Performance and integrity analysis of the vector tracking architecture of GNSS receivers

Abstract

Frequent loss or attenuation of signals in urban areas and integrity (or reliability of system performance) are two principal challenges facing the Global Navigation Satellite Systems or GNSS today. They are of critical importance especially to safety or liability-critical applications where system malfunction can cause safety problems or has legal/economic consequences. To deal with the problem of integrity, algorithms called integrity monitors have been developed and fielded. These monitors are designed to raise an alarm when situations resulting in misleading information are identified. However, they do not enhance the ability of a GNSS receiver to track weak signals. Among several approaches proposed to deal with the problem of frequent signal outage, an advanced GNSS receiver architecture called vector tracking loops has attracted much attention in recent years. While there is an extensive body of knowledge that documents vector tracking’s superiority to deal with weak signals, prior work on vector loop integrity monitoring is scant. Systematic designs of a vector loop-integrity monitoring scheme can find use in above-mentioned applications that are inherently vulnerable to frequent signal loss or attenuation. Developing such a system, however, warrants a thorough understanding of the workings of the vector architecture as the open literature provides very few preliminary studies in this regard. To this end, the first aspect of this research thoroughly explains the internal operations of the vector architecture. It recasts the existing complex vector architecture equations into parametric models that are mathematically tractable. An in-depth theoretical analysis of these models reveals that inter-satellite aiding is the key to vector tracking’s superiority. The second aspect of this research performs integrity studies of the vector loops. Simulation results from the previous analysis show that inter-satellite aiding allows easy propagation of errors (and faults) among satellite loops in vector tracking mode. Hence, the basic single fault requirement of the traditional Receiver Autonomous Integrity Monitoring or RAIM is violated with the pseudorange measurements of the vector architecture. This work develops a vector loop RAIM scheme that addresses above limitation. The designed vector loop RAIM algorithm is validated via a high fidelity simulation of an aircraft making an instrument approach.