Conf-7905143 Workshop on Satellite Power Systems (SPS) Effects on Optical and Radio Astromony April 1980 U.S. Department of Energy Office of Energy Research Satellite Power System Project Division DOE/NASA Satellite Power System Concept Development and Evaluation Program
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CONF-7905143 Dist. Catg. UC-13, 34b Workshop on Satellite Power Systems (SPS) Effects on Optical and Radio Astronomy April 1980 Battelle, Seattle Conference Center Seattle, Washington May 1979 Edited by: P. A. Ekstron and G. M. Stokes Pacific Northwest Laboratory Richland, Washington 99352 Under Contract No. KD-03-82029 Prepared for: U.S. Department of Energy Office of Energy Research Satellite Power System Project Division Washington, D.C. 20545 DOE/NASA Satellite Power System Concept Development and Evaluation Program
CONTENTS Page INTRODUCTION AND SUMMARY............................................ vil WORKSHOP OPENING STATEMENT ........................................ xix INTRODUCTION .................................................................................................... 1 BRIEFING DOCUMENT ........................................................................................... 9 INTRODUCTION ........................................................................................... 11 GENERAL CHARACTERISTICS OF THE SPS REFERENCE SYSTEM . . . 12 SATELLITE OPTICAL EFFECTS ................................................................ 13 A. DIFFUSE REFLECTION ................................................................ 13 B. SPECULAR REFLECTION OF SUNLIGHT .............................................. 14 C. DIFFUSE SKY BRIGHTNESS................................................................. 15 D. SATELLITE THERMAL INFRARED EMISSION ..................................... 17 E. IONOSPHERIC INFRARED AND OPTICAL EMISSIONS ... 17 RADIO EFFECTS..................................................................................................... 19 A. SATELLITE MAIN POWER BEAM................................................................. 19 B. SATELLITE HARMONIC RADIATION .............................................. 20 C. SATELLITE NOISE RADIATION ....................................................... 20 D. RECTENNA POWER BEAM SCATTERING .............................................. 22 E. RECTENNA HARMONIC RADIATION .............................................. 22 F. RECTENNA NOISE RADIATION ....................................................... 22 APPENDIX A - TIME-VARYING BRIGHTNESS OF THE SOLAR COLLECTING ARRAY 24 INVITED PRESENTATIONS ON SPS EFFECTS ON OPTICAL ASTRONOMY ... 27 LIMITATIONS OF THE BRIEFING DOCUMENT'S CHARACTERIZATION OF THE SPS REFERENCE SYSTEM - G. M. Stokes........................................................ 29 COMMENTS ON THE EFFECTS OF INCREASED DIFFUSE SKY BRIGHTNESS ON FAINT OBJECT ASTRONOMICAL OBSERVATIONS - J. S. Gallagher and S. M. Faber 33
EFFECTS OF THE SATELLITE POWER SYSTEM ON GROUND-BASED ASTRONOMICAL TELESCOPES - P. B. Boyce............................................. 39 INFRARED ASTRONOMY - D. A. Harper................................... 45 POSSIBLE IMPACTS OF THE SPS ON THE SPACE TELESCOPE - E. J. Groth . 47 REPORT OF THE OPTICAL ASTRONOMY WORKING GROUP .................... 51 THE NATURE OF ASTRONOMICAL OBSERVATIONS ......................... 54 THE ORIGIN OF SPS EFFECTS ON OPTICAL ASTRONOMY .... 64 IMPACT THRESHOLDS OF THE SPS ON OPTICAL ASTRONOMY .... 69 EFFECTS ON OPTICAL ASTRONOMY ................................... 75 RECOMMENDATIONS AND REMEDIES ................................... 77 REPORT ON SPS EFFECTS ON AERONOMY........................................ 81 INTRODUCTION .................................................. 83 EFFECTS ON AERONOMY OBSERVATIONS - K. Clark .................... 85 INVITED PRESENTATIONS ON SPS EFFECTS ON RADIO ASTRONOMY ... 95 MICROWAVE POWER TRANSMISSION SYSTEM - G. D. Arndt .... 97 SPS NOISE AND HARMONICS - R. M. Dickinson........................ 103 SPS-GENERATED FIELD STRENGTHS AT 2.45 GHz TYPICAL EFFECTS - W. Grant ............ 119 INTERFERENCE EFFECTS ON RADIO ASTRONOMY EQUIPMENT - W. C. Erickson 123 POSSIBLE OVERLOAD AND PHYSICAL DAMAGE OF A RADIO ASTRONOMY RECEIVER CAUSED BY THE SPS - H. Hvatum....................................... 127 POTENTIAL IMPACT OF OUT-OF-BAND RADIATION FROM THE SATELLITE POWER SYSTEM AT ARECIBO OBSERVATORY - M. M. Davis........................ 131 THE EFFECTS OF THE PROPOSED SATELLITE POWER SYSTEM ON THE VLA - A. R. Thompson ........... 135 SATELLITE POWER SYSTEM EFFECTS ON VLBI - B. F. Burke ... 143 CONSIDERATIONS REGARDING DEEP SPACE COMMUNICATIONS AND THE SPS - N. de Groot ........... 147
