In this contribution, we study the phase-only ambiguity resolution and positioning\nperformance of GPS for short baselines. It is well known that instantaneous (single-epoch) ambiguity\nresolution is possible when both phase and code (pseudorange) data are used. This requires, however,\na benign multipath environment due to the severe effects multipath has on the code measurements.\nWith phase-only processing, one would be free from such severe effects, be it that phase-only\nprocessing requires a change in receiver-satellite geometry, as a consequence of which it cannot be\ndone instantaneously. It is thus of interest to know how much change in the relative receiver-satellite\ngeometry is needed to achieve successful phase-only ambiguity resolution with correspondingly high\nprecision baseline solutions. In this contribution, we study the two-epoch phase-only performance\nof single-, dual-, and triple-frequency GPS for varying time spans from 60 s down to 1 s. We\ndemonstrate, empirically as well as formally, that fast phase-only very-precise positioning is indeed\npossible, and we explain the circumstances that make this possible. The formal analyses are also\nperformed for a large area including Australia, a part of Asia, the Indian Ocean, and the Pacific\nOcean. We remark that in this contribution \"phase-only\" refers to phase-only measurements in the\nobservation model, while the code data are thus only used to compute the approximate values needed\nfor linearizing the observation equations.
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