As global navigation satellite system (GNSS) applications become more widespread, the need for service assurances and associated service monitoring continues to grow. In the early days of satellite navigation, the emphasis was on the navigation solution and its accuracy. Once the service approached operational capability, the emphasis shifted to signal monitoring to ensure a guaranteed level of performance. The first GPS Standard Positioning Service (SPS) performance standard was published in 1993, in the same year that GPS initial operational capability (IOC) was declared. The original five‐station monitor network was expanded, and receiver technology was upgraded to improve satellite signal monitoring, and to reduce the user range error (URE). Performance standards for other GNSSs that are available include BeiDou, and Galileo, while others are anticipated to be based on the performance standards being drafted by the International Committee on GNSS (ICG) Working Group on Systems, Signals and Services.
The next step in signal monitoring is the development of performance specifications to identify all GNSS parameters that need to be monitored. The first edition of the GPS Civil Monitoring Performance Specification (CMPS) was published in 2005. In the international community, this activity is pursued under the Joint ICG‐IGS (International GNSS Service) International GNSS Monitoring and Assessment (IGMA) Trial Project. The development of monitoring standards and their implementation are an ongoing process for GNSS service providers.
Signal Quality Monitoring (SQM) is a subset of all monitoring requirements and primarily covers the transmitted carrier waveform and code performance. Within the SQM subset, there are a number of aspects of the signal that are of interest to the user, ranging from received signal power to potential failure modes and mechanisms. For high‐integrity, safety‐of‐life applications such as aircraft landing operations, SQM requirements are well developed since their full definition was required for the approval process.
This chapter starts with an explanation of the importance of signal quality monitoring, followed by a description of SQM requirements, and an overview of current monitoring systems. Next, SQM algorithms and methods are summarized for six parameters: signal power, cross‐correlation, cycle slip, excessive acceleration, code‐carrier divergence, and signal deformation.