Reliable resolution and validation of carrier-phaseambiguities is a major challenge when employing fixedambiguity Precise Point Positioning (PPP). The currentPPP ambiguity validation and resolution methods areanalyzed and tested in this paper for the case of usingGPS L1 and L2 signals without external ionosphericcorrections. Based on the Integer Least-Squares usingDoubly Non-Central F-distribution based ratio testthreshold (ILSDNCF) with a 1200 s carrier-phase locktime requirement is chosen as the most reliable. It yields arate of incorrect ambiguity resolution of 12.7%. It isdefined that ambiguity resolution is incorrect, if themagnitude of the 3D position error is larger than 10.7 cm,which is the wavelength of the narrow-lane signalcombination, at the initial ambiguity resolution epoch.Ambiguity resolution is defined as correct, if the 3Dposition error is smaller or equal than 10.7 cm at the initial ambiguity resolution epoch, and undecided, ifambiguity resolution cannot be done at all.However, the rate of incorrect ambiguity resolutionachieved with this method is too high for many real-lifeapplications. This paper, therefore, develops,enhancements to improve PPP performance. Theenhanced PPP method uses a time window basedambiguity validation and both GPS and GLONASS toimprove GPS ambiguity resolution. The achieved rate ofincorrect ambiguity resolution is 6.0% when using GPSalone and 5.3% when using both GPS and GLONASS.The rate correct ambiguity resolution is 80.8% whenusing GPS alone and 82.2% when using both GPS andGLONASS. Compared to the most reliable existingmethod, the correct ambiguity resolution rate is increasedby 8.9% when using GPS alone and 10.8% when usingboth GPS and GLONASS.In terms of the horizontal, vertical and 3D position errorsat the initial ambiguity resolution epoch, employing theenhanced PPP method using GPS and GLONASSmeasurements reduced the errors by 40.0%, 23.8% and31.7%, respectively, compared to the most reliableexisting PPP method. In terms of the solutionconvergence time, there is no significant differencebetween the enhanced PPP method and existing methods.On integrity monitoring, improvements are developed toenable a more realistic computation of protection levels.
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