Some general features of inertial and hybrid inertial systems are examined in relation to the navigational problem in its two essential aspects:

(i) The present situation.

(ii) The action required.

As the fundamental D.R. system, inertial navigation has the virtue of independence but suffers from time-dependent errors. The engineering complexity and delicacy of adjustment of purely inertial navigation systems of long-period accuracy detract from its general application where other aids are available. On the other hand, increasingly stringent demands on flight-path computation and control justify an examination of the benefits to be derived from exploitation of the particular short-period merits of an inertial element used in conjunction with other aids.

Principles of mixing are discussed in relation to quantity and quality of information inputs. Examples are then given of fallacies which may arise from the omission of essential information or from ignoring cross-coupling, and of the impracticability of mixed systems which assume too much of the quality of the input information.

After summarizing the main features of inertial, radio/radar and astro aids, examples are given of practical and useful mixed systems which do not require the highest precision of inertial components. For example, auto-astro and inertia/doppler/ground fix contain the necessary and sufficient ingredients for combining long-period accuracy with short-term memory, speed of response in manoeuvre and instantaneous outputs such as are required for precise control and for flight path monitoring.

Summarizing, it may be said that the value of true and hybrid inertial systems in the military context is evident. For more general application we may hope to see a gradual evolution of small, accurate and reliable central reference systems which will permit the employment of inertial methods suitable for mixing, especially with doppler in an accurate D.R. system. The outputs will be suitable not only for flight-path monitoring, but as providing information of a quality very well suited for integration into the control system and so into any overall system of flight automation.