1989017910 "r PB89-159701 AtmospheRelevantric to PropaOpticgaationl CommunIssicautioes ns _ i (U.S.) National Oceanic and Atmospheric _i Administration, Boulder, CO i Jan 89 I ........................_i989017910-002 _U89-159701 _£_T OF"Cl NOAA Technical Memorandum ERL _TL-159 * 1_[ * %,o--=_ / ATHOSPHERIC PROPAGATION ISSUES RELEVANT TO OPTICAL COI_IUNICATIONS James H. Churnside Kamran Shatk Wave Propagation Laboratory Boulder, Colorado January 1989 noaa-----ATMOIM_IRIC ADMINISTRATION I'-La-borMori"-_" REPROOUCEDBY U,$, DEPARTMENT OF COMMERCE NATIONALTECHNICALINFORMATIONSERVICE SPRiNGFiELD,VA.22161 I IIIII .... _ '.............. 1989017910-003 BIBLIOGRAPHIC INFORMATION PB89-159701 Report Nos: NOAA-TR-ERL-WPL-159 Title: Atmospheric Propagation Issues Relevant to Optical Communications. Date: Jan 89 Authors: J. H. Churnside, and K. Shalk. : Performing Organization: National Oceanic and Atmospheric Administration, Boulder, CO. Wave Propagation Lab.**Jet Propulsion Lab., Pasadena, CA. Type of Report and Period Covered: Technical memo., L gupp!emenrary Notes: Prepared in cooperation with Jet Propulsion Lab., Pasadena, CA. NTIS Field/Group Codes: 45C, 84B Price: PC A04/HF A01 Availability: Available from the National Technical Information Service, Springfield, VA. 22161 Number of Pages: 57p Keywords: *Spacecraft communication, *Atmospheric scattering, *Optical communication, Light transmission, Detection, Atmospheric refraction, Turbulence, i Lasers, Attenuation, Fog, Haze, Wavelength, Molecular spectra, Adsorption, Irradiance, Space flight, Mathematical models, Particulates. Abstract: Atmospheric propagation issues relevant to space-to-ground optical ! communications for near-earth applications are studied. Propagation effects, current optical communication activities, potential applications, and communication techniques are surveyed. It is concluded that a direct-detection space-to-ground llnk using redundant receiver sites and temporal encoding is likely to be employed to transmit earth-senslng satel.ite data to the ground some time in the future. Low-level, long-term studies of link availability, fading statistics, and "_ I turbulence climatology are recommended to support this type of application. I i ' ' " 1989017910-004 _i NOAA Technical Memorandum ERL WPL-159 ATMOSPHERIC PROPAGATION ISSUES RELEVANT TO OPTICAL COF_VFUNICATIONS } James H. Churnslde Wave Propagation Laboratory Kamran Shalk i Jet Prcpulsion Laboratory :_ California Institute of Technology i::i Pasadena, California :._ .._ i! Wave Propagation Laboratory Boulder, Colorado . January 1989 _X UNITED STATES NATIONALOCEANICAND EnvironmentalResearch i DEPARTMENTOF COMMERCE ATMOSPHERICADMINISTRATION Laboratories ._ C.WllllamVerity WilliamE. Evans VernonE. Derr, Secretary Under Secretaryfor Oceans Director and Atmosphere/Administrator 1989017910-005 NOTICE i -i Mention of a commercial company or product does not constitute i an endorsement by NOAA Environmental Research Laboratories. i Use for publicity or advertising purposes of information from this publication concerning proprietary products or the tests of such products is not authorized, This work has been sponsored by the Jet Propulsion Laboratory under the auspicies of the National Aeronautics and Space Adrninistration (NASA) as a pa,t of the NASA Propagation Experimenters (NAPEX) program. Fnl _al¢ by ihe Nallonll Technical Infnrman_,n .%r',,:t. 321.( Porl Ro_'al Road Spain|field. %'A 2._161 -ii- ...........................................................................,......•............................................ 98961 z91.0_o06 _#r_ .... _ =,-._._ .... _ ._k-_-._r_....... -_._-_ -_.--_,_,__'_'_'_ _"- ¸- " : .... ' ...... _r '_.... -_--7 " CONTENTS _, Abstract ........................................................... 1 1. Introduction .......................................................... ! _ 2. Atmospheric Propagation Effects ................................. 3 2.1 Molecular Absorption ....................................... 3 2.2 Particulate Scattering ........................................ 7 _; 2.2.1 Scattering by air molecules .............................. 7 _ 2.2.2 Scattering by haze and thin clouds ....................... 9 2.2.3 Scattering by thick clouds and fog ....................... 10 2.3 Refractive Turbulence ...................................... 14 2.4 Background Light ......................................... 18 3. Survey of Optical Communication Activities ...................... 21 3.1 Theoretical Studies ........................................ 21 3.2 Experimental Programs ..................................... 24 4. Optical Communication Applications ............................. 27 5. Optical Communications in the Atmosphere ...................... 