REPORT OF THE RADIO ASTROMONY WORKING GROUP ......................... 153 SUMMARY STATEMENT ............................................. 155 ACTUAL PROPERTIES OF THE MICROWAVE POWER TRANSMISSION SYSTEM . 158 ASSIGNMENT OF SPS HARMONIC FREQUENCIES ......................... 159 TIME-VARIABILITY OF SPS OFF-AXIS RADIATION AND INTRINSIC MULTIPLE SATELLITE EFFECTS ............................................. 159 THE "RUSTY BOLT EFFECT"............................................ 160 INTERFERENCE REJECTION PROPERTIES PECULIAR TO SYNTHESIS ARRAYS (AS THE VLA)...................................................... 161 SITING CONSIDERATIONS ........................................ 162 ON MOVING RADIO ASTROMONY TO THE LUNAR FAR SIDE . . . . 164 APPENDIX A - CCIR REPORTS ON RADIO ASTRONOMY (Reports 223-4 and 224-4)................................................................ A.l APPENDIX B - CCIR REPORTS ON DEEP-SPACE RESEARCH (Reports 365-3 and 685)..................................................................... B.l APPENDIX C - EFFECT OF SOLAR POWER SATELLITE TRANSMISSIONS ON RADIO ASTRONOMICAL RESEARCH ............................................. C.l APPENDIX D - NATIONAL ACADEMY OF SCIENCES REPORT ON SPS EFFECTS . . D.l APPENDIX E - ENVIRONMENTAL CONSIDERATIONS FOR THE MICROWAVE BEAM FROM A SOLAR POWER SATELLITE ............................................. E.l ACKNOWLEDGMENTS .................................................. 255
WORKSHOP ON SATELLITE POWER SYSTEMS EFFECTS ON OPTICAL AND RADIO ASTRONOMY Held at BATTELLE SEATTLE CONFERENCE CENTER May 1979 Edited by GM Stokes PA Ekstrom August 1979 Participants GD Arndt - National Aeronautics and Space Administation B Balick - University of Washington NF Barr - SPS Project Office - DOE P Boyce - American Astronomical Society BF Burke - MIT KC Clark - University of Washington KC Davis - Pacific Northwest Laboratory M Davis - Arecibo Observatory NF de Groot - Jet Propulsion Laboratory RM Dickinson - Jet Propulsion Laboratory PA Ekstrom - Pacific Northwest Laboratory WC Erickson - University of Maryland SM Faber - Lick Observatory, University of California JS Gallagher - University of Illinois W Grant - Institute for Telecommunication Sciences EJ Groth - Princeton University JP Hagen - Pennsylvania State University DA Harper - Yerkes Observatory, University of Chicago WE Howard - National Science Foundation (Division of Astronomical Sciences) AT Moffett - Owens Valley Radio Observatory RO Pi land - National Aeronautics and Space Administration GM Stokes - Pacific Northwest Laboratory RA Stokes - Pacific Northwest Laboratory GW Swenson - University of Illinois Observatory AR Thompson - VLA Project - National Radio Astronomy Observatory A Valentino - Argonne National Laboratory
INTRODUCTION AND SUMMARY BACKGROUND This report summarizes the proceedings of a workshop on the potential impact of the conceptual satellite power system on astronomy. The workshop addressed two questions: "What will the SPS look like to an observer?” and, "What will that mean to astronomy?" It was organized by the U.S. Department of Energy (DOE) and Pacific Northwest Laboratory (PNL) (operated by Battelle Memorial Institute) and held at the Battelle Seattle Conference Center on May 23 and 24, 1979. This workshop and report were produced under the electromagnetic compatibility subtask of the environmental assessment portion of the joint DOE/NASA Satellite Power Systems (SPS) project. The SPS concept has been suggested as a possible new energy source which, if fully developed, could provide a source of power equal to all the electrical energy generated in the United States in 1975. The energy would be collected by building and operating satellites equipped with large solar arrays in geostationary orbits around the earth. In the present version, solar energy would be converted to microwaves and transmitted from space to earth. Earth receiving stations would convert the microwave energy to electricity, which could be fed directly into utility networks. Each satellite/ receiving station combination would provide approximately 5 GW of electric power. Other transmission systems, such as lasers, also are being considered, but this workshop considered only rhe microwave version. The workshop considered the SPS design concept described in the "Reference System Report" of October 1978 (DOE/ER-0023). The Reference System’s purpose is to serve as a common basis for further technological development
(systems definition and critical supporting investigations), preliminary environmental and societal assessments, and comparative analyses of the SPS concept and other national energy ventures. For the most part, the Reference System is based on fully-matured engineering precepts (methods, materials, practices, etc.) and realizable projections of future improvements. However, it is by no means an optimized engineering design, and does not account for newly emerging technologies which might become standard practices in the post-2000 era. Continuing systems definition undoubtedly will change many of the current characteristics of the Reference System. Some of those changes can already be reasonably perceived, but others most likely will occur that cannot yet be appreciated. Thus some potential problems associated with the present Reference System may subsequently become moot, and new ones will be recognized as development continues. Despite its current limitations, the Reference System is an important tool for identifying and evaluating significant side effects which conceivably could accompany SPS. The general features of the current microwave-based SPS Reference System which are of concern to astronomy are: • The Reference System consists of 60 satellites each of whose solar collecting area is 55 km^2 . • The energy would be transmitted to earth as microwaves at a frequency of 2.45 GHz.
