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As a consequence of an ever-increasing world-wide energy demand and of a need for a “clean” energy source, the solar power satellite (SPS) concept has been explored by scientists and engineers in the United States, Japan, and Europe. An SPS constitutes a method of generating electricity from solar energy using satellites and transporting it to the ground via electromagnetic waves. Several candidate systems have been proposed. However, so far no system has been either constructed or tested in space, and it is currently unknown when one might be.

The purpose of this URSI white paper is to provide knowledge about the SPS concept based on evidence, and an open forum for debate on the scientific, technical, and environmental aspects of the SPS concept. [URSI white papers are documents issued by URSI scientific experts on controversial subjects involving aspects of radio science. Although under the responsibility of URSI, they do not necessarily reflect all the views of individual URSI Member Committees nor Commissions (see Appendix 1: URSI White Papers)].

In a typical SPS system, solar energy is collected in space by a satellite in a geostationary orbit. The solar energy is converted to direct current by solar cells, and the direct current is in turn used to power microwave generators in the gigahertz frequency (microwave) range. The generators feed a highly directive satellite-borne antenna, which beams the energy to the Earth. On the ground, a rectifying antenna (rectenna) converts the microwave energy from the satellite into direct current, which, after suitable processing, is fed to the terrestrial power grid. A typical SPS unit – with a solar panel area of about 10 km², a transmitting antenna of about 2 km in diameter, and a rectenna about 4 km in diameter – may yield an electric-power output of about 1 GW. Two critical aspects that have motivated research into SPS systems are the lack of attenuation of the solar flux by the Earth’s atmosphere, and the twenty-four-hour availability of the energy, except around midnight during the equinox periods.

Among the key technologies involved in SPS systems are microwave generation and transmission techniques, wave propagation, antennas, and measurement and calibration techniques. These radio-science issues fall within the scientific domain of the International Union of Radio Science (URSI). URSI’s ten Scientific Commissions (Appendix 2) cover a broad range of aspects involved in an SPS system, ranging from the technical aspects of microwave power generation and transmission to the effects on humans and potential interference with communications, remote-sensing, and radio-astronomy observations.

This has led URSI to organise an open forum for the debate of the radio-science aspects of SPS systems and related technical and environmental issues. The present white paper is intended to draw attention to these aspects of SPS systems. It is not URSI’s intention to advocate solar power satellites as a solution to the world’s increasing energy demands, or to dwell on areas outside of URSI’s scientific domain, such as the whole issue of the space engineering to launch, assemble, and maintain an SPS system in space, the economic justification, and public acceptance. URSI is well aware that if a practical SPS system is feasible, the realisation of such a system is far in the future. Many of the required technologies currently exist, but some of these must be substantially advanced, and others must be created.

Microwave power transmission is an important technology for SPS systems, since its overall efficiency is one of the critical factors that determines the interest in such systems from an economic standpoint. Ideally, almost all energy transmitted from the geostationary orbit should be collected by the rectifying antennas on the ground. In that respect, an overall dc-to-microwave-to-dc power efficiency in excess of 50% is needed (see Section 2.4), which requires the development of suitable microwave power devices. Accurate control of the antenna beam is essential, and measurement and calibration are important issues. Even if these technologies can be successfully developed, there remains the challenging task of combining the outputs of thousands or even millions of elements to form a focused beam. Proper safety measures have to be developed to be certain that the transmitted microwave beam remains within the rectenna’s area. Maintenance of the space systems may be very difficult and expensive in the harsh environment of a geostationary orbit. Ensuring the long-term stability of huge structures in space in the presence of solar radiation pressure and tidal forces is an unsolved problem.

The influence and effects of electromagnetic emissions from an SPS, and, in particular, the microwave power transmission, are radio-science issues that concern URSI. Atmospheric effects on the microwave beam, and linear and non-linear interactions of the microwave beam with the atmosphere, ionosphere, and space plasmas, are among the numerous issues that must be investigated and evaluated. Undesired emissions – such as harmonics, grating lobes, and sidelobes from transmitting antennas and rectennas – must be sufficiently suppressed. This is true not only to avoid wasting power, but also to avoid interference with other radio services and applications and with remote sensing and radio astronomy, in accordance with the provisions of the Radio Regulations of the International Telecommunication Union (ITU). The evaluation of possible effects on human health and the incorporation of appropriate safety measures are essential for legal operation and public acceptance of this power-generation technique.

