Abstract: To implement very weak global positioning system(GPS) signal tracking when the tracking loop is unlocked and data bit edge is unknown, this article analyzes GPS signal tracking models and output characterization of correlators in 1 ms intervals. Dynamic programming algorithm is applied for GPS bit synchronization. In addition, two combination carrier tracking schemes are proposed based on square root unscented Kalman filtering (SRUKF) and designed bit synchronization module, both of which are designed for very low carrier to noise ratio (CNR) signals. To satisfy the bit synchronization operability and realize weak signal tracking, a special SRUKF error tracking model is established and bit synchronization module is performed in advance. The SRUKF works alone after bit synchronization. The testing results indicate that the methods can successfully detect bit edge position with high parametric estimation whether or not the tracking loop is locked. When CNR decreases to 22 dB/Hz, the co-operational tracking scheme is still able to perform.

Key words: global positioning system, bit synchronization, optimal path dynamic programming, square root unscented Kalman filtering, weak signal, carrier tracking