Three and Four Carriers for Reliable Ambiguity Resolution

The use of three carriers to perform high precision carrier phase based positioning now has been accepted widely for the modernized GPS as well as for the planned Galileo satellite navigation system. In principle, instantaneous (one-epoch) ambiguity resolution becomes feasible for a broad range of applications. A boost of system availability and reliability is recognized as well. For Galileo, in addition to the open access service (OS) a commercial service (PRS) is planned. Besides other improvements a fourth carrier frequency E6 will be available. It has to be proved that the corresponding fees are justified by the actual gain in service quality. Based on a contract with ESA/ESTEC, an extensive hardware-simulation of the planned threeand fourcarrier options for Galileo/GPS was carried out. A modified Hardware satellite-signal-simulator provided GNSS data containing three or four carrierfrequencies which were tracked by an AGGA-2 receiver. The range of simulation parameters covers various levels of ionosphere, troposphere and multipath. Up to 10 GPS or Galileo satellites were tracked simultaneously on baselines up to 82km. The simulation scenarios cover surface, airborne and static applications. The new Factorized Multi-Carrier Ambiguity Resolution (FAMCAR) approach for efficient combination of multi-carrier data was used to analyze OTF ambiguity resolution performance. This paper presents final experimental results including detailed analyses of reliability, availability and accuracy of ambiguity resolution and carrier based positioning for the generated data sets using new processing algorithms. The influence of major error sources and signal design on the system performance is evaluated in depth. Furthermore the statistical significance of such experiments, especially with the scope on high reliabilityapplication is discussed briefly. A final comparison between the two, three and four carrier solutions concludes the paper. It reveals significant improvements especially for baselines over 35 km comparing the twoand the threefrequency solution. The benefit of the fourth carrier is less pronounced, but the effort of providing it may still be justified for applications with very high requirements on reliability and availability. The impacts of multipath and ionosphere (for longer distances to the reference station) are mitigated by the use of at least three carriers. Still, they remain to be the limiting factors for carrier-based positioning performance.