• Materials would be assembled at work stations and staging areas in low earth orbit (LEO), and the satellites would be constructed in geostationary orbit (GEO). • Some energy would be lost in each stage of energy conversion and transmission. That is, the solar array would not absorb all of the energy striking it, the transmitting antenna would not radiate all of the energy absorbed by the solar array, the ground receiving station would not receive all of the energy transmitted, and the receiving station would not collect all of the energy illuminating it. WORKSHOP ORGANIZATION During the process of organizing and conducting the workshop, three distinct kinds of documents were generated. All three bound together form the workshop proceedings. The first is the Workshop Briefing Document which was the basis for the discussions, invited contributions, and reports. This document was prepared by Battelle on the basis of the SPS Reference System Report and on calculations made to elucidate system features relevant to astronomy. The primary goal of the Briefing Document, like the Reference System Report, was to provide a set of consistent parameters for discussing SPS. A tabulation of the most important parameters used in the Briefing Document is shown in Table 1. The Briefing Document is by no means final, of course, since it is based almost entirely on the current Reference System. The second document is the report on topics that the workshop organizers felt would be important areas of discussion. These reports were prepared by the participants and were based on the characterization of the system as described in the Briefing Document.
TABLE 1. Data Relevant to the SPS Reference System Geosynchronous Orbit Altitude Radius Satellite Spacing in Orbit Inclination of Planned Orbit Planned Number of Satellites Delivered Power Per Satellite - Rectenna Pair Dimensions of Solar Collector Blanket Area of Solar Collector Blanket Area of Transmitting Antenna Array Power Transmission Frequency Solid Angle Subtended by Satellite seen from Surface of Earth at Sub-Satellite Point . Angle Subtended by Satellite Solid Angle Subtended by the Transmitting Array Angle Subtended by Transmitting Array Angle Subtended by Earth seen from Synchronous Orbit Angle Subtended by Sun, Average Solid Angle Subtended by Sun, Average Satellite Solid Angle as a Fraction of Sun Solid Angle Antenna Solid Angle as a Fraction of Sun Solid Angle Illuminance of Brightest Moonlight as a Fraction of Noon Sunlight Illuminance of Venus at Maximum Brilliance (m = -4.3) as a Fraction of Noon Sunlight Effective Aperture of an Ideal Isotropic Antenna at 2.45 GHz (X = 0.122 m) X2/4n
Following an introductory statement, the invited contributions were presented and discussed. The group was then divided into radio and optical working groups for more detailed consideration of SPS effects on astronomy. The form and content of the working group meetings were largely left to the members of each group with one exception. Both groups were specifically asked to comment on the possibility of moving the affected portions of astronomical observations to facilities located in space or on the far side of the moon. The reports of the working groups make up the final document in the workshop proceedings. The objective of the meeting was to identify the potential impacts of the SPS on astronomy and to do so in a fashion that would allow system designers to recognize and modify those aspects of the system that create potential problems to the extent it may be possible to do so. The actual content of these various documents fell into two natural divisions: optical and radio effects. From one viewpoint, this division is in keeping with an astronomical tradition of dividing the profession according to the portion of the electromagnetic spectrum that is observed. From another viewpoint, the division expresses the separate effects of the passive and active properties of the satellite system. In particular, the major optical effects caused by SPS would be functions of the system’s structures in orbit and would continue even if the system were turned off. Radio astronomy, however, would be particularly affected by the active portion of the system — the intended microwave transmission of energy from space to earth. In terms of design, both optical and radio astronomy impacts are primarily a result of unintentional side effects. The optical effects would occur because the SPS solar blankets would reflect some of the light that strikes them. The radio effects would occur because a small portion of the transmitted energy
would not be confined to the narrow beam from the orbiting antenna to the earth rectenna, or to the assigned portion of the radio spectrum. BASIC CONCLUSIONS OF THE WORKSHOP The effects on astronomy discussed below must be understood in the general context of how astronomical research is conducted. Participants at the workshop continually emphasized that virtually all of our knowledge about the Universe outside of the Solar System has been obtained by studying the electromagnetic emissions of celestial objects. Because the most distant objects are also the faintest, all branches of astronomy have attempted to develop the most sensitive detectors possible. Because these detectors are so sensitive, they are limited by interference due to other sources of radiation. The effect of the SPS would be to substantially increase the amount of man-made interfering radiation. This would further limit the astronomer’s ability to observe faint objects and thus the size of the measurable Universe. The primary effect on optical astronomy is attributed to increased sky brightness. The increase in sky brightness comes from sunlight which would be reflected from the SPS solar cell blanket. The amount of light scattered from a satellite is measured by its diffuse albedo, which is simply the amount of scattered light expressed as a percentage of the total incident light. Using the lowest estimates of light scattering for the conceptual SPS design, an albedo of 4 percent, each satellite would be as bright as the planet Venus at its brightest. This would make the satellites the third brightest objects in the sky, only the sun and the moon being brighter. The magnitude of the effect is a function of SPS design parameters. Any increase in the brightness of the sky results in a proportional reduction in the effective aperture of a telescope when it is being used on
