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H Matsuoka, M Nagatomo & P Collins, October 7, 1999, "An Equatorial SPS Pilot Plant", Paper IAF-99-R.3.06 presented at Session on Affordable Large Power Systems of the 50th IAF Congress, Amsterdam, October 7, 1999..
Also downloadable from equatorial sps pilot plant.shtml

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An Equatorial SPS Pilot Plant
Hideo Matsuoka*, Makoto Nagatomo** & Patrick Collins***

The general public are strongly hoping for some inspiring initiatives to be announced in the year 2000, to give them confidence that their political leaders are abreast of the rapid changes occurring in the world today, and well prepared to face the major challenges expected in the following decades. Ensuring future supplies of clean energy on a scale many times larger than today is perhaps the greatest problem, and is deeply interconnected with the problems of environmental pollution, economic growth, population growth and unemployment.

It is increasingly recognised that, as the technologies for utilising solar energy in its various forms continue to improve, they will contribute increasingly to world energy use. One of the candidate ways of using solar power on a large scale is to transmit power generated continuously from sunlight in space by orbiting satellites to the Earth's surface using microwave beams. This system was proposed more than 30 years ago, and since then the various technologies required have been developed to a point of readiness.

The start of a first pilot plant project to demonstrate the delivery of solar-generated microwave power from satellites in Earth orbit to users living on Earth would be an exciting, publicly visible demonstration of a new way of tapping the limitless, clean solar energy available to us. Such a real demonstration will also be far more convincing proof to engineers and managers from the electricity generation industry that the technology of this system is now mature than theoretical explanations or experiments performed in space.

A 10MW solar power satellite ( SPS) pilot plant is being designed in Japan to operate in orbit 1100 km above the equator and provide the first ever supply of electric power for thousands of homes in some of the poorest regions of the Earth. In doing so it will also generate a mass of data on the system's operation, and provide a test-bed that electricity supply companies will be able to use to perform a range of experiments that they need to be convinced of the system's feasibility.

To date the authors have made field research visits to ten developing countries along the equator, meeting government officials and researchers, and visiting candidate sites for microwave power receiving antennas (rectennas) up to 1 km or more in diameter. All the countries visited have expressed keen interest in participating in the project, and more detailed case studies of each candidate rectenna site are being planned. It is highly desirable that the technologically more advanced countries should collaborate with the less developed countries near the equator to bring this project to reality.


As research in atmospheric science and climatology has advanced, recognition of the likely extremely high cost of permitting "greenhouse gas" emissions to grow indefinitely has spread worldwide. Consequently, although world oil prices are currently at a historic low in real terms (contrary to some predictions made during the 1970s) the need to curb emissions of CO2 from fossil fuel combustion is increasingly accepted, and is already the subject of global negotiations.

However, it is widely recognised that the international agreement to restrain emissions of CO2 reached at the Kyoto Summit in 1997 was only a small step towards solving the underlying problem, and not a fundamental solution. First, this is because the agreements aimed mainly at improving the efficiency of the existing energy industry, with its huge dependence on the fossil fuels coal, oil, gas and uranium, all of which face major pollution problems. Secondly, few of the developing countries joined the Kyoto agreement, although they represent most of the world's population, and are expected to be responsible for producing most of the world's atmospheric pollution within a few decades.

Clean Energy from Space

The possibility of transmitting microwave power beams to terrestrial receiving antennas from large solar power stations in space is an important candidate to become a major source of clean electric power for people on Earth in the future. Over the past 30 years a great deal of experimental work has been done that shows the technical potential of SPS as a new power source. A great deal of analytical work has also shown the benefits to world economic growth that will arise from the growth of a range of space-based commercial activities which the successful development of power supply from space will generate. Furthermore, microwave power beams from space could become a particularly attractive source of power for many developing countries, as described below.

