In this chapter we consider how external aiding data can be used in acquisition, tracking, and positioning. We use aiding from Inertial Navigation Systems (INS). In general any system which provides measurements of vehicle dynamics such as velocity and acceleration can be used. In particular, we can use relatively cheap inertial measurement unit (IMU) sensors, such as MEMS accelerometers and gyros, to provide suitable information in a wide range of applications, including those less technically demanding. However, we base our examples on a rather more demanding INS for an airborne navigation complex. The theory, approach, and algorithms are applicable to low-end consumer applications in general, however, rather than only to aviation and space applications, which would have been the case just a decade ago at the time of writing.

Principles of GNSS and INS integration

The GNSS gives stable navigation solutions as long as the receiver maintains a signal lock on four or more satellites. We discussed in Chapter 1 that information about height allows us to discard one variable and reduce the number of satellites required to three. Furthermore, a stable enough clock (usually not a part of a conventional receiver) allows reduction of this number to two. In the case of aircraft, attitude information is essential. Clock stability in general is not good enough to assume a constant receiver clock error. Therefore, if the number of satellites is reduced to fewer than four due to signal blockage, intentional/unintentional interference, etc., the position and timing solution will be lost. Vulnerability of GNSS radiowave propagation is a critical issue for aviation safety.