Single Frequency Ionosphere-free precise point positioning: A cross-correlation problem?

This research investigates the feasibility of applying the code and the ionosphere-free code and phase delay observables for single frequency Precise Point Positioning (PPP) processing. Two observation models were studied: the single frequency ionosphere-free code and phase delay, termed the quasi-phase observable, and the code and quasi-phase combination. When implementing the code and quasi-phase combination, the cross-correlation between the observables must be considered. However, the development of an appropriate weight matrix, which can adequately describe the noise characteristics of the single frequency code and quasi-phase observations, is not a trivial task. The noise in the code measurements is highly dependent on the effects of the ionosphere; while the quasi-phase measurements are basically free from the effects of the ionospheric error. Therefore, it is of interest to investigate whether the correlation between the two measurements can be neglected when the code measurements were re-introduced to constrain the initial parameters estimation and thereby improving the phase ambiguities initialization process. It is revealed that the assumed uncorrelated code and quasi-phase combination provided comparable if not better positioning precision than the quasi-phase measurement alone. The level of improvement in the estimated positions is between 1- 18 cm RMS.