faint sources. The predicted increases of sky brightness from sixty satellites suggest that at a minimum any observatory would be prevented from effectively observing faint sources in a 10 degree by 70 degree band defined by the line of satellites. There would also be a noticeable effect on observation over a region more than 60 degrees by 90 degrees (approximately half of the night sky). For radio astronomy and deep space research there are three potential major effects. Microwave radiation leaking from a single satellite’s power beam could temporarily overload or permanently damage sensitive receivers used for radio observation. This effect would prevent successful operation of centimeterwave radio telescopes located too close to SPS power receiver (rectenna) locations or to regions of high leakage. Necessary avoidance distances may be hundreds of kilometers, and even at those distances some problems may remain. The effect would also prevent successful operation of such telescopes pointed too near the line of power satellites. The magnitude of this effect can be influenced to a limited extent by the design of the radio telescope, and by the design of the SPS. The second major effect arises if power beam leakage from two or more satellites were received simultaneously by a single radio telescope. Depending upon SPS design, the result could be a slow, partly random variation in receiver properties. This could be extremely difficult to distinguish from natural astronomical processes. As a result, multi-satellite power beam leakage effects could do markedly greater harm. The third major effect arises from unintentional radio emissions associated with massive amounts of microwave power, or with the presence of large, warm structures in orbit. These emissions from power satellites would
make the satellites appear as individual stationary radio sources, unlike natural radio sources. Emissions originating at the power receiving (rectenna) arrays could be much like other terrestrial sources of interference. Emissions in the allocated radio astronomy bands are subject to constraints under international treaty. Emissions at other frequencies can also harm a substantial number of important radio astronomy observations that occur at spectral lines and frequencies of opportunity outside the protected radio astronomy bands. While the potential effects of SPS on astronomical research are quite diverse, particularly as they apply to the radio and optical regimes of the electromagnetic spectrum, there are two important effects of common origin that would affect both areas of research. The satellites would be in geostationary orbits and occupy the same portion of the sky at all times. Therefore, a fixed region of the sky would not be usable for astronomical research. The size of the region depends on the design of the satellites, the particular observation being made, and the kind of instrumentation being used. The second effect is that the source of electromagnetic interference and light pollution would be high in the sky. As a result, the general strategy of placing observatories in remote locations to avoid local interference and light pollution effects would be very little help in mitigating SPS effects on astronomical observations. Finally, optical effects resulting in increased sky brightness would affect not only optical astronomy, but aeronomy as well. Aeronomers study the physics and chemistry of the upper atmosphere by observing naturally occurring optical emissions such as airglow. This is difficult to distinguish from other increases in night sky brightness. It was concluded that a substantial fraction of faint airglow studies are incompatible with the current SPS Reference System.
RECOMMENDATIONS Beyond the conclusions noted above, the working groups made several recommendations for further study to account for information which is not yet adequate for a complete assessment of SPS effects on astronomy and aeronomy. Satellites as a source of light pollution are a phenomenon new to optical astronomy. While an attempt was made to assess as much of the potential impact as possible, the optical astronomy working group recommended four areas for future study: 1. Diffuse Reflectivity - Most of the effects discussed by the group are a direct function of the diffuse reflectivity. Baffling systems should be investigated as a way of lowering the reflectivity. Each evolving design for power satellites should include a calculated meaningful estimate of the reflectivity and its potential for change over the orbital lifetime of the system. If active baffling were adopted, the effect of its failure or any system failure on the reflectivity should be estimated. 2. Low Earth Orbit Structures - It was recommended that the design of these structures be brought to a level which would permit their reflectivity to be computed and the impact assessed.* 3. Atmospheric Effects - Calculations of the effect of the satellites on sky brightness should be carried out for those meteorological conditions appropriate to real observatories.* * Since the workshop, the Department of Energy has initiated a study to characterize reflected light from the SPS Reference System, including structures in low earth orbit. Also, a study of tropospheric light scatter has been started.
4. Ionospheric Heating - Ionospheric and atmospheric heating calculations should be used to estimate the effects of emitted optical and IR radiation on astronomy. Although a substantial basis already exists for quantitative evaluation of SPS radio interference, uncertainties concerning properties of SPS and radio astronomy equipment still remain and in several cases preclude quantitative estimates of effects. Among the areas the radio group recommended for further it study were: 1. Noise Radiation - Uncertainties in the noise levels in the protected bands make it clear that noise measurements for any planned system should be made at an early stage. 2. Effect on the Very Large Array and Arecibo Facilities - Current engineering data are not adequate to determine the level of 4.9 GHz second harmonic interference to these two unique facilities. This potential problem should be given careful study. 3. Rectenna Siting - Rectenna sites have associated leakage. Its effect on existing facilities should be investigated. 4. Reradiated Energy - Rectenna arrays would reradiate energy at various frequencies in the radio spectrum. Their properties are not yet sufficiently defined to allow a meaningful assessment of the consequences of this radiation. Both the optical and radio working groups further recommended an ongoing panel to continually evaluate the impact of SPS as system designs evolve. * All these areas are being considered in the current electro-magnetic compatibility task of the SPS Environmental Assessment Program.