Finally, this paper identifies specific radio-science issues requiring further studies. It is stressed that only some of these questions can be solved by laboratory work, simulations, and system analysis. Testing of elements of such large systems in space is mandatory before a possible demonstration SPS unit can be considered, and broad international consensus is likely to be required before an SPS demonstration system can be launched.

2. Solar Power Satellite Systems

2.1 The SPS Concept

A solar power satellite is a very large-area satellite in an appropriate orbit (see Section 2.6), which would function as an electric power plant in space. The satellite would consist of three main parts: a solar-energy collector, to convert solar energy into dc electric power; a dc-to-microwave converter; and a large antenna array, to beam the microwave power to the ground. For the production of 1 GW of dc power, the solar collector would need to have an area of 10 km², and would consist of either photovoltaic cells or solar thermal turbines. The dc-to-microwave converter could be realised using either a microwave-tube system or a semiconductor system, or a combination of both. For transmitting the power to the ground, frequency bands around 5.8 GHz or 2.45 GHz have been proposed, which are within the microwave radio windows of the atmosphere. The antenna array to transmit the energy to the ground would require a diameter of about 2 km at 2.45 GHz, and its beam direction would have to be controlled to an accuracy of significantly better than 300 m on the Earth, corresponding to 0.0005°, or less than 2 arc seconds (for a geostationary orbit of the satellite).

In addition to the SPS orbiter, a ground-based power-receiving site has to be constructed, consisting of a device to receive and rectify the microwave power beam, i.e. to convert it back to dc electric power. This device is called a rectenna (rectifying antenna). The dimensions of the rectenna site on the ground depend on the microwave frequency and the size of the transmitting antenna. A model system, operating at 2.45 GHz, would use a rectenna site with a diameter of 4 km and a satellite-based transmitting antenna with a diameter of 2 km (see Section 2.4). The peak microwave power-flux density at the rectenna site would then be 300 W/m², if a Gaussian power profile of the transmitted beam is assumed. The beam-intensity pattern would be nonuniform, with a higher intensity in the centre of the rectenna and a lower intensity at its periphery. For human safety requirements, the maximum-allowable microwave power level has been set to 10 W/m² in most countries, and the SPS power-flux density would be constructed to satisfy this requirement at the periphery of the rectenna. After suitable power conditioning, the electric output of the rectenna would be delivered to the power network.

The combination of an SPS in orbit and the ground-based rectenna will be called an SPS “unit” in the following. On a global scale, a very large number of 1 GW units may be necessary for a practical SPS system. More details about the SPS concept can be found in [1].

For the sake of completeness, it should be mentioned that besides microwave power transmission, laser power transmission has also very recently been suggested [1, Appendix D.9]. In such a scenario, highly concentrated solar radiation would be injected into the laser medium (direct solar pumping) and transmitted to Earth. On the ground, the laser light would be converted to electricity by photovoltaic cells. It is obvious that such a system would be fundamentally different from a “classical” SPS using microwave power transmission: In space, there would be the light-concentration system and the lasers instead of a photovoltaic-cell array and the transmitting antenna; on the ground, there would be a photovoltaic-cell array instead of the rectenna. Since the technological challenges and problems for such laser-based systems have not yet been sufficiently explored, and since many subcomponents are at a low technology-readiness level, this approach will not be treated in this white paper.

2.2 The Aim and Purpose of this White Paper

There are SPS-related issues that are highly controversial. Although several space agencies have pursued SPS studies and research (see the next section), very critical papers have been published that concluded that an SPS is impractical and will never go into operation (e.g., [2]). A more pro-SPS reply to this criticism [3] was based on the economic issues raised in [2]. Among the controversial issues is the question of the space engineering and technology that are necessary for the launch, and the assembly and the maintenance of an SPS system, all of which to a great extent are not yet possible. Other heavily debated issues are related to economic justifications (in comparison with other power sources), are related to the question of whether an SPS can provide a base-load “clean” power system on a global scale, are related to military applications, and are related to public acceptance. All of these issues are beyond URSI’s scientific domain and will therefore not be discussed in this white paper. Social issues of an SPS may perhaps be addressed by the International Council for Science (ICSU).

