Overview

An inertial navigation system (INS), sometimes known as an inertial navigation unit (INU), is a dead-reckoning navigation system, comprising an inertial measurement unit and a navigation processor, as shown in Figure 1.3. The IMU, described in the last chapter, incorporates a set of accelerometers and gyros and, assuming a strapdown configuration, produces measurements of the specific force, , and angular rate, , of the body frame with respect to inertial space in body-frame axes. Alternatively, it may output integrated specific force, , and attitude increments, .

This chapter focuses on the navigation processor. This may be packaged with the IMU and the system sold as a complete INS. Alternatively, the navigation equations may be implemented on an integrated navigation processor or on the application's central processor. Marine, aviation, and intermediate grade inertial sensors tend to be sold as part of an INS, while tactical grade inertial sensors are usually sold as an IMU. In either case, the function is the same, so the term inertial navigation system is applied here to all architectures where a three-dimensional navigation solution is obtained from inertial sensor measurements. Other uses of inertial sensors in navigation and dead reckoning techniques using other types of sensor are described in Chapter 10.

Figure 5.1 shows a schematic of an inertial navigation processor. This integrates the IMU outputs to produce a position, velocity, and attitude solution. The navigation equations comprise four steps: attitude update, transformation of the specific-force resolving axes, velocity update, and position update. In addition,...