12.1 Introduction

In a practical implementation, the accuracy to which a strapdown inertial navigation system is able to operate is limited as a result of errors in the data which are passed to it prior to the commencement of navigation, as well as imperfections in the various components which combine to make up the system. The sources of error may be categorised as follows:

• initial alignment errors;

• inertial sensor errors;

• computational errors.

Many of the contributions to the errors in these different categories have been described in Chapters 4 7, 10 and 11.

Any lack of precision in a measurement used in a dead reckoning system such as an inertial navigation system is passed from one estimate to the next with the overall uncertainty in the precision of the calculated quantity varying or drifting with time. In general, inertial navigation system performance is characterised by a growth in the navigation error from the position co-ordinate values which are initially assigned to it. It is common practice to refer to an inertial navigation system in terms of its mean drift performance; a one nautical mile per hour system is a typical performance class of a system. This would be typical of an inertial navigation system used in a commercial aircraft.

During the early stages of design and system specification, it is necessary to estimate navigation system performance under the conditions in which that system will be called upon to operate. A combination of analysis and simulation techniques are commonly used to predict...