Spatiotemporal characteristics of GNSS carrier phase diffraction error in occlusive environments

Global Navigation Satellite System (GNSS) positioning performance is significantly degraded because of the signal interferences caused by buildings, trees and slopes, especially in precise positioning applications in water conservancy projects. Besides the multipath and non-line-of-sight (NLOS) receptions, the diffraction effect frequently occurs in urban and natural canyons to cause large errors when the GNSS signals transmitting path is close to the obstruction edge. We first analyzed the characteristics of diffraction errors and proposed an obstruction adaptive elevation masks (OAEMs) determination method to eliminate the diffractions in the data processing. It shows that the diffraction error has an approximately linear pattern and largely dependents on the horizontal distance of the diffraction point to the phase center of the GNSS antenna and the diffraction elevation. It can be far beyond the theoretical limitation of multipath error and achieve several phase wavelengths, and could have various patterns in time series, such as trend, period and regular or unregular fluctuations, according to the features or distributions of obstruction edges. We proposed an OAEMs determination method by detecting diffractions in time series subsections and mapping and stacking them in a hemispherical map. The results show that it can eliminate the large diffraction errors to reduce the outliers and large fluctuations in the time series, and the positioning precision and reliability could be improved to a large extent. Especially in vertical directions, the precision can be improved by more than 40%.