Correction of fractional cycle bias of pseudolite system for user integer ambiguity resolution

Abstract

Synchronized pseudolite systems allow single-point positioning (SPP) using carrier-phase measurements with centimeter-level accuracy. Locata, developed by the Locata Corporation, emerged from the development of synchronized pseudolite technology. Numerous studies have claimed to achieve Locata SPP with centimeter-level accuracy; however, the ambiguities of Locata observations have been typically estimated as floating-point values. Similar to precise point positioning of global navigation satellite systems, pseudolite SPP that is based on ambiguity fixing can improve the positioning accuracy and shorten the convergence time, compared to the float solution. However, integer ambiguity fixing may be prevented due to the clock offset among the pseudolite transmitters. To recover the integer nature of single-difference ambiguity, a fractional cycle bias (FCB) correction method is proposed; this method is based on two-way time synchronization. The FCB correction method employed to estimate and correct the bias by a pseudolite network itself has two steps: two-way FCB measurement and half-cycle bias estimation. A ground-based testbed is constructed to test and verify the proposed method. It is shown that the FCB between the pseudolite transmitters have a stability better than 15 ps even considering the influence of variable multipath delay; the bias can be estimated with high precision and reliability. Statistical results support the theoretical findings that half-cycle bias may be caused by two-way clock offset measuring via carrier phase measurement. A two-dimensional positioning experiment is performed to evaluate the performance of the proposed FCB correction method. The single-difference ambiguities are fixed to integer values using the LAMBDA method with the fixed failure-rate ratio test. The results indicated that the positioning accuracy is at the sub-centimeters to centimeter-level compared with the real-valued solutions for all surveyed points.