MITIGATION Due both to the lack of specific SPS data relative to interference with astronomy and the limited time available for the workshop, mitigation possibilities were not considered in detail. A specific request was made, however, to consider the use of space-based facilities to compensate for interference with earth-based ones. The discussion of space astronomy as a mitigation strategy was quite different for each of the two working groups. The optical group noted that the development of the technology required for the SPS should make it both easier and cheaper to construct and maintain space telescopes. It is also recognized that a great deal of the future of astronomy will depend on developing space astronomy beyond current and planned levels and that some kinds of astronomy can only be done from space. Two problems were noted in association with any proposal that space astronomy might serve as a substitute for lost capability of ground-based telescopes. First, ground-based facilities have historically been used to complement those studies made from space, and SPS could affect that interaction by decreasing the effectiveness of ground-based facilities. Second, it is important to recognize that astronomy is an observational rather than an experimental science. It has not always been obvious what the critical observations are or what the best instruments to pursue them will be. As a result, the diversity of astronomy has been an important source of vitality in research. The working group felt that while astronomy from space is important and desirable, creating a single space facility to replace ground-based facilities would not preserve this vitality.
The radio working group, however, concluded that the reconstruction and operation of several hundred million dollars’ worth of existing ground- based radio facilities on the lunar farside would be so expensive that it is not realistic. The following points were noted with regard to mitigating possible SPS effects on earth-based observations: 1. Because of SPS’s orbital location, distance and terrain cannot be used to isolate observatories from the source of interference as has historically been done for earth based interferers. However, effects on radio observatories could be minimized by providing maximum separation between them and SPS rectenna sites (expected to be local interference sources) and locations of strong SPS microwave leakage. 2. Modification of existing radio astronomy receiving systems to reduce SPS interference, e.g., by addition of filters, is possible but would result in some degradation of receiver performance. As mentioned earlier, SPS is not yet fully developed. Mitigating strategies appropriate to reducing potential impacts on astronomy should be accounted for principally by satellite and rectenna designs and engineering practices, including compliance with regulations governing the shared use of the electromagnetic spectrum. The early recognition of potential problems, such as is possible through workshops like the one reported here, is especially important in providing guidance for future SPS development and electromagnetic compatibility.
WORKSHOP OPENING STATEMENT Ladies and Gentlemen, welcome. In the interest of precision, let me read a short opening statement. It is the last time I plan to read to you. Our job at this workshop is to elucidate the probable effects of the proposed Satellite Power System on observational astronomy. Each of you invited participants is here for at least two reasons, one technical and one political. Each of you has some technical information or special expertise to contribute to our efforts. Each of you also has experience as a working scientist in one of the areas of observational astronomy which may be affected. You are here not only to contribute information and skill, but also to represent the concerns of your colleagues, to make their voices heard in the complex process of deciding what, how, and whether a Satellite Power System should be. The output of this workshop will be a report entitled Satellite Power System Effects on Optical and Radio Astronomy. This opening statement will be its preface. About a month ago each of you received a briefing document that outlined our best information on the nature and emissions of the SPS satellites and ground-based components. That document, including any corrections suggested here, will form the first major section of the report. A number of you have been asked to prepare presentations on various SPS effects and to bring with you written versions of those presentations. These will form the second major section. The remainder of the report will consist of those additional items that you contribute or develop here during the workshop. We have provided the first section and organized the second. The third is yours to write. As I have said, each of you has an axe to grind. You were chosen because you know and care about some area of observational astronomy. When we called around, asking your colleagues who should represent this or that aspect of the SPS-effects problem, yours were the names which kept cropping up. Presumably you consented to come and work here because you hope to help preserve the environment in which you do your research. To accomplish that goal, we need to answer two questions: "What will the SPS look like to an observer?" and, "What will that mean to astronomy?"
In answering the latter question, bear in mind that this report will be read by many people who do not much care about astronomy, and do not necessarily understand the significance and value of data on an object such as a galaxy. It will also be read by persons who care so much about astronomy that no other consideration--such as a society's need for energy--weighs very heavily. If our report is to be influential, both groups must be able to agree that the issues have been addressed fairly. As you bear in mind this diverse audience, remember as well that any eventual SPS built 20 years from now will likely differ from the Reference System in many ways we cannot now anticipate. The most useful kinds of statements will be those which are not only obviously fair and penetrating, but also easily applied to all future system variants that may be proposed. We have our work cut out for us. Thank you. (These remarks delivered by R. A. Stokes.)