Instead, this white paper will focus on the radio-science aspects of an SPS. Among the key radio-science technologies involved in the SPS concept are microwave generation and transmission techniques, antennas and beam control, and the very challenging task of protecting other services to the levels required by the International Telecommunication Union (ITU). Of the various scientific organisations or unions concerned with international development and applications of these technology areas, URSI has an important role to play, because it covers most aspects of the above-mentioned radio techniques. The scientific competence of URSI’s ten Commissions (see Appendix 2) encompasses aspects of microwave power generation (Commissions B, C, and D), antennas (Commission B), calibration (Commission A) and transmission (Commissions G and H), the effects of electromagnetic emissions on humans (Commission K), the potential interference with communications (Commission C and E), remote-sensing (Commissions E and F) and radio-astronomy (Commission J) observations, and, to some extent, solar-cell technology (Commission D). Thus, URSI can provide a continuing forum for development, discussion, and debate on technical issues related to SPS systems.

In keeping with what has been said above, it is not the intention of this document to advocate an SPS as a “clean” solution to the world’s increasing power demand (as is argued, for instance, in [1]). However, it is conceivable that an SPS, and, more generally, microwave power transmission, may be used in the future for special purposes. Among such possible scenarios are bringing energy to remote areas on the globe that are difficult to otherwise access, sending energy from spacecraft to spacecraft, or providing energy to the dark side of the moon (in compliance with Recommendation ITU-R RA.479, recognising a shielded zone on the moon). Possible spin-offs from SPS-related research have been considered elsewhere [1, Section 3.6].

A number of the issues related to radio science that are addressed here are also of relevance to the process that the International Telecommunication Union (ITU) has initiated towards an ITU-R Recommendation and/or Report on wireless power transmissions, to be completed by 2010 at the latest [4].

It should be stressed that an SPS is not imminent. Many changes in technology can be expected before an SPS is launched. Major technological problems still have to be solved, even before a demonstration project could be realised. On the other hand, the radio-science aspects of an SPS encompass many interesting scientific, engineering, and technological challenges. To identify, to describe, and to discuss these items is the main aim of this white paper.

2.3 The History of SPS Research

The first concept of an SPS system was proposed by P. Glaser in 1968 [5], after a series of experiments on microwave power transmission [6a, 6b]. Following this article, the United States conducted an extensive feasibility study in 1978-1980. The feasibility study was a joint effort of NASA (National Aeronautics and Space Administration) and the Department of Energy. A reference model was proposed in 1979, known as the NASA/DoE reference model (Figure 1, [7]). Research on an SPS was suspended in the US in 1980, due to high estimated costs. Given a pre-set policy to re-evaluate the SPS concept after an appropriate time interval, the Fresh-Look-SPS concepts were published in 1977 as an improved SPS reference system. This included the “Sun Tower” SPS concept (Figure 1, [8]). This is a constellation of medium-scale, gravity-gradient-stabilised, microwave-transmitting space solar power systems. Each satellite resembles a large Earth-pointing sunflower, in which the face of the flower is the transmitting array, and the “leaves” on the stalk are solar collectors. The Sun Tower is assumed to transmit at 5.8 GHz from either a low Earth orbit or a geostationary orbit, and to operate sun-synchronously at a transmitted microwave power level of about 200 MW. NASA stated that due to its extensive modularity, the low-Earth-orbit concept entails the use of relatively small individual system components, which could be developed at a moderate price, ground-tested in existing facilities, and could be demonstrated in a flight environment during a sub-scale test.

An SPS system using mirrors for sunlight concentration on the solar cells, the Integrated Symmetrical Concentrator, was also proposed. It uses 24 or 36 plane mirrors of 500 m diameter for a concentration factor of two or four (Figure 1, [9]).

