Radio and Optical Space Communications

ABSTRACT /? 5 3 _,Coherent electromagnetic energy has been used for many years atradio frequencies for communications and radar, more recently overextremely long distances in space. The invention of a high-coherencylight source has produced a quantum jump in the technology of com-munication systems at optical frequencies. In this Report the theoryand state of the art of laser sources is reviewed, with the possible appli-cation of these devices to space communications and long-range radar(or lidar). Sample optical and radio-frequency systems are analyzedin detail. Practical system and component problems which arise inoptical communication and lid"ar systems are discussed.I. INTRODUCTION Man has been communicating at light frequencies withsome objects in space for a good many years, althoughon a strictly one-way basis. We refer to astronomicalobservations of stars and other luminous sources and thebistatic radar observations of planets using the Sun as anilluminator. A sizeable amount of very important infor-mation has been collected in that way. Thirty years agoobservations of astronomical sources were extended tothe radio range of frequencies (Jansky), and sixteen yearsago an extraterrestrial object (the Moon) was broughtunder observation by radio-frequency radar techniquesusing an Earth-based illuminator (U. S. Signal Corps,DeWitt and Stodola; Z. Borg, Hungary). The principalobjective of all of the space communications has been toincrease man's knowledge of the Universe. Recently,beginning in 1958, a new experimental technique of spaceexploration by rocket-propelled vehicle has been added(Sputnik, et al.), which promises to be very useful, at leastthroughout the solar system. As with previous techniques,communication plays an essential role in the space vehiclemethod of exploration. To date, communication withspace vehicles has been provided almost entirely byradio-frequency devices, and quite satisfactorily, too.Recently, the invention of a high-coherency light source(predicted by Schawlow and Townes, 1958, first demon-strated experimentally by Maiman, 1960), has produceda quantum jump in the technology of communicationsystems at optical frequencies. As a result of that inven-tion, interest in the use of light in its traditional role ofproviding space communication has also taken a (high-energy) quantum jump. The purpose of this paper is todiscuss the probable role of light frequencies in providingcommunications for space exploration by spacecraft inthe next several years. We will also discuss lunar andplanetary radar/lidar.The use of Earth-based radar to explore heavenlybodies (apparently limited to the Sun and near-Earthplanets) is related to spacecraft communications for anumber of reasons. The results of radar measurementshelp to define the orbital and physical characteristics ofthe Moon or planets and some of the conditions of com-municating between them and Earth-information whichis very useful to a spacecraft designer. The ground-stationequipment of a lunar/planetary radar observatory bearsa remarkable resemblance to the equipment needed foreffective spacecraft radio communications (for example,the 1961 Venus radar experiments performed by the JetPropulsion Laboratory used the basic antenna, trans-mitter, receiving, and data handling equipment of theNASA/JPL Goldstone Deep Space InstrumentationFacility). Because the advent of the new high-coherency lightsource (the laser) is the real impetus for this paper, weshall begin with a discussion of its characteristics andsome of the equipment which would be used with it.