Analysis and elimination of GNSS carrier phase diffraction error in high occlusion environments

In city canyons or natural valleys, carrier phase diffraction effect occurs when Global Navigation Satellite System (GNSS) signal approaches to the edge of buildings, trees and slopes etc., resulting in large diffraction errors, which is one of the important error sources for the low ambiguity fixing rate (AFR), the reduction of accuracy and frequent gross errors. In this study, the diffraction error estimation method was proposed, and the time-varying feature of the diffraction errors were comprehensively studied. It shows that the diffraction error estimated with double-differencing (DD) model generally increases or decrease monotonously, and according to the sign of the error, we can identify which station the diffraction effect occurs at. Large diffraction error usually corresponds to a low signal-to-noise ratio (SNR), with less than 40 dB-Hz, and the differenced SNR of the reference station and the monitoring station is generally greater than 5 dB-Hz. Based on this feature, we proposed to remove the diffraction error by a SNR mask and a differential SNR strategy. Based on an experiment of data processing in a high-occlusion complex environment, the SNR mask and differential SNR can effectively eliminate the large diffraction errors to achieve millimeter-level positioning accuracy and more than 90 % AFR, which is much better than the elevation- and SNR-based down weighting method, and the geographic cut-off elevation method. Meanwhile, the differential SNR strategy is more effective in practical monitoring applications.