Figure 1. An artist’s impressions of various current SPS models: NASA/DoE SPS Reference Model (top left), Sun Tower (NASA, top centre) [8], Integrated Symmetrical Concentrator (top right) [9], JAXA 2003 Free Flyer Model (middle left) [18], Tethered-SPS (USEF, middle right) [19], and Sail Tower (ESA, bottom) [10].

The European Space Agency (ESA) proposed a Sail Tower SPS (Figure 1, [10]), the design of which is similar to that of NASA’s Sun Tower SPS. However, the Sail Tower SPS uses thin-film technology, and an innovative deployment mechanism developed for 150 m × 150 m solar sails. The power generated in the sail modules is transmitted through the central tether to the antenna, where microwaves at 2.45 GHz are generated in mass-produced inexpensive magnetrons. The energy emitted would be 400 MW. In 2003, the Advanced Concepts Team (ACT) of ESA initiated a three-phased, multiyear program related to solar power from space (including laser power-transmission concepts) [11]. In addition, a European Network on Solar Power from Space was established. It provides a forum for all relevant and interested European players in the field of SPS, including industry, academia, and institutions.

Japanese scientists and engineers started their SPS research in the early 1980s. They conducted a series of microwave power-transmission experiments, such as the world’s first rocket experiment with powerful microwave transmission in the ionosphere [12, 13], experiments on the ground [14], computer simulations [15], theoretical investigations [16], and system studies for a demonstration experiment [17]. After a conceptual study phase, two Japanese organisations have recently proposed their own models. JAXA (Japan Aerospace Exploration Agency) proposed an SPS 5.8 GHz/1 GW model (Figure 1, [18]), which is different from the NASA/DoE model. It is based on a formation flight of a rotating mirror system and an integrated panel, composed of a photovoltaic-cell surface on one side and a phased microwave-array antenna on the other side. Formation-flying mirrors are used to eliminate the need for rotary joints. The Institute for Unmanned Space Experiment Free Flyer (USEF) proposed a simpler model (Figure 1, [19]). The USEF model is a tethered SPS, which is composed of an integrated panel similar to JAXA’s, but suspended by multi-tether wires emanating from a bus system above the panel.

The leading group in Japan in basic SPS-related research is based at Kyoto University. Many projects on microwave power transmissions have been conducted, and several important papers have been published (e.g., [12-16]). To a large extent, this white paper is based on an extensive review of SPS issues prepared by an URSI Inter-Commission Working Group [1] under the leadership of the Kyoto group.

International collaboration was established at a Japan-US SPS workshop [20], an International Conference on SPS and Microwave Power Transmission [21], by the International Astronautical Congress (IAC) Space Power Committee, and by an URSI Inter-Commission Working Group.

2.4 A Coherent Set of Numerical Values

A set of typical numerical values was extracted from the various concepts of SPS mentioned in the previous section. This set forms the basis of the discussion in this white paper.

Assuming that an SPS unit will generate 1 GW effective power on the ground, the characteristic efficiencies are summarised in Table 1. The figures are given for a 2.45-GHz unit; corresponding values for a 5.8-GHz unit are not fundamentally different. Therefore, in order to generate 1 GW at the ground, one needs to collect about 14 GW in space. Since the solar radiation power flux is equal to 1.37 kW/m², one needs a solar-panel area of approximately 10 km². The transmitted RF power is GW. Taking into account the RF collection efficiency of 87%, the RF power received at the ground level is GW. The efficiency of the microwave power transmission (dc-microwave-dc) is the product of the efficiencies given in lines 2-4 of Table 1, i.e. 54%. (Actually, 54.18% was demonstrated and certified in a NASA laboratory test [22]).