BACKGROUND This document reports the proceedings of a workshop on the potential impact of the proposed Satellite Power System (SPS) on astronomy. The workshop was organized by the U.S. Department of Energy (DOE) and Pacific Northwest Laboratory (PNL) (operated by Battelle Memorial Institute) and held at the Battelle Seattle Conference Center on May 23 and 24, 1979. The workshop was conducted and the report prepared under the electromagnetic compatibility subtask of the environmental assessment portion of the joint NASA/DOE Satellite Power System project. While the possible effects of the SPS on radio astronomy had been discussed in an initial assessment of the electromagnetic compatibility of the SPS (PNL-2482), it was decided that it would be useful to convene a workshop to more broadly assess the effects on astronomy in general. It was felt that such a workshop would be the most effective way to involve the astronomical community as a whole in the assessment of SPS. The goal was to keep the investigation of the SPS as open a process as possible. Second, it had become apparent at both DOE and PNL that there were potential adverse effects of the SPS on optical astronomy that did not fall under the clear responsibility of any of the subtasks in the environmental assessment. Responsibility for optical effects was subsequently added to PNL1s subtask. ORGANIZATION OF THE WORKSHOP The Workshop Briefing Document (see p. 9)provides the basis for the discussions, invited contributions, and reports that appear in this proceedings. This document was prepared by P. A. Ekstrom and G. M. Stokes on the basis of the October 1978 SPS Reference System Report (DOE/ER-OO23) and on calculations they made to elucidate system features not well covered in the report. The primary objective of both the Briefing Document and the Reference System Report is to provide a set of numbers that could serve as a point of reference for discussion of SPS. It is essential to understand that the INTRODUCTION
Briefing Document is by no means final. While a conscientious attempt was made to identity and quantify those system parameters that are important to astronomy, the document is based on the reference system. As mentioned in the Briefing Document, major design changes are 1ikely--perhaps as a result of this assessment effort--and these changes may affect assumed system properties in a major way. For example, G. D. Arndt's invited presentation includes design changes that reduce grating side lobe intensities by a factor of 10 from those given in the Reference System report. Many of the workshop participants were asked to prepare reports on specific topics that we felt would be important areas of discussion. These reports were to be based on the characterization of the system as described in the Briefing Document. Each participant was provided with a copy of both the Briefing Document and the Reference System Report about three weeks prior to the workshop. The workshop itself was convened with a statement read by R. A. Stokes of PNL which serves as the preface to this report. Following this introductory statement, the invited contributions were presented and discussed. This process took most of the first day, after which the participants were divided into optical and radio working groups for more detailed consideration of SPS effects on astronomy. The form and content of the working group meetings were largely left to the members of each group with one exception: Both groups were specifically asked to discuss and comment on the possibility of moving the affected portions of astronomical observations to facilities located in space or on the far side of the moon. ORGANIZATION OF THE REPORT The entire process of convening, conducting and summarizing this workshop fell into two natural divisions from the outset. We have, with reasonable accuracy, described these two sections as optical and radio effects, respectively. From one viewpoint, this division is in keeping with an astronomical tradition of dividing the profession according to the portion of the electromagnetic spectrum that is observed. From another viewpoint, the division expresses the separate effects of the passive and active properties
of the satellite system. In particular, the major optical effects caused by the system are a function of the very existence of the system's structures in orbit and would continue even if the system were turned off. Radio astronomy, however, bears the brunt of the active portion of the system--the intentional transmission of energy to the ground in the form of microwave radiation. In terms of system design, the impacts on both optical and radio astronomy are a result of the fact that SPS is not "perfect". The optical effects arise because the SPS solar blankets will not absorb 100% of the light that strikes them; the radio effects come from the inability to confine all of the transmitted energy either to the narrow cone that connects the orbiting antenna to the rectenna on the ground, or to the assigned portion of the radio spectrum. Another difference between radio and optical astronomy is manifest in their history of dealing with problems such as those presented by the SPS. Radio astronomy is a member of a very large, international community that uses the radio frequency portion of the electromagnetic spectrum. Because these users include both emitters and receivers of radiation that could potentially interfere with each other, the spectrum is managed by the International Telecommunications Union (ITU) which exists by virtue of international treaty. The management process consists of assigning portions of the spectrum to specific classes of users and setting strict standards on the extent to which other users may interfere with certain portions of the spectrum. Radio astronomy has several such bands assigned to it that must be viewed as regions protected by international law. Radio astronomy also has a tradition of vigorous action against those who infringe on these bands. This history of spectrum management has created within radio astronomy a collection of individuals who are specialists in this area. Many of the workshop participants are such specialists and as a result many of the individuals in the radio astronorny working group had served together on similar committees, discussed the effects of satellites on radio astronomy in other contexts, and discussed SPS previously, with some involved in the writing of the NAS/CORF report. The legal basis for protection of radio astronomy and the experience of the radio workshop participants had several effects. The standards on interference in the protected bands that the SPS must meet already exist.
The group felt that the SPS would have an extremely difficult task meeting these standards. Partly as a result of focusing on the protected bands, the effects on radio observations outside the protected bands may not have been treated in as much detail as might be eventually required. This viewpoint was, in fact, probably the most appropriate one for a workshop of this duration. The section in the radio working group report on the "Rusty Bolt Effect" illustrates the fact that all of the ways in which the system could create radio frequency interference in protected bands will not be known until the SPS is actually turned on. Optical astronomy has no such history of enforced protection of observations from interference. Optical astronomers who are interested in faint sources have simply tried to avoid strong sources of light in the planning of their observations and the construction of observatories. They observe at places that are as far removed from large cities as was practical at the time of the construction of the observatory. As with out-of-band radio astronomy, terrain shielding is the method by which optical astronomers attempt to protect their observations from interference. Because of this difference in the history of optical and radio astronomy, existing light pollution standards for optical astronomy are few and have sanction under municipal law only in isolated instances. There are very few light pollution specialists in optical astronomy. As a result, much of the discussion of the optical group centered on setting reasonable standards for light pollution that could be used in this assessment of SPS effects. The concept of "impact thresholds" was developed in this context, and whereas it does not have the quantitative basis that the radio regulations do, the concept should be useful in assessing satellite designs. A final major difference between the optical and radio working group reports can be found in the extensive discussion of the conduct of optical astronomical observations. The optical working group felt that it was necessary to provide sufficient background to allow someone unfamiliar with astronomy to understand the working group report. On the other hand, since the radio working group felt that the discussion of astronomy in the CCIR 224-4 report served the same function for its report, the CCIR report is included here as an appendix to the radio astronomy section.
