Navigation system integrity refers to the ability of the system to provide timely warnings to users when the system should not be used for navigation. The basic GPS (as described in Chapter 3) provides integrity information to the user via navigation message, but this may not be timely enough for some applications, such as civil aviation. Therefore, additional methods of providing integrity are necessary.
Two different methods will be discussed—GPS-only receiver (TSO-C129-compliant) autonomous integrity monitoring (RAIM) and use of ground monitoring stations to monitor the health of the satellites, as is done via SBAS and GBAS (TSO-C145-compliant receivers). Three RAIM methods have been proposed in recent papers on GPS integrity [150, 151, 152, 153]:
We will briefly discuss the RAIM methods, then discuss SBAS and GBAS integrity design.
For the GNSS navigation problem described in Chapter 2, Section 2.5, there are four unknowns (three position coordinates [x, y, z] and clock bias Cb) and more than four satellites in view (e.g., six satellites). One can solve the position and time equations for the first four satellites, ignoring noise, and find the user position. This solution can then be used to predict the remaining two pseudorange measurements, and the predicted values could be compared ...