Design of a Unified MLE Tracking for GPS/Galileo Software Receivers

摘要

This paper presents an efficient and practical way of general purpose high performance signal tracking software for a multi-frequency GNSS receiver by using a MLE technique. The cost function of the MLE for estimating signal parameters such as code phase, carrier phase and Doppler frequency is used to derive a discriminator function to create error signals from incoming and reference signals. Assuming a code-free signal and a white additive Gaussian noise, the cost function of MLE is expanded to a simple form of quadratic cost function only for Doppler frequency, which makes it possible to generalize the MLE equation for arbitrary spreading codes and modulation schemes. In order to save computational time, an iterative solution algorithm is modified to a non-iterative feed-forward solution algorithm by using an assumption of no coupling between signal parameters, where the non-iterative solution algorithm uses the gradients of MLE cost functions for creating input error signals in signal tracking block diagram. The use of the block diagram approach for signal tracking makes it possible to provide a well-engineered partitioning of the required computations and to decrease the required processing time by forcing the tracking module to process without any iteration. The designed signal tracking algorithm is incorporated with a 4th order Kalman filter, which has code phase, carrier phase, Doppler frequency and Doppler frequency rate as its state variables, to reduce noise effect on the signal parameter estimates. Finally, a unified approach is also adopted to implement the signal tracking software for tracking a various kind of multi-frequency GNSS signals using the same lines of source code with different configuration setting, i.e., the architectural design method which uses a set of abstract base classes serving as a basis to build a specific receiver implementation is used for maximum reusage of source code.