Quantity	Efficiency	Reference
Solar-power-to-dc-power efficiency	13%	[1, Section 2.4.1.2]
dc-power-to-RF-power efficiency	78%	[1, Section 2.4.1.2]
RF collection efficiency	87%	[1, Section 2.4.1.2]
RF-power-to-dc-power (rectenna)	80%	[1, Section 2.3.6,1] (average of 70% and 90%).
Total efficiency	7%

In order to define the rectenna characteristics, a reasonable value for the power flux at the centre of the rectenna system has to be assumed. Different values have been proposed, between 230 W/m² and 1000 W/m² [1, Table 2.3.2]. Here, a conservative value of 300 W/m² (which is less dangerous from a biological point of view: see [1, Section 4.3]) is adopted for the central power flux. Assuming a Gaussian distribution of the power at the ground, and assuming further that the power flux at the edge of the rectenna is equal to 10 W/m² (for safety reasons, which are discussed in [1, Section 4.3]), after a simple calculation one arrives at a radius of the rectenna of km. If L is the altitude of the geostationary orbit (km) and if is the radius of the transmitting antenna, one has that approximately , where is the RF wavelength (m at 2.45 GHz). Therefore, m (a more accurate estimate would arrive at 1200 m: see [1, Section 3.1.1]). The assumed sizes are summarised in Table 2.

Quantity	Size
Solar-cell array	10 km² area
Transmitting antenna on satellite	2.4 km diameter (for 2.45 GHz)
Rectenna	4 km diameter (independent of frequency in the above estimate)

The last number to be introduced is the desired pointing accuracy of the transmitting antenna. In most projects, one assumes that the allowed displacement of the centre of the beam is a small fraction of the diameter of the rectenna system. In [1, Section 2.1.1], the adopted value of this displacement was 300 m, so that the required pointing accuracy for a geostationary power station is 0.0005°. It should be noted that the above estimate for the rectenna size does not take into account any safety margin due to the pointing accuracy of 300 m.

2.5 Economic Issues

As already stated in Section 2.2, economic-related issues are outside of URSI’s scientific domain. Some important aspects are therefore touched on only briefly in this section, with some figures quoted from the available literature.

There are four main factors that determine the power-production costs of an SPS system: photovoltaic module efficiency and costs, mass-specific power production (W/kg) of the solar modules and the transmission system, microwave power-transmission efficiency, and launch costs. The target is an efficiency of about 50% for the total microwave power transmission dc-microwave-dc conversion (see Section 3.1), and a specific power output of 1 kW/kg for the whole microwave power-transmission system. The published SPS cost estimates are based on a launch cost of USD150/kg [1, Section 2.1.4]. All these assumptions lead to an estimated energy-generation cost of approximately USD0.1-0.2 per kWh for an SPS system[23]. These estimates remain controversial. For example, present-day launch and space-assembly costs are greater than two orders of magnitude higher than the desired USD150/kg (present-day launch costs are USD10,000/kg [24]). While NASA expects the launch costs to decrease by a factor of 100 by 2025 and by a factor of 1000 by 2040 [25], ESA is less optimistic. In a corresponding ESA report, the energy-generation costs for a 500 GW SPS system were estimated to be USD0.40/kWh, assuming transportation costs of USD1,500/kg, and a mass-specific power production of 0.2 kW/kg [26]. In the same report, it was stated that transportation costs may be reduced to USD200/kg in the future.

A direct comparison of the output power from a space-based solar power unit with that from a terrestrial photovoltaic array with equal area is not straightforward. On one hand, a simple estimate of the energy output yields an advantage of about a factor 2.5 for the SPS. For the SPS system, 1.37 kW/m² solar power flux in space × 0.07 overall SPS efficiency (Table 1) × 24 h = 2.3 kWh/m²/day. For a terrestrial solar-cell array, 5 kWh/m²/day average solar power flux at a sunny place (Arizona [27]) × 0.17 solar cell efficiency = 0.85 kWh/m²/day.

