This chapter describes the concept of integrity for GNSS applications. Previous chapters show how integrity is addressed for specific GNSS system architectures (e.g. Chapter 12 GBAS, Chapter 13 SBAS) or in response to specific threats (e.g., Chapter 10 Signal Quality Monitoring). Integrity, in very simple terms, refers to the level of trust that can be placed in the outputs of GNSS or any other navigation system. For some applications, errors beyond specified limits would result in great levels of harm to users, possibly including the loss of human life. For these applications, it is necessary to go beyond quantifying accuracy (the extent of typical errors within confidence intervals of 50% to 95%) to estimating error bounds that apply at very low probabilities.
Section 23.2 of this chapter defines integrity in the context of other navigation system parameters (accuracy, continuity, and availability) and explains the trade‐off between integrity and continuity/availability. Section 23.3 explains the key variables used to quantify integrity and discusses the context in which integrity risk probabilities are estimated. Section 23.4 explains how and why integrity is evaluated by users in real time by computing protection levels for the PVT outputs that matter to particular applications. Sections 23.5 and 23.6 examine the building blocks of protection level calculations: rare‐event bounds under nominal conditions in Section 23.5 and bounds under faulted conditions in Section 23.6. Section 23.7 shows how these principles are applied to RAIM, which is a monitoring technique that exploits redundancy in user measurements. Section 23.8 expands this to what is known as “Advanced RAIM” or ARAIM, which can handle larger sets of possible failure hypotheses. Section 23.9 briefly summarizes the chapter.