Improving Optimality of Deeply Coupled Integration of GPS and INS

Benefits of integrating an inertial navigation system (INS) and a Global Positioning System (GPS) have been well established. A robust integrated receiver that processes the information from all the visible satellites and uses a Kalman filter to close the phase lock loops has been proposed recently in the form of an Ultra Tightly Coupled (UTC) receiver. One of the complexities of the UTC receiver is establishing the mathematical relationship between the states (position, velocity, attitude) from INS, and measurements from the GPS. Many forms of ultra tight coupling method have been proposed to realize these highly non-linear relationships in the integrated system. Many forms of Extended Kalman Filter have been proposed by linearizing these relationships that result in sub-optimal receivers. In this paper, a non-linear filter is proposed for a UTC receiver in order to mitigate the affect of this linearization. The pre-filter that models the relationship between the GPS signal quantities and receiver measurements in a federated integrated design is implemented using a Particle Filter. This filter eliminates the need for the linearization of the relationship, which leads to a near optimal integration that improves the estimation accuracy of the receiver. The application of this technique is described and simulation results are presented.