Design and implementation of Kalman filter for GPS receivers

Abstract

Objective: This paper discusses the design and development of a Kalman filter for effective tracking of code and carrier in a GPS receiver. The objective is to incorporate Kalman filter in the tracking channel of a GPS receiver. Methods/Statistical Analysis: The tracking channel keeps synchronizing continuously, the received satellite signal and the locally generated code and carrier frequencies, using tracking loops. The first step is to develop a tracking channel and the next step is to develop a Kalman filter model suitable for the design. The reliable and standard tracking loops for tracking code and carrier are Delay Locked Loop (DLL) and Costas Loop respectively. When these loops lose the lock, i.e., when the signals are weak, the receivers can no longer retrieve the navigation data unless the code and carrier comes in lock again. This causes disturbance in tracking, i.e., the receiver can no longer determine its position until the signal becomes strong. The proposed design can solve this problem by using a Kalman filter in a phase lock loop. Findings: The Kalman filter, with its linear recursive filtering provides greater accuracy in estimating the phase/frequency of the received incoming signal with the initial state of the system, noise statistics of the system and the corresponding measurement errors. The replacement of Low Pass Filter (LPF) with the Kalman filter in the tracking channel provides a better estimation of tracking. The analysis reveals that the Kalman filter is the best replacement tool that can strongly influence the area of navigation in a way that minimizes the error in tracking. Conclusion: Thus the design ensures the continuous tracking of GPS signals by preventing the need for frequent requisition even under weak signal conditions.