On the other hand, a detailed economic comparison of the costs turns out to be very complicated and dependent on many factors, such as launch costs (see above), SPS concept, power-consumption profile (base-load versus non-base-load power-supply systems), storage technology (for base-load power supply), terrestrial power-transmission system (depending on the location of the terrestrial power plant), energy payback times, and others. ESA conducted several corresponding studies (including also terrestrial solar thermal plants) (e.g., [28, 29]. One of these came to the conclusions that (i) for a base-load power supply, SPS systems above 5 GW and launch costs between USD824 and USD1023/kg would be required for an SPS to be competitive with terrestrial plants; (ii) for non-base-load power supplies, SPS systems above 50 GW and launch costs between USD206 and USD2146/kg would be required for an SPS to reach a competitive level with terrestrial plants [28]. More-detailed results of these comparisons are presented and discussed in [1, Section 2.4.3 and Appendices E.5-E.7].

2.6 Key SPS Technologies

The most important key technology concerns the infrastructure to launch, assemble, transport, and maintain the SPS system. Since this topic is beyond URSI’s scientific domain, it will not be dealt with here.

The key elements in the dc power generation for the SPS system are the solar cells. Thin-membrane (amorphous) silicon solar cells are expected to be the most suitable type for the SPS system because of their good performance for a given weight, and because of conservation of natural resources, although their conversion efficiency is lower than the figures for Si cells (17.3% [7]) and GaAs cells (20% [7]). Mass-production feasibility is also an important aspect in choosing the most suitable solar-cell type. A sunlight concentrator would enhance the power output. Therefore, two types of power-generation systems have been studied: (a) a massive light-concentration type [9], and (b) a super-light-weight thin-membrane type [30]. An increase of the total power-conversion efficiency is to be greatly desired. However, it should be noted that solar cells in space deteriorate, due to accelerated solar-wind particles and solar radiation. Radiation-hardened cells are already available for long-term space missions, but at considerably higher costs than cells for terrestrial use.

The thermal design and control of the SPS system will also be of importance, particularly if sunlight concentration is applied. One method for thermal control of the generator is blockage of the infrared radiation from the sun, either by effective reflection or by band-elimination filters for infrared radiation.

The radio science and technology of an SPS system, such as the microwave power transmission, microwave power devices, rectennas, and beam control, will be discussed in detail in Section 3.

A very important detail of an SPS is the proper orbit in space. A geostationary orbit has been proposed for most of the systems envisioned so far. However, a more-remote orbit, an L2-halo orbit [31], was also considered. It is generally assumed that the SPS is assembled at a low Earth orbit, with subsequent transportation to a geostationary orbit. Modern SPS concepts rely on robotic assembly and maintenance systems, rather than human astronauts for the assembly task. For transportation, suitable orbit-transfer vehicles have to be developed to transport a very large structure from a lower to a higher orbit. Solar electric-propulsion orbital-transfer vehicles have been suggested for this purpose. Some corresponding prototype propulsion systems, such as a magneto-plasmadynamic thruster, a Hall thruster, and a microwave-discharge ion engine, have been tested ([1, Section 2.3.1.2).

It should also be noted that the selection of the final working orbit of an SPS may have important implications for the antenna design and its characteristics (far-field or Fresnel region).

Other key issues of SPS technology are lifetime and maintenance. The limited lifetime of solar cells has already been mentioned, but a long-term radiation hazard also exists for any solid-state device on the SPS, such as dc-to microwave converters, for instance. In addition, there is the problem of the long-term mechanical stability of the very large structures of the solar panels and the microwave transmitting antenna. The long-term influence of tidal effects and radiation pressure have to be examined. In principle, both effects can deform the structure as well as change its orientation. In particular, the radiation pressure exerts a force that changes continuously in direction with respect to the line joining the satellite and the rectenna. This may pose serious problems concerning the control of the orbit and the orientation of the RF beam. The amplitude of this force is of the order of 100 N for a solar-cell area of 10 km² (2 × solar radiation power flux × 10 km²/velocity of light). Regarding maintenance, the present-day experiences for low Earth orbits with the Hubble space telescope and the International Space Station indicate that maintaining and servicing a much larger system in a much higher orbit may be very difficult and much more expensive than for low Earth orbits. A completely new approach to space maintenance may be required to maintain assets at geostationary orbit. Currently, progressive replacement is the only viable option.