Once the optical astronomy working group convened, it became increasingly clear that optical aeronomy, represented by K. Clark of the University of Washington, deserved attention beyond that which could be provided in the optical astronomy report. As such, aeronomy has been separated out as a third topic of the workshop. This separation is quite appropriate because a large fraction of the natural background sky brightness comes from aurorae and airglow phenomena studied by aeronomers. Since the effects studied produce relatively diffuse light emission which is especially difficult to distinguish from other increases in sky brightness, the thresholds for impacts on aeronomy are lower than they would be for optical astronomy. BASIC CONCLUSIONS OF THE WORKSHOP While the effects of the SPS on astronomical research are quite diverse, particularly as they apply to the radio and optical regimes of the electromagnetic spectrum, there are two important effects that have a common origin that will affect both areas of research. The geostationary character of the satellite orbits means that the satellites will occupy the same portion of the sky at all times. Therefore, a fixed region of the sky will be unusable for astronomical research. How large this region is depends on the design of the satellites, the particular observation being made, and the kind of instrumentation being used. A further effect is that placing the source of electromagnetic interference and light pollution relatively high in the sky will essentially eliminate terrain shielding as a strategy to combat pollution and interference effects. The primary effect on optical astronomy was attributed to increased sky brightness. Any increase in the brightness of the sky results in a proportional reduction in the effective aperture of a telescope when it is being used on faint sources. The predicted increases of sky brightness suggest that at a minimum any western hemisphere observatory will be prevented from effectively observing faint sources in a region that covers 10° in declination and 70° in hour angle surrounding the line of satellites. There will also be, again as a minimum, a noticeable effect on observation over a region that covers more than 60° in declination and 90° in hour angle, approximately half
of the night sky. The magnitude of this effect is a strong function of SPS design parameters, and it appears that for more likely design parameters, in particular for a likely increase in the diffuse albedo of the satellites, the effect will be much greater. For radio astronomy and deep space research there are three major effects. Microwave radiation leaking from a single satellite's intentionally generated power beam can temporarily overload or permanently damage the sensitive receivers used for radio observation. This effect will prevent successful operation of centimeter-wave radio telescopes located too close to power receiver (rectenna) locations or to regions of high leakage (grating side lobes). Necessary avoidance distances may exceed hundreds of kilometers. The effect will also prevent successful operation of such telescopes pointed too near the line of power satellites. The magnitude of this effect can be influenced to some extent by the design of the radio telescope, and to a smaller extent by the design of the SPS. The second major effect arises when power beam leakage from two or more satellites is received simultaneously by a single radio telescope. The result will be a slow, partly random variation in receiver properties that can be extremely difficult to distinguish from the astronomical process being observed. As a result, power beam leakage effects will do markedly greater harm when more than one satellite is operating simultaneously. The third major effect arises from unintentional radio emissions unavoidably associated either with the generation and handling of massive amounts of microwave power, or with the presence of large, warm structures in orbit. When these emissions originate in the power satellites, they make the satellites appear as individual radio sources that do not move as do natural radio sources. When the emissions originate in the power receiving (rectenna) arrays, they can be much like other terrestrial sources of interference. When these emissions lie in the allocated radio astronomy bands, they are subject to the most stringent regulations under international treaty. These regulations may prove extremely difficult for the system to meet. When the emissions occur at other frequencies, they are subject to less stringent
standards but can still do comparable harm to the substantial amount of important radio astronomy observation that occurs at spectral lines and frequencies of opportunity outside the protected radio astronomy bands. Although a substantial basis already exists for quantitative evaluation of the SPS radio interference, specific uncertainties concerning properties of the SPS and of radio astronomy equipment still remain and in several cases preclude quantitative estimates of effects. The report of the radio working group offers those quantitative estimates that can be made now, makes a number of recommendations for further investigation, and urges that an ongoing panel be constituted to evaluate the impact of SPS as the design of the system evolves.
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SPACE POWER SATELLITE BRIEFING DOCUMENTRADIO AND OPTICAL ASTRONOMY EFFECTS
SPACE POWER SATELLITE BRIEFING DOCUMENTRADIO AND OPTICAL ASTRONOMY EFFECTS INTRODUCTION This briefing document was prepared for participants in a workshop on Satellite Power System (SPS) effects on optical and radio astronomy held May 1979 in Seattle, Washington. The document draws much of its information from, and is meant to be used in conjunction with, the SPS Reference System Report, D0E/ER-0023, dated October 1978. The aim of this Briefing Document is to collect information relevant to SPS effects on optical and radio astronomy, to present the information in a manner that is both useful to the astronomical observing communities, and is as independent as possible of future changes in the SPS design. In most cases, the numerical quantities of greatest interest are not given in the Reference System Report and had to be calculated or estimated based on information that was available. In such cases, the numbers were calculated in the simplest manner consistent with a useful result. Results of more than one significant figure are seldom offered for a quantity. Greater emphasis has been given to the identification of qualitative effects, and some effects are mentioned for which no magnitude estimate is currently available. In view of probable SPS design changes and uncertainties in the properties of some system components, it is important to regard the information presented here as reference numbers used to focus discussion, and not as definitive results. These same uncertainties mean that the most useful and influential statements on SPS effects will be those phrased in one of two ways: they either should be parametric in the level of SPS emission or should offer design criteria to be observed if corresponding effects are to be avoided. Persons evaluating SPS effects are encouraged to report their conclusions in one of these two forms whenever possible.
