This paper describes GPS carrier phase (CP) multipath characterization and error reduction techniques and their application in single aircraft relative positioning. In particular, the single aircraft relative positioning sce- nario has applications for high-accuracy multi-sensor sta- bilization where CP multipath is a major error source that limits system performance. We will briefly review the requisite multipath theory and discuss models for quanti- fying the error characteristics. Field-test data will be used to validate the multipath models considering the underlying assumptions. A basic geometric reflection point theory will be presented to demonstrate the physical environmental sensitivity of CP multipath to parameters such as surface flatness and antenna height. Several different quantities will be described as multipath indicators for time-domain detection and will be compared with a frequency-domain technique. A new multipath detection approach will be introduced that is suitable to track multipath from time-varying reflection/diffraction points. Finally, the narrow-lane antenna baseline processing technique will be presented as a real-time approach to mitigate CP noise and multipath errors that is well suited for a very short baseline single aircraft high accuracy relative positioning system. Field-test results and analysis will be used to illustrate the key concepts in this paper and to help characterize the total navigation system performance.