3. SPS Radio Technologies

3.1 Microwave Power Transmission

Wireless communication uses radio waves as carriers of information. However, in the microwave power-transmission system, radio waves would be used as carriers of energy. In principle, the energy-carrying microwaves would be monochromatic waves, without any modulation. The microwave power transmission would use power densities at the surface of the transmitting antenna that are three or four orders of magnitude higher than the corresponding levels in wireless-communication systems, and up to 25 orders of magnitude higher than power densities received by the radio-astronomy and remote-sensing services.

The main parameters of the microwave power-transmission system for the SPS system are the frequency, the diameter of the transmitting antenna, the output power (beamed to the Earth), and the maximum power-flux density. In addition to the system parameters described above, the weight per unit power of the microwave devices is also of importance [1, Section 3.2].

Efficiency is very important for the microwave power-transmission system. Assuming the SPS transmitting-antenna-to-rectenna propagation path is optimum, the following efficiencies will be important: dc-to-radio-frequency (RF) conversion, RF-to-dc conversion, and beam-collecting efficiencies. Conversion efficiencies higher than 80% for both RF-dc and dc-RF conversions are necessary to make the cost of the SPS system reasonable (see Section 2.4).

Various types of transmitting antennas have been considered, such as slotted-waveguide antennas, dipole antennas with reflectors, and microstrip antennas. The most suitable antenna type depends on the chosen microwave generator and amplifier, but also on weight. A possible concept seems to be the active integrated antenna technique, combing the dc power generation, microwave conversion, and radiation and control in one multi-layered plate [32].

As mentioned in Section 2.4, the diameter of a transmitting antenna array of a 1 GW SPS system would be about 2 km. The average microwave power-flux density at the array of the SPS would then be about 300 W/m² on the surface of the transmitting antenna. A phased antenna array is planned for the SPS system, in order to obtain high-efficiency beam collection under the condition of fluctuating SPS attitudes. Depending on the frequency of the microwave power transmission, e.g. 2.45 GHz or 5.8 GHz, the number of antenna elements per square meter would need to be of the order of 100 or 400, where the power delivered by a singe element would be 10 W or 2.5 W, respectively [1, Section 3] . Thus, the total number of elements could be of the order of several hundreds of millions (this number could be substantially reduced if single klystrons of more than 1 kW output power were used to feed one antenna element). Such a large phased array has neither been developed nor constructed up until now, even on Earth. It is uncertain if simple scaling of already realised arrays is possible, or whether it may lead to unexpected problems.

Hence, realising the SPS system will require overcoming many engineering challenges, such as arrays with a dc-RF conversion efficiency higher than 80%, a phase-shifting system with very low root-mean-square errors for accurate beam control, phase synchronisation over millions of elements, and very-low-cost mass production of these elements.

3.2 Microwave Power Devices

Many possibilities have been proposed for the microwave generators, such as microwave vacuum tubes (klystrons, magnetrons, travelling-wave tube amplifiers), semiconductor transmitters, and combinations of both technologies. These types of generators have been compared with respect to their efficiency, output power, weight, and emitted harmonics [1, Section 2.3.4.2]. The dc-to-RF conversion efficiency for microwave vacuum tubes can be as high as 65% to 75%; the power of a single tube can be more than 100 kW. For semiconductor transmitters, the best achievable efficiency is 40%, the power from a single transmitter being below 100 W. Better efficiencies may be possible with new devices, such as wide-bandgap devices using GaN, which have significant power output, in particular at microwave frequencies of 2.45 GHz and 5.8 GHz [1, Section 2.3.4.2 (4)].

Compared to semiconductor technologies, a microwave tube has higher efficiency, lower cost, and a smaller power-to-weight ratio (kW/kg), even if one includes the power source, the dc-dc converter, the cooling system, and all the other elements needed to drive the system. Some of the SPS concepts are based on a microwave power transmitter with microwave tubes, such as klystrons and magnetrons. For example, a new concept for a microwave transmitter has been developed. It is called a phase-controlled magnetron, and it satisfies both the requirements of high efficiency and beam controllability [33]. A hybrid tube-semiconductor system is also a possible solution currently under investigation [34].

