This approach should provide excellent isolation from low frequency Earth motion by suspending the gyro test turntable in a set of instrumented and controlled gimbals. Further studies by the Electronics Research Center are under way to investigate the addition of a passive damping system design such as a spring-damper system for high frequency isolation. The result of this effort is expected to be incorporated in the design of precision inertial test instruments for the Electronics Research Center Guidance Laboratory Facility.

Manual Sextant Sighting.-Hand-held navigation devices are known to have existed since 300 BC. The early instruments could determine position to within only 300 nautical miles, but as the systems evolved and were used at sea, this error was reduced to 20 n.mi in circa 1730 AD. The accuracy of the modern marine and aircraft sextant, first used during the 1940's, is approximately one nautical mile.

In anticipation of future manned lunar and interplanetary missions, the Ames Research Center has been concerned with the development and simulation of manual space navigation techniques. One of the results of these efforts has been the development of a hand-held space sextant, figure 329, built by the Kollsman Instrument Company and having an accuracy equivalent to 100 yards for Earth-based navigation.

Potential applications of this sextant are as a backup for spacecraft mounted semi-automatic instruments or as a primary navigation instrument for lunar and planetary missions. It has surpassed performance expectations as borne out by a sextant-sighting experiment conducted on the Gemini XII mission. Operational accuracies using such an instrument depend on many variables; for example, spacecraft window errors, as well as a man's ability to use the instrument, enter into the over-all accuracy prediction. The estimated capability of the sextantman combination was an error of 10 arc seconds. Results of the Gemini experiments for four sighting periods with the helmet off, consisting of 57 separate sightings between two stars, showed that the average error was 6.6 arc seconds. A sighting period with the helmet on and visor down consisting of 15 separate sightings resulted in a slightly increased error of 7.5 arc seconds. Additional hand-held sextant sighting experiments using lunar limb and landmark sightings are planned during an Apollo development flight.

Guidance Computer Technology.-The requirements of future long-term unmanned missions point to the necessity for systems requiring on-board guidance computation with long operating life and high reliability. This has led to studies to determine the methods by which computer reliability in guidance and control subsystems can be enhanced.

As a result of these studies, the Jet Propulsion Laboratory has initiated the development of self-testing and repair computer technology using redundancy techniques. The Westinghouse Corporation has developed, for the Jet Propulsion Laboratory, a triply redundant sequencer configuration shown in figure 330.

This experimental equipment is a functional duplicate of portions of the central sequencer used on the Mariner IV, but constructed here in redundant form incorporating an error detecting code for diagnosis and switching of redundant sub-units. This device will permit study of practical problems such as effects of voltage variations and noise on the system and will allow assessment of the effectiveness of self-repair and redundancy techniques in advanced computer design. The results of these studies and earlier guidance computer technology development on magnetic logic computers by the Jet Propulsion Laboratory are currently being applied to the development of a dually redundant, magnetic logic sequencer for the next Mariner mission.

Lunar Surface Navigation.-For manned exploration of the Moon beyond the immediate vicinity of the spacecraft, it will be necessary to provide the surface vehicle with means of navigating to a desired exploration site and returning to the spacecraft. Marshall Space Flight Center has carried out a planned investigation using system studies analysis and computer simulations to determine the requirements and possible methods of implementing such a navigation system. This effort has led to a dead-reckoning navigation system concept using a gyrocompass for heading reference, a computer to process compass heading data and vehicle odometer information to provide navigation information to the

The system also provides for the incorporation of a sextant for position fixing to correct system drifts. In order to resolve questions related to the human operation role and compatability between the navigation system and vehicle operation, a system as described above has been fabricated and installed in a field test vehicle as shown in figure 331. The vehicle experimental tests will

be conducted at the U.S. Geological Survey facility at Flagstaff, Arizona, under simulated lunar terrain conditions.

Method for Accurate Space Pointing Adopted.-I mentioned earlier that an important area for technical advancement is in providing satellites for space astronomy. This requires a system capable of achieving a high pointing accuracy. In addition, future large orbital spacecraft will need continuous rate and attitude control for maneuvering and holding a desired position.

A control system to do these tasks must be capable of generating large torques for maneuvers and very small torques for attitude and pointing control. It must also provide compensation for imbalances from motions of the astronauts or equipment, gravity gradient, and possibly for small aerodynamic forces.

Studies of these control demands for long duration missions lead to the conclusion that a device capable of storing momentum and exerting torque when needed would be very attractive. A large gyroscope or heavy flywheel is a good device for this purpose.

The Langley Research Center, working with the Bendix Corporation and the Garrett AiResearch Corporation, has developed a system of stored momentum for attitude control called a control moment gyro, or CMG for short. It uses constant-speed flywheels oriented through the use of two gimbals. A realignment of the spin axis of a flywheel by a low-torque motor creates a torque and reaction on the spacecraft causing it to change position or attitude. Through the use of three such flywheels with their spin axes normal to each other, we have a means for controlling and stabilizing the spacecraft in any position as desired. The results of this work have been so promising that the CMG has been adopted

the capability of the system and this will lead to future applications such as a manned orbital telescope or manned scientific laboratory, as illustrated by figure 332.

Reentry Vehicle Indirect Viewing Display.-Horizontal landing reentry vehicles face a difficult problem during the final phases of landing approach and flare. With current aerodynamic configurations, adequate pilot visibility is impossible to achieve through conventional means. Structural loads and aerodynamic heating encountered during reentry into the Earth's atmosphere preclude the use of conventional canopy type windows. The weight penalties associated with providing a jettisonable heat shield for a large canopy seriously detract from the desirability of this technique.

A program at the Flight Research Center has resulted in the flight evaluation of an optical system designed to relieve the landing visibility problem, Figure 333. This optical system provides the pilot with a 140 by 90 degree unobstructed field of view while exposing only eight square inches of transparent material to the external environment. The flight evaluation of this system was conducted on an F-104B aircraft and included approaches and landings very closely approximating the conditions that will be experienced by lifting reentry vehicles upon landing. The flight program, as shown in the data at the bottom of the figure, proved the feasibility of this concept of providing adequate landing visibility by indirect means. Continuing efforts are being directed toward further refinement of this concept and the exploration of other possible optical techniques.

Visual Simulation of Aircraft Landing.-Ground-based simulation of the visual scene for take-off and landing, as shown in Figure 334, is technologically limited. Resolution and dynamic range of the visual display are the primary constraints encountered during simulation of the landing approach of high performance vehicles. During an approach and landing, the pilot makes continuous fine judgments based on multiple cues derived from the visual scenes. The scene available through current high resolution television and quality optics is adequate for only