However, before clean power can be delivered from space to Earth by commercial satellite power stations, a pilot plant system is unquestionably needed. All new energy sources have to be demonstrated in practice before electricity supply companies invest in them. No matter how advanced theoretical analyses and laboratory demonstrations may be, the first actual implementation of a new engineering system always reveals new aspects and new problems that were not foreseen in theoretical and experimental studies. As a result, the first demonstration system is not profitable - and commercial investors are famously wary of investing in technological development. As a new electric power system, this is particularly true of a system of which much of the technology is outside the experience.of electricity companies, being designed for operation in space.

SPS Pilot Plant

It would therefore be only a continuation of normal energy industry practice for governments to fund a solar power satellite ( SPS) pilot plant. The operation of such an SPS pilot plant will provide engineers with a test-bed that they can use for a wide range of experiments concerning different operating conditions, including load following, response to transients, reliability, electromagnetic noise and others, as described in (1). It could also be used to test the design, manufacture, assembly, operation and reliability of a range of different sub-system and component designs.

Over and above these uses for experimental testing, an SPS pilot plant should be designed to provide data on economic aspects as well as engineering matters. Hence, on the basis of operating experience, electricity companies and energy policy makers should be able to judge the future economic promise of SPS systems. It could thereby demonstrate to members of the electricity industry, investors, manufacturers, governments and the media that the SPS concept is now technically mature, environmentally benign and potentially profitable.

In addition, by providing electricity supply to ordinary users, a pilot plant system can be used for market priming. Even if standard figures-of-merit such as the satellite's specific output in kW/kg and specific cost in $/W are not yet low enough to attract commercial investment, provided that engineering progress is expected to meet the necessary targets in due course, subsidising operations of an early system has great value from the point of encouraging rapid market penetration of subsequent commercial or near-commercial systems.

This is a well-known form of industrial support and is currently used, among others, by the Japanese Ministry of International Trade and Industry (MITI) to expand the market for roof-mounted domestic photovoltaic electricity generation systems. These are not yet economically competitive with commercial electricity supply, but they are confidently expected to become so within a few years as the technology continues to improve and further scale economies are achieved - in part through the initial subsidised sales. The same logic is a strong additional reason for using an SPS pilot plant to deliver useful electricity supplies to users, rather than just for performing experiments.

Public Support

From the results of public opinion polls, an SPS pilot plant project is also likely to be popular with the general public, who have high expectations of some promising new initiatives to give them confidence that the 21st century is going to bring improvements in global living standards and life-styles world-wide - not increasing constraints and growing stresses from an increasingly crowded world of limited resources.

It is very unfortunate that, more than 30 years after its inception, the SPS concept is still little known outside professional circles. For example, very few public exhibitions concerning energy held at science museums even refer to energy from space - although there are always sections relating to nuclear fusion, research on which has received many tens of billions of US$ of government funding without becoming a realistic energy option.

From the point of view of receiving government support, it is a weakness of the SPS system that it cuts across the responsibilities of several different government departments, including energy, telecommunications, technology, environment, industry and foreign policy, as discussed in (2). Consequently the total amount of government support for SPS research has not yet reached even 1/1000 of that given to other new energy technologies. For example, governments have given and continue to give subsidies of hundreds of billions of US$ to various different nuclear power generation technologies, but in total no more than a few tens of $millions have been devoted to SPS research over the past 30 years, most of that in the USA during the 1970s.

However, this ratio does not reflect public opinion, which is increasingly unfavourable towards nuclear power generation and favourable towards the use of solar energy in its various forms, the use of which are growing steadily despite disproportionately little government support. The public, particularly the young, also generally support space development, and so it is likely that, if given the opportunity, the public would strongly support an SPS pilot plant project.


For simple physical reasons, an SPS demonstrator satellite in low orbit is economically much more attractive than one in geo-stationary orbit ( GEO). For example, in order to focus a beam of microwave energy onto a terrestrial antenna of a given diameter, a trans-mitting antenna in geo-stationary orbit would need to be nearly 36 times larger in diameter, and so about 1000 times larger in area than a transmitting antenna at 1000 km altitude.

