Automatic navigation of aircraft may be accomplished in a number of ways. Where radiative contact with the ground is satisfactory, systems such as the conventional radio ranges and the more recently developed hyperbolic grid techniques are economical and normally reliable. However, in some regions of the earth and under certain atmospheric conditions, these types of radio aids may not be reliable. Other systems that involve radiative contact with the ground include radio mapping techniques and Doppler navigation; these generally require more expensive airborne equipment, although they are less subject to atmospheric disturbances. But since in military applications radiation from an aircraft furnishes a potential means of enemy detection, such techniques are relatively undesirable. At the present time, Doppler systems are the least subject to this objection. Inertial navigation makes use of acceleration detection and integration for obtaining information on the progress of the aircraft over the surface of the earth. It is independent of radiative contacts, and therefore free from such detection. On the other hand, it is subject to errors resulting from instrumental imperfections. In particular, drift of the essential gyroscopes leads to cumulative errors in indicated position; consequently, pure inertial autonavigators are limited in the flight time over which their indications are satisfactorily accurate. Use of photoelectric telescopes (star trackers) in combination with an inertial system provides a tie to basic inertial space, such as to minimize or eliminate the cumulative effect of gyro drift. Such a combination is known as a stellar inertial autonavigator.