An Improved Method of Ambiguity Resolution in GNSS Positioning

High precision navigation with positioning accuracy of decimeter or even centimeter is becoming more and more important in many fields. Accurate carrier phase observations are the prerequisites for this requirement. Because they have much lower measurement noises than the code observations which are primarily used nowadays. However, the existence of integer ambiguity when counting cycles of carrier phases prohibits the straightforward application of the measurements. Once the integer ambiguity has been resolved, the application of carrier phase measurement is almost equivalent to pseudorange however with much higher precision. Therefore ambiguity resolution is key for fast and high precision positioning. The success rate of ambiguity resolution is affected by the ionospheric delay and observation noise. In previous methods, ionospheric errors are simply ignored. To resolve ambiguity reliably and quickly, we propose a modified method for precise point positioning.We analyze the characteristics of the dual-frequency combinations of original observations and the ones with longer wavelengths and lower noises are preferable. The wide-lane and sub-wide-lane combinations with higher success rates are chosen for ambiguity resolution. Extra pseudoranges are included to eliminate the ionospheric delay which hampers ambiguity resolution seriously. After the combined ambiguities have been resolved, the original ambiguities of each frequency can be calculated from the linearly independent equations. Based on real Global positioning system(GPS) navigation data, performances of the modified method are tested and compared with that of the traditional method. The results show that the modified method is less affected by ionospheric delay and can obtain more accurate ambiguity resolution.