For the high-efficiency power transmitters, a design that generates a low amount of harmonics, and low-loss phase shifters, are particularly important and would need to be developed. Manufacturability would be one of the important considerations in the implementation of particular technologies for the microwave power-transmission system. Since the SPS requires huge investments, even in electronic parts, the availability of particular materials and the manufacturability need to be examined. From a manufacturing point of view, recent semiconductor technologies could be useful for SPS systems. However, their reliability in space would need to be investigated. For the microwave power-transmission technology, the reduction of the weight per unit of generated power would also be of importance to ensure a reasonable cost for a given performance.

In any case, thousands of microwave tubes or millions of solid-state amplifiers and oscillators have to be phased and controlled, which is a large technical challenge.

3.3 Rectennas

The rectenna (located on the Earth) receives the microwave power from the SPS and converts it to dc electricity (e.g., [35]). The rectenna is composed of an RF antenna, a low-pass filter, and a rectifier. It is a purely passive system (apart from a low-power pilot beam: see Section 3.4) and needs no extra power. A low-pass filter is necessary to suppress the microwave radiation that is generated by nonlinearities in the rectifier. Most rectifiers use Schottky diodes. Various rectenna schemes have been proposed, and the maximum conversion efficiencies anticipated so far are 91.4% at 2.45 GHz [36] and 82% at 5.8 GHz [37]. However, the actual rectenna efficiency will also depend on various other factors, such as the microwave input power intensity and the load impedance.

The single elements of the rectenna can be of many types, such as dipoles, Yagi antennas, microstrip antennas, or even parabolic dishes.

The rectenna array, with a typical radius of approximately 2 km, is an important element of the radio technology for which high efficiency is essential. The efficiency depends on the input power, and the input-power flux density is not constant over the entire rectenna site for the SPS system. Further research will be required into rectennas that maintain high efficiency under various input-power conditions. Recently, development has started on a low-power (only 100 µW or less), high-efficiency rectenna system for the perimeter of the rectenna site [38]. Studies and experiments have also been performed for a hybrid technique [39].

3.4 Control and Calibration

Another important issue concerning the space-based microwave antenna is the necessarily high precision of the control of the beam direction. This is important for two reasons: to maximise the energy transferred to the Earth; and to limit radiation in undesired directions, in order to avoid adverse effects on existing telecommunications, passive radio-detection systems, and biological systems. This goal may be achieved with the concept of a retrodirective array, in which the rectenna sends a pilot signal to the SPS in order to indicate its position before the power beam is transmitted. This pilot beam is then used to direct the power beam back along exactly the same path as the pilot beam: in the retrodirective direction. The effect of this is to automatically remove perturbations to the direction of the propagating beam, assuming that the perturbing factors along the propagation path do not change during the round-trip transit time. For this to work, retrodirective beam-forming techniques have to be developed in order to suppress sidelobes and to maximise the transmission efficiency. In addition, control measures have to take the delay of commands into account, which is a considerable fraction of a second for an SPS in geostationary orbit.

Emergency procedures should be defined and have to be applied when the beam direction is not contained within the predefined angle of 0.0005°. Ordering an interruption of the RF transmission may be a possible solution, but the detrimental effects that could be caused by a sudden interruption of the dc-to-RF conversion onboard the satellite have to be evaluated, not forgetting that the load to the grid will also need to be managed carefully.

The centre of the microwave beam should be confined to a region within 0.0005° of the centre of the rectenna. This corresponds to less than one-fourth of the 8-arc-seconds half-power beamwidth of a 1000-m-diameter parabolic SPS antenna. Achieving such pointing accuracy and stability would currently pose a major technical challenge. The required beam-control accuracy of the SPS microwave power-transmission system may be achieved using a very large number of power-transmitting antenna elements, and by limiting the total phase errors over the antenna array to a few degrees. Technologies to achieve these goals are presently under study [18]. Beam-collection efficiency is as important as the beam-control accuracy, and the efficiency depends on the power lost in sidelobes and grating lobes.

Measurement and calibration are important issues for the SPS and microwave power-transmission systems, because the SPS’s microwave power-transmission system requires accurate beam control with a large phased array