithm that modifies filter process noise based on the weapon's vibration environment.