175 / 2023-08-31 16:15:05

Analysis and elimination of GNSS diffraction error in high occlusion environments

GNSS diffraction error, high occlusion environments, real-time kinematic positioning, SNR Mask, SNR differencing constraints

The GNSS performance is significantly degraded in urban or natural canyons because of the signal interferences caused by buildings, trees, and slopes. Besides the multipath and non-line-of-sight receptions, the diffraction effect frequently occurs in urban and natural canyons to cause large errors when the Global Navigation Satellite System (GNSS) signals transmitting path is close to the obstruction edge. It will result in a large diffraction error, which is the main reason for the low ambiguity fixing rate, the reduction of positioning accuracy and the frequent gross error in real-time kinematic positioning. To solve this problem, the estimation and extraction methods of diffraction errors are first studied, and the time-varying characteristics of diffraction errors and their relationship with SNR are analyzed. The results show that the diffraction error has an approximately linear pattern and was largely dependent 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. The diffraction error generally follows a normal distribution and has a linear character of positive increasing or negative decreasing. The positive increasing or negative decreasing characteristics of diffraction errors extracted from the double difference residuals are caused by the double difference operation of GNSS observations. According to the positive and negative characteristics of errors, we can identify the diffraction effect that occurs at the reference station or monitoring station. The larger diffraction error corresponding signal-to-noise ratio to the is usually less than 40 dB/Hz, and the absolute difference value of the signal-to-noise ratio of the reference station and the monitoring station is generally greater than 5 dB/Hz. Based on this, we proposed two diffraction error removal strategies with an SNR mask and the SNR differencing constraints between stations. The results of GNSS data testing in two groups of high-occlusion environments show that the SNR mask and SNR differencing constraints can effectively eliminate the influence of large diffraction errors, and the real-time dynamic positioning can achieve more than 90% of the ambiguity fixing rate and millimeter-level positioning accuracy. In addition, averaging the ambiguity-fixing solutions of real-time dynamic positioning can be treated as the solutions for the deformation monitoring of slow-moving deformable objects, and even 0.5mm accuracy can be reached when the baseline is short.