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This paper deals with astronomical and geophysical considerations applicable to optical measurements made with selfcontained equipment in a space vehicle in order to determine its position and/or velocity. Various measurements made from the earth, their temporal variations and their accuracy as applied to space navigation are discussed. Information regarding stellar positions, magnitudes, spectra and frequency distribution is presented. A discussion of planetary positional ephemerides, diameters, phase curves, albedos and spectral characteristics is given. The advantages of using asteroids vis-Ã -vis planets are discussed. There are several problems in observing terrestrial occultations of stars from a satellite due to the presence of the earth's atmosphere. Pertinent atmospheric characteristics, such as cloudiness, absorption, scattering and emission of radiation are discussed in the above context.