GENERAL CHARACTERISTICS OF THE SPS REFERENCE SYSTEM The Reference System consists of 60 satellites in synchronous orbit, each 2 consisting of a large (54-km ), solar photovoltaic cell array and a microwave beam generator used to transmit power to an antenna-rectifier ("rectenna") q array on the earth's surface. Each satellite transmits 6.7 GW (10 W) at 2.45 GHz, and delivers a nominal 5 GW to the utility power grid. Pages 10-46 of the Reference System Report, D0E/ER-0023, provide a good introduction to the system, and should be read by anyone unfamiliar with the details of the Reference System. Table 1 lists data relevant to the Reference System drawn from the Reference System Report. The table also presents calculated values based on that data and on data from the American Ephemeris and Nautical Almanac and the RCA Electro-Optics Handbook. The values in Table 1 are the basis for most of the calculated quantities appearing in later sections of this document. TABLE 1. Data Relevant to the SPS Reference System
SATELLITE OPTICAL EFFECTS A. DIFFUSE REFLECTION Each satellite is oriented so that the solar cell array approximately faces an observer at the subsatellite point once each day, at local midnight. This condition combines the maximum projected area visible to the observer with the darkest sky and represents a worst case. For this worst-case situation, an attenuation factor for scattered sunlight can be obtained by assuming that sunlight intercepted by the solar array is scattered in a Lambertian (cosine) pattern with a diffuse albedo a. If the solid angle subtended by the satellite is ft , the expected illuminance at the Eartn's surface, expressed as a fraction of noon sunlight, is aft /II = -8 1.38 x 10 a. For a 4% albedo, approximately 1/4 that of lunar material, the satellite will appear as bright as Venus at its most brilliant. The actual albedo is a combination of the diffuse reflectance of the solar cell surface, whicn will be small for any efficient cell, and the combined specular reflections from variously oriented pieces of satellite structure. Discussions with a solar cell manufacturer indicate that current Si cells absorb 93 to 96% of all incident visible light, leaving no more than 7% total reflectance, most of which is probably specular. However, the end- of-life degradation in cell and concentrator efficiency is presumably a result of increased surface roughness and other damage. This would increase diffuse reflectivity. The uncertainties encountered here are typical of those to be found tnroughout this effort to assess SPS effects. Pending the availability of better data, the value a = 0.04 mentioned above can be recommended as a reference value, and the satellite taken as approximately as bright as Venus ever is. The combined light from 60 satellites will then be approximately as bright as that of the moon halfway between new and quarter phase. The solar blanket subtends an apparent angle of 1/2 x 1 arc minutes, and will appear to the naked eye as a point source under all but ideal conditions.
B. SPECULAR REFLECTION OF SUNLIGHT Both the solar cell blanket and the transmitting antenna array have large, flat specularly-reflecting surfaces. Although the antenna array is much 2 2 smaller, 0.8 km versus 54 km , its expected reflectance is much greater, leading to comparable estimated illumination levels at the Earth's surface for reflections from each surface. The antenna's aluminum front surface is expected to have a specular reflectance above 0.9. Of the various possible reflecting interfaces in the solar blanket, the boundary between vacuum and front cover sheet is easily analyzed and can be used to set a lower bound to array reflectance. The silicon cell option employs a borosilicate glass cover sheet with a reflectance of 0.04. The GaAlAs option employs synthetic sapphire with a reflectance of 0.063, but at a concentration ratio of two, so that only half of the blanket area is cover sheet. The effective reflectance for the GaAlAs option is therefore 0.032. We adopt a mean value of 0.036. The reflected spot of light on the surface of the Earth may be regarded as a pinhole camera image of the Sun approximately 330 km in diameter. It will be reduced in brightness from that of noon sunlight by the product of the specular reflectance of the surface and the ratio of solid angle subtended by the satellite to that subtended by the Sun's disk. For the solar blanket, the _5 illumination as a fraction of noon sunlight is 2.3 x 10 , or approximately ten times that of brightest moonlight. For the antenna array, the fractional _ 6 illumination is 8 x 10 , roughly four times that of brightest moonlight. If the solar blanket were held precisely facing the Sun, then specular reflection from it could fall on the Earth only for the brief period when the satellite was in the Earth's penumbra, and would be visible only at local sunset or sunrise. If the satellite's attitude is controlled only well enough to avoid significant power loss, the reflected spot could fall on a much larger portion of the Earth's night side. The transmitting antenna is constrained by beam-forming requirements to point with high precision directly toward its rectenna array. The path of the reflected spot across the surface of the Earth is, therefore, completely determined once the longitude of the satellite and location of the rectenna are specified. Livingston (L. E. Livingston, Visibility of Solar Power Satellites from the Earth, Document JSC-14715, L. B. Johnson Space Center,
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