Inertial navigation systems have become of great importance in the navigation of military aircraft and in recent years there has been a growing interest in their application to civil transports. A special problem arises with this class of equipment in that the axes require to be set before navigating and a particular difficulty arises with the azimuth. This involves penalties in special pre-flight routines or in extra cost and complexity of equipment which are of concern to the airline operator who is attempting to assess the usefulness of inertial techniques. The purpose of.this paper is to discuss the general principles on which azimuth alignment schemes can be based with reference to the likely penalties involved.

The paper is primarily concerned with systems where aircraft accelerations are measured by accelerometers mounted on a gyro stabilized platform and then integrated to obtain velocity. The velocity may be either used directly in a navigation computer or mixed first with velocity from another source, such as doppler radar, as described in Ref. 1. However, all dead reckoning systems require a knowledge of heading and in cases where a better accuracy is required than can be obtained by reference to the Earth's magnetic field, a low-drift gyroscope can be used as an azimuth memory. Some of the alignment schemes discussed would be applicable to these heading references as well as to stable platforms with accelerometers. With most inertial systems, alignment to the Earth's magnetic field would be made before fine alignment commenced.