29 5.1 Scintillation ............................................... 29 5.2 Opaque Clouds ........................................... 30 i 5.2.1 Dispersed direct link .................................. 30 _ 5.2.2 Clustered direct link ................................... 31 5.3 Weather lViodels and Sirnulations ............................ 32 5.4 Other Diversity Techniques ................................. 33 6. Conclusions and Recommendations .............................. 33 7. References ................................................... 34 ' - Ill- 1 1989017910-007 Atmospheric Propagation Issues Relevant to Optical Communications James H. Churnside Wave Propagation Laboratory _ National Oceanic and Atmospheric Administration Boulder, Colorado 80303 Kamran Shaik Jet PropulsionLaboratory California Institute of Technology Pasadena, California 91109 ABSTRACT. Atmospheric propagation issues relevant to space-to-ground communications for near-earth optical applications are studied. Propaga- tion effects, current optical communication activities, potential applica- tions, and commtinication techniques are surveyed. It is concluded that a direct-detection space-to-ground link using redundant receiver sites and i, temporal encoding is likely to be employed to transmit earth-sensing sat- : ellite data to the ground some time in the future. Low-le_'el, long-term studies of link availability, fading statistics, and turbulence climatology r are recommended to support this type of application. ( 1. INTRODUCTION The transfer of information from space to the surface of the earth by means of light is not a new idea. For thousandsof years, visual observationsof the position of the sun, the moon, and the stars have been used to obtain information about the time of day and the season of the year. Similar observations were used to obtain naviga- tional information. Other observations, especially those made with magnifying optics in the last several hundred years, have gained information about the nature of extraterres- i trial bodies transmitted from those bodies to earth through the atmosphere at optical ! frequencies. i'_ From this body of experience (and other more casual observations), the qualitative ' features of the space-to-earth optical communication channel are known. For exarnple, ! we know that the atmosphere can produce a significant amount of background light ! : i + that can interfere with an optical signal. During the day, scattered sunlight is so bright + _* that stars cannot be seen. Even at night, a full moon is bright enough that only brighter stars are visible, and scattered city light can have the same effect. We also know that clouds can produce severe fades. Under thick clouds, no stars _ are visible. Even sunlight is perceived as a diffuse illumination rather than a distinct +: - I- 1989017910-008 1 !' disk. which implies that the inforrnation content of the optical signal is severely re- _ duced. Under other conditions, less severe faciescan be produced. Thin clouds or heavy haze can obscure faint stars and blur the disk of the sun, but still allow trans- mission of most of the optical information. This type of fade, whether severe or mod- erate, can persist for days. Refractive turbulence, caused by srnall-seale temperature fluctuations in the atmos- p, ere, can also cause fading of an optical signal. This effect, observed as twinkling of starlight, is much faster than aerosol-induced fading The area affected by each fade is also much smaller. If the signal is averaged over a finite disk, such as the sun, moon, or a planet, the twinkling is much less than that observed from a point source, such as a star. One property of the atmosphere that affects the quality of optical communication is not readily apparent to the unaided eye, optical radiation is directly absorbed by the atmosphere. Spectroscopic analysis of extraterrestrial sources has demonstrated that the arnot'.nt of absorption depends on the wavelength of the light and on the composition of the atmosphere. For many wavelengths the amount of water vapor in the atmos- phere is particularly in_portant. With the development of spaceflight and of the laser, the possibility of using this channel for ground-to-space and space-to-ground commurficatiorls was recognized. Several potential advantages were cited. The first was the enormous bandwidth. Green light has a frequency of 600,000 GHz and even a 1% modulation bandwidth implies incredible data transfer rates. This capability was expected to become more and more attractive as the radio spectrum became more crowded and more regulated under in- creasing demand for telecommunication services. Two aspects of optical communication links were especially attractive to military planners. One is the narrow beam width. A 10-era