Assessment of Measurement Distortions in GNSS Antenna Array Space-Time Processing

Antenna array processing techniques are studied in GNSS as effective tools to mitigate interference in spatial and spatiotemporal domains. However, without specific considerations, the array processing results in biases and distortions in the cross-ambiguity function (CAF) of the ranging codes. In space-time processing (STP) the CAF misshaping can happen due to the combined effect of space-time processing and the unintentional signal attenuation by filtering. This paper focuses on characterizing these degradations for different controlled signal scenarios and for live data from an antenna array. The antenna array simulation method introduced in this paper enables one to perform accurate analyses in the field of STP. The effects of relative placement of the interference source with respect to the desired signal direction are shown using overall measurement errors and profile of the signal strength. Analyses of contributions from each source of distortion are conducted individually and collectively. Effects of distortions on GNSS pseudorange errors and position errors are compared for blind, semi-distortionless, and distortionless beamforming methods. The results from characterization can be useful for designing low distortion filters that are especially important for high accuracy GNSS applications in challenging environments

Space-Time Processing Methods to Enhance GNSS Signal Robustness under Electronic Interference

Open access GNSS signals have enabled the development of receivers that find their use spanning multitude of user segments. Owing to the long distance travel from the satellite to the user, the received signal level is very weak on or near earth. This is due to the free-space loss and, to a small extent, atmospheric losses. The signals arriving at the RF front-end can be affected by the presence of signals from other communication systems. Since GNSS has to coexist with other such systems, it is not abnormal to expect that even in normal operations, receivers experience interference. Additionally, undesirable signals can appear in the GNSS frequency band due to other man-made high power signal transmissions. The levels of these disturbing signals will determine the impact that they may have on the performance metrics of the receiver. Antenna array processing techniques are studied in GNSS as effective tools to mitigate interference in spatial and spatiotemporal domains. Analyzing the performance of array based mitigation methods involves many challenges, such as prohibition to propagate test interference signals and the challenge involved with the design and execution of cost-effective experimental setups. To reduce this burden, a new approach is proposed and developed and tested herein. Without specific filter design considerations, the array space-time processing (STP) results in distortions. This research also focuses on characterizing these degradations for different controlled signal scenarios and for live data from an antenna array. The capability of antenna array STP to mitigate the interference from near zone pseudolites and provide subsequent enhancements is studied in the latter part of the thesis. An array simulator is developed during the research and is effectively used for assessing STP measurement distortions. The characterization results show that distortions due to STP are significant and can lead to erroneous pseudorange measurements. From the simulation results, it is concluded that the antenna array STP methods are beneficial for interference mitigation in GPS-pseudolite combined signal environments

Millimetre Level Accuracy GNSS Positioning with the Blind Adaptive Beamforming Method in Interference Environments

The use of antenna arrays in Global Navigation Satellite System (GNSS) applications is gaining significant attention due to its superior capability to suppress both narrowband and wideband interference. However, the phase distortions resulting from array processing may limit the applicability of these methods for high precision applications using carrier phase based positioning techniques. This paper studies the phase distortions occurring with the adaptive blind beamforming method in which satellite angle of arrival (AoA) information is not employed in the optimization problem. To cater to non-stationary interference scenarios, the array weights of the adaptive beamformer are continuously updated. The effects of these continuous updates on the tracking parameters of a GNSS receiver are analyzed. The second part of this paper focuses on reducing the phase distortions during the blind beamforming process in order to allow the receiver to perform carrier phase based positioning by applying a constraint on the structure of the array configuration and by compensating the array uncertainties. Limitations of the previous methods are studied and a new method is proposed that keeps the simplicity of the blind beamformer structure and, at the same time, reduces tracking degradations while achieving millimetre level positioning accuracy in interference environments. To verify the applicability of the proposed method and analyze the degradations, array signals corresponding to the GPS L1 band are generated using a combination of hardware and software simulators. Furthermore, the amount of degradation and performance of the proposed method under different conditions are evaluated based on Monte Carlo simulations

Assessment of Measurement Distortions in GNSS Antenna Array Space-Time Processing

Antenna array processing techniques are studied in GNSS as effective tools to mitigate interference in spatial and spatiotemporal domains. However, without specific considerations, the array processing results in biases and distortions in the cross-ambiguity function (CAF) of the ranging codes. In space-time processing (STP) the CAF misshaping can happen due to the combined effect of space-time processing and the unintentional signal attenuation by filtering. This paper focuses on characterizing these degradations for different controlled signal scenarios and for live data from an antenna array. The antenna array simulation method introduced in this paper enables one to perform accurate analyses in the field of STP. The effects of relative placement of the interference source with respect to the desired signal direction are shown using overall measurement errors and profile of the signal strength. Analyses of contributions from each source of distortion are conducted individually and collectively. Effects of distortions on GNSS pseudorange errors and position errors are compared for blind, semi-distortionless, and distortionless beamforming methods. The results from characterization can be useful for designing low distortion filters that are especially important for high accuracy GNSS applications in challenging environments.Peer Reviewe