GNSS Signal Diffraction Effects in High Precision Positioning.

Doktorsavhandling, 2026

High-precision GNSS is fundamental to modern geodesy. Standard processing assumes line-of-sight signal propagation, treating residual non-line-of-sight effects as multipath or random noise. This thesis shows that diffraction caused by signal bending around obstacles, is a distinct, deterministic error source that biases key geodetic parameters (coordinates, troposphere) in ways that averaging or double-differencing cannot remove. Unmodeled diffraction introduces slowly varying carrier-phase biases that affect station coordinates (especially the vertical component), hinder ambiguity resolution, and mimic tropospheric zenith delays. Under common field conditions (e.g., a 1 m obstacle at 1 m distance), biases can reach decimeter levels, leading to false interpretations of tectonic motion, seasonal deformation, or local subsidence. Because diffraction correlates with satellite geometry, it aliases into tropospheric estimates, degrading climate-related GNSS applications. The DOP metric alone is insufficient, and conventional multipath mitigation (choke rings, absorbers) is largely ineffective against diffraction. This thesis makes four novel contributions. First, it clearly distinguishes diffraction from multipath using GNSS SNR observables and carrier-phase residuals (from PPP and double differences), resolving a long-standing ambiguity. Second, a controlled movable-wall experiment quantifies the inverse distance dependence of diffraction error, validates knife-edge models at far-field (2 m) while revealing their near-field limitations (1 m), and derives an operational guideline (obstacle subtended angle <11°) for PPP and RTK. Third, a first-of-its-kind phasor-based, data-driven interference model for near-field cylindrical interference achieves correlations >0.61 and mm-level residuals, turning empirical site-calibration maps into interpretable physical models. Fourth, diffraction analysis is extended to tropospheric parameter estimation, uncovering previously undocumented errors in estimated zenith total delays and gradients. The thesis includes three introductory chapters (GNSS error sources, diffraction theory, post-processing software) and five appended papers. Together, they reclassify diffraction from a neglected phenomenon into a quantifiable, modelable one, with direct implications for monument design, site selection, and interpretation of long-term coordinate time series in high-precision geodesy.