Method and system for initial quaternion and attitude estimation

Techniques are provided to determine an initial quaternion transform that is used to transform measurements received from accelerometer, rate sensor and inertial reference subsystems from a vehicle coordinate frame to an inertial navigation frame. Methods disclosed determine corrective angular rates to use as a feedback signal to rotate the quaternion transform to counter errors that result when transforming a gravity vector and an inertial reference vector (e.g., a magnetic North reference vector) that are being measured by the accelerometer and an inertial reference subsystems, respectively. The initial quaternion determination is performed during a time period when the only substantial acceleration on the vehicle is due to gravity. The initial quaternion can be used for processing various guidance, navigation and control functions.