Improved Triple-Frequency GPS/GALILEO Carrier Phase Ambiguity Resolution Using a Stochastic Ionosphere Modeling

摘要

GALILEO and Modernized GPS are expected to be fully operational and widely available by 2008. These global navigation satellite systems (GNSSs) will provide three civil frequencies per system. Previous studies have shown that, when taken as a combined system, three frequency GNSS will bring significantly stronger geometry and improved availability. These studies also show that triple frequency integrated GPS and GALILEO can provide accurate float solutions and effective ambiguity resolution. Therefore, fast and reliable ambiguity resolution was possible for short baselines reaching 20 km under fair ionospheric conditions. Expanding the filters used in previous research, the addition of ionospheric delays as states to the ambiguity filter, allow for a further improvement in terms of ambiguity resolution, which can lead to instantaneous ambiguity resolution over medium length baselines. Three frequency systems offer new possibilities for ionospheric modeling, as each double difference slant ionospheric delay will be contained in three single-frequency double difference measurements. This allows for a significant improvement in carrier phase float ambiguity estimation. This ionosphere model can be further enhanced by implementing GPS and GALILEO tight coupling. Tight coupling is the use of double differenced observations between systems that have overlapping carrier frequencies. Forming double difference carrier phase measurements only requires that the phase measurements be at the same carrier frequency. As a result, it is possible to difference carrier phase observations between systems as long as they have a common frequency (E1/L1 and E5a/L5 for GALILEO and GPS). The resulting measurements are called extra-measurements. In order to keep all the observations linearly independent, only one extra-measurement can be used per common frequency per epoch. This results in a tighter coupling of the two systems and has been shown to enhance ambiguity resolution performances when a conventional ambiguity filter is used.