Low Earth orbits have the disadvantage that the satellite will not remain fixed above a single receiver, but will move continuously around the Earth. Thus, in order to provide a continuous power supply to users on Earth it will be necessary to include electric power storage in the ground segment. (This would not be satisfactory for a commercial system, but is acceptable for a pilot plant.) Among the range of low Earth orbits, those directly above the equator have the important operational advantage that a satellite can deliver power to the same receiving antenna on every orbital revolution, that is every 90 minutes or so. This is very different from orbits inclined to the equator, from most of which it is possible to deliver to the same antenna no more than once every few days at best.

A study on such an SPS pilot plant system has been under way in Japan since the late 1980s (3). A guideline for the study was developed in order to focus researchers' work on designing a system suitable for use by the terrestrial electricity industry, which included 6 requirements, as described in detail in (4, 5). Based on these requirements, a system design was developed, as shown in Figure 1, which came to be known as "SPS 2000" in recognition of the goal that, after adequate preparation, the project would start in the year 2000.

Basic Requirements

  1. The first construction will be started no later than 2000.
  2. Commercial and versatile tech-nologies will be used for this system.
  3. The electricity generated by the SPS 2000 shall be commercially competitive with existing small scale utility electricity (NB this excludes launch costs).
  4. 2The SPS 2000 satellite will be placed on a low altitude equatorial orbit by commercial launch systems.
  5. Prospective customers for the electric power utility service are residents in the equatorial zone.
  6. Design a basic model with 10MW microwave power allowing for system growth in the future.
SPS 2000" solar power satellite pilot plant design

As the SPS 2000 study project progressed, in 1994 it was felt that it was time to start to plan the system's operation. This requires collaborative research with potential users in countries around the equator. In order to maximise the many benefits of operating a pilot plant, it was decided that power receiving antennas (rectennas) should be sited in as many different countries as possible.

There are a number of constraints on the selection of suitable sites for rectennas, as described in (6, 7). From the system design, rectennas need to be within 3 degrees latitude or about 300 km of the equator, and about 1200 km apart west-east. As it happens, most of the people living in this zone are at a relatively early stage of economic development, and many tens of millions of them live without electric power, and so even relatively small amounts of power will be genuinely useful. Consequently the authors have been performing field research to identify candidate rectenna sites around the equator.

To date this has involved visiting ten countries, meeting government representatives with a range of responsibilities, establishing relations with researchers and business-people, and obtaining agreements to participate in the project by hosting a rectenna and collaborating in the system design, construction, operation and data collection. The results of these field research visits have been described in a series of reports, which discuss candidate rectenna sites that were visited during the field trips, as listed in Table 1 (8). A number of detailed case studies are currently under preparation.

Country Site Terrain Use of power

Tanzania Namanga bush border village supply
Papua New Guinea Manus island tropical forest high school + villages
Brazil Alcantara + ? forest link to village grid
Indonesia Halmahera coastal plain villages
Ecuador Galapagos (?) open fields local grid
Maldives Gan atoll shallow lagoon villages
Malaysia UTM campus hillside research
Sarawak state tropical forest villages
Colombia Tuquerres agricultural land native villages
Kiribati Tarawa atoll tidal lagoon villages
Phoenix islands lagoon villages
Nauru tbd over mining spoil link to local grid

Table 1: Provisional SPS 2000 Rectenna Sites (8)

Potential sites are also under consideration in Gabon, Peru, Jarvis island (USA) and Somalia, though these may not all be possible for various reasons.

The planning, construction and monitoring of several of these rectennas may also involve collaboration with researchers in neighbouring countries, such as Kenya and Uganda in the case of Tanzania, Sri Lanka in the case of Maldives, Singapore and Thailand in the case of Malaysia, and others. There is also interest among equatorial south American countries in collaborating over their participation in the project.

From contacts with interested researchers, there is also a possibility of technical cooperation with researchers in China, India and Ghana. There will also be a need for discussions with all countries over which the satellite will pass, not only those which participate by hosting a rectenna. Some of these countries might also participate in the project by monitoring the satellite on its passes through their region of longitude.

Because the project is inherently international the United Nations could clearly play a very important role in overseeing the initial stages of what could grow into a major new global industry. International agreements will also be needed on a number of issues of space and international law, and these will particularly involve developing countries. This is because SPS operators looking towards Earth from geo-stationary orbit will sell their power supplies to the global market - and the greatest growth in energy demand in future is expected to be in currently less-developed countries.

Moreover, it is notable that many of these countries will have an economic advantage in purchasing power supplies from orbiting solar power satellites: this is because the price that rectenna operators will offer for supplies of microwave 'fuel' will be based on the difference between alternative means of generation and the cost of their rectenna (1). Both land and labour are relatively cheap in developing countries, and so rectennas in these countries will have much lower costs than rectennas in more advanced countries where land and labour are much more expensive. But existing means of electricity generation in developing countries are expensive, since prices are to a large extent set by global markets. Consequently in many of these countries rectenna operators will be able to offer higher prices to purchase microwave power supplies from space than electricity supply companies operating rectennas in more economically developed countries. This is an additional incentive for planning a pilot plant system for users in developing countries.


International cooperation among advanced countries to develop an SPS pilot plant could be politically as well as economically advantageous, due to the growing problem both in the USA and elsewhere concerning what activities government space agencies should focus on after the international space station is assembled: on current plans there will be little further work for NASA's space shuttle and astronauts (9).

It is well known that NASA's leaders want the US government to pay for them to carry out a crewed mission to Mars. The situation has been described by former NASA associate administrator for space science Wesley Huntress in the following terms: "[Werner von Braun] identified that goal 50 years ago as a mission to Mars... Human space flight in NASA still lives by that 1950s Werner von Braun plan. The agency clings to that vision as a missionary to a Bible.... today, at the end of the century, there is neither national will nor mandate to realize such a vision" (10). Indeed, the US Congress is not keen to pay to send a team to visit Mars, primarily because it would produce little or no benefit for the economy, and the US President has stated explicitly that "...benefits to us here on Earth" such as advances in medicine and Earth sciences are more important to the US public than a Mars mission (11).

Consequently the US Congress has for years been pressing NASA to concentrate on activities that would earn some return on the $14 billion of taxpayers' money that it uses every year, and has recently tripled funding for SPS research to $15 million. Since opinion polls show that the US public are concerned about the global energy problems foreseen a few decades in the future, the possibility of employing NASA's astronauts to help build an SPS demonstrator - for example, by offering crewed assistance in assembling Japan's SPS 2000 satellite, which is the only SPS system currently at an advanced stage of planning - is currently receiving attention.

Crewed Intervention

The availability of crewed intervention during the assembly process could reduce the risks of the SPS 2000 system, since it is not possible to adequately test the deployment of its 300 metre structure - taller than Tokyo tower - in the gravity field on Earth: being designed for a micro-gravity environment it would be much too flimsy to support its own weight. However, crewed intervention would alter one of the basic assumptions underlying the SPS 2000 study, and so would change the optimum system design. Most importantly, neither US nor Russian crewed launch vehicles can reach orbits above the equator from their existing launch sites, and so a different sequence for launch and deployment would be required. Three possible sequences need consideration:

  1. Equatorial Launch + Orbit Raising: A new equatorial launch site for a space shuttle (and/or Soyuz and/or Proton) might be built at Brazil's Alcantara or some other equatorial launch site. This would involve launching the satellite segments into an assembly orbit at a few hundred km altitude, and then raising the satellite's altitude to the planned 1100 km with ion-engines powered by the satellite's own solar panels. This would increase the satellite's complexity and cost; however, many future SPS concepts use self-propelled orbit-raising, and so a pilot plant test of this operation could be of value.

  2. Inclined Orbit Assembly + Orbit Transfer: Alternatively, the satellite components could be launched from existing launch sites to an inclined orbit where they would be assembled with input from astronauts and/or cosmonauts, after which self-powered ion-engines would both raise the satellite's orbit to 1100 km altitude and shift it to the equatorial plane.

  3. Non-Equatorial Orbit: Another proposal is that the satellite could be assembled in inclined orbit as in b), and a non-equatorial orbit used for the satellite operation. If the satellite had no self-propulsion capability it would have to remain in the initial orbit at low altitude reachable by existing crewed vehicles. This would have major problems: the satellite would be severely damaged by the denser debris at lower altitude; its lifetime would be shorter due to faster orbital decay; and the duration of each power delivery period would be shorter. Consequently, raising the orbital altitude with ion-engines is very desirable. However, in that case changing the inclination to equatorial orbit would also be feasible, although it would use more propellant. Consequently the use of an inclined orbit for the satellite's operation does not seem advantageous.

It remains to be seen whether an initiative to agree international collaboration for crewed intervention in the assembly of an equatorial SPS pilot plant will gather support in Japan, the USA and elsewhere. However, without near-term agreement on a new task that is popular with the general public NASA's and other space agencies' budgets could decline sharply after the space station is assembled.

Although the initial proposal has been for US-Japan cooperation, it seems likely that if the idea gathers support, Russia, Europe and Canada would all wish to participate. If so, there could be a danger that the project would change from the current SPS 2000 objective to be as cheap as possible, to becoming as expensive as the inter-national space station. In that case it would be a question whether this might still be better than no SPS at all.

The Japanese space industry faces a similar problem to NASA: after H2A and JEM are developed, there is no consensus on what government space spending should aim at. Space science of course has its own justification, but that represents only about 10% of government space spending. At a time when the Japanese government's financial deficit is reaching crisis-level, it is surely undesirable to fund space technology development projects that offer no possibility of becoming commercially profitable. This poses important questions for Japan's space leadership, but as a potentially important energy project, the building and operation of an SPS pilot plant would certainly have economic value.

Recent cost-estimates suggest that the SPS 2000 satellite would cost some hundreds of $millions, and that using today's launch vehicles its launch will cost some $1-2 billion. Thus the total project cost would be about 10% of one year's expenditure by the US, European and Japanese space agencies - or 2% per year if spread over 5 years. Clearly this would not be a significant burden, either for taxpayers, nor for government space budgets. Indeed, by giving the space agencies a project that has public support it could benefit them greatly. However, in view of the problem referred to above that SPS has weak political support because it falls between the responsibilities of different government departments (2), it lies in the hands of the space community to design and propose such a collaborative project to follow ISS.

International Work-sharing

The overall breakdown of work between different participating countries would depend on their political and economic objectives. Based on the discussion above, the different countries' roles might work out somewhat like those shown in Table 2, with the space infrastructure being the responsibility primarily of the advanced countries, and the terrestrial infrastructure being primarily the responsibility of developing countries, with Japan also involved in coordinating the rectennas.

Japan satellite production, launch costs, rectennas
USA astronaut-tended assembly, + some launches?
Russia, Ukraine launches, cosmonaut-tended assembly?
Europe launches, + ?
Canada orbital assembly, rectennas?
Equatorial countries rectenna sites, operating data, collective planning

Table 2: Possible Work Shares among Participating Countries
Further Developments

As described above, the SPS 2000 project was planned to lead on to a range of further developments, in the same way that other power generation technologies have developed through a sequence of progressively more advanced "generations". In order to maximise the system's value to the electricity industry, this should remain an important objective of any international SPS pilot plant system.

Once there is a series of equatorial rectennas operating on Earth, their operators will be in a position to purchase microwave "fuel" from other power satellites (1), since each SPS 2000 rectenna will be receiving power for only a few percent of the time (ie through a pass lasting about 200 seconds each orbit of 100 minutes).

Even if the first pilot plant system is successful, it is not certain that the next SPS satellite could be commercially profitable. However, there are many possible scenarios whereby governments might provide partial subsidies to a second generation of satellites if necessary, as they have to electricity generation systems. Since the major objective of SPS research is to develop an economically competitive new power source, and government participation should be planned to ensure the achievement of this over-riding objective.

In negotiating with companies planning successive new satellites to deliver to the system of rectennas, the community of rectenna operators will play an important role in establishing specifications for the microwave power beams that they will accept, and laying the groundwork for future international standards for this new industry, as described in (12).


A low-orbit SPS pilot plant is highly desirable from the point of view of both energy policy and environmental policy, in order to test out this new, potentially major, environmentally benign source of electric power. It could also be politically attractive in the near future, since such a project could fill the gap in plans for government-funded space activities that exists for the period after completion of the assembly of the international space station.

Since there is no urgent need to carry out a project such as a crewed visit to Mars within the next few decades, any such project should take place after the development of low-cost reusable launch vehicles. By contrast, humans' energy problem and global environ-ment problem are fundamental challenges for the maintenance of economic growth and even order in the world economy, with its ever-growing population.

In addition, the present economic difficulties currently faced in many countries, exemplified by record levels of unemployment simultaneously in Japan, Russia, Germany, France, and many developing countries in South East Asia and South America are evidence above all, of the need to develop new industries. Solar power satellites would generate sales revenues from a new space-based service, and would thereby create a wide range of new business opportunities in space. In addition, Germany and several other countries in Europe have renounced the use of nuclear electricity generation, but they have yet to take any major initiative towards developing a major new power source to replace it. Participation in SPS 2000 could be a positive and popular move in this direction.

Many of the benefits of realising SPS as a new energy source will be world-wide, independent of which countries are most involved in production, operation and utilisation of the system, since it will expand world energy supply; help to solve global problems of environmenal pollution; and create a major new field for economic growth which will benefit both more and less economically developed countries.

The symbolic importance that the media and general public attach to the year 2000 provides a unique opportunity for an effective political and economic initiative in this field - by acting not only to test a new, potentially major, environmentally clean electricity source, but also by doing so in a visible way that provides the public with powerful promise of developing into a major new field for business activity and growth. Consequently a commitment to build the SPS 2000 system could earn its political supporters both domestic and international recognition, and valuable electoral benefits.

  1. P Collins, R Tomkins and M Nagatomo, 1991, "SPS 2000: a commercial SPS test-bed for electric utilities", Proceedings of Inter-Society Energy Conversion Engineering Conference, American Nuclear Society, pp 99-104; also downloadable from
  2. P Collins, 1997, "Widening the Base of SPS", Equatorial Times No 5, pp 6-7; also downloadable from
  3. M Nagatomo and K Itoh, 1991, "An Evolutionary Satellite Power System for International Demonstration in Developing Nations", Proceedings of SPS91, pp 356-363; also downloadable from
  4. M Nagatomo et al, 1994, "Conceptual Study of A Solar Power Satellite, SPS 2000", Proceedings of ISTS, Paper No. ISTS-94-e-04; also downloadable from
  5. M Nagatomo, 1996, "An Approach to Develop Space Solar Power as a New Energy System for Developing Countries", Solar Energy, Vol. 56, No 1, pp 111-118; also downloadable from
  6. H Matsuoka and P Collins, 1996, "Equatorial Cooperation for SPS 2000 rectennas", Proceedings of ISAP '96, pp 421-4.
  7. P Collins, 1998, " SPS 2000 and its International Importance", Space Energy and Transportation, Vol 1, No 4, pp 279-287.
  8. H Matsuoka et al, 1994-99, " Field Research for Solar Power Satellite Energy Receiving Stations", Matsuoka Laboratory Working Papers 1-10, Tokyo University / Teikyo Heisei University.
  9. B Berger, 1999, 'New Role Sought for Shuttle After Space Station', Space News, Vol 10, No 32, pp 8, 19.
  10. F Ordway, 1999, 'Space Future - By Way of Space Past', Ad Astra, Vol 11, No 4, pp 26-32.
  11. Anon, 1999, 'Human Mars Mission Not a Priority, Clinton Says', Space News, Vol 10, No 30, p 2.
  12. H Matsuoka, 1999, " Global Environmental Issues and Space Solar Power Generation: Promoting the SPS 2000 Project in Japan", Technology In Society, Vol 21, No 1, pp 1-17.
H Matsuoka, M Nagatomo & P Collins, October 7, 1999, "An Equatorial SPS Pilot Plant", Paper IAF-99-R.3.06 presented at Session on Affordable Large Power Systems of the 50th IAF Congress, Amsterdam, October 7, 1999..
Also downloadable from equatorial sps pilot plant.shtml

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