316 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1960 



point because of the earth's rotation. For the purposes of tliis guid- 

 ance method, gravity is regarded as merely one component of the 

 resultant specific force, Avithout any particular attention to its direc- 

 tion as it is related to location on the earth. Time is also an input to 

 the system, but it is one of several factors for the computer, rather 

 than the means for determining the orientation of the earth. 



Satellites may be put into orbit by inertial guidance, but inertial 

 equipment is not essential but may be helpful during long periods of 

 coasting flight. Very probably, inertial devices will be useful in 

 sensing angular- velocity inputs for control purposes. It is also likely 

 that gyroscopic stabilization w^ill be utilized to assist radiation- 

 contact devices carried by satellites in tracking the earth and other 

 celestial bodies. 



INTERPLANETARY GUIDANCE 



Interplanetary vehicles must operate for long periods of time with 

 only tenuous radiation contacts wdth either their points of departure 

 or their destinations. During the midcourse phases of long inter- 

 planetary trips, it may be desirable for the guided vehicle and its 

 equipment to be able to operate without outside assistance from manned 

 installations. Conditions will be close to ideal for radiation contacts 

 with celestial bodies, and techniques are well developed for acquiring 

 and tracking sources of optical wavelengths. Data from the operation 

 of optical trackers, combined with accurate time from devices based 

 on the natural-frequency vibrations of atoms or elastic bodies and 

 with almanac data stored in digital computer memories, provide all 

 the information needed for the accurate navigation of space vehicles. 

 In effect, celestial-space coordinates are used for this purpose, with 

 locations of the guided vehicle determined from the angular relation- 

 ships among lines of sight to the sun, the stars, the observable planets, 

 and satellites such as the moon. These lines of sight can be automat- 

 ically sought out by optical trackers and may be maintained either by 

 continuous tracking or by the use of gyroscopically controlled refer- 

 ence members with their orientations set from radiation-contact 

 information. 



For example, the process of navigation might start by acquiring and 

 maintaining the line of sight from the vehicle to the sun by means of a 

 heliotracker (pi. 4). The second step would be searching for a se- 

 lected star over a conical surface about the heliocentric line by means of 

 a star tracker whose angle setting is based on an accurate indication of 

 time and data stored in the memory of a computer. The angle meas- 

 ured between the lines of sight to the sun and this star establishes in 

 celestial coordinates one of the cones of position shown in plate 4. 

 This process is repeated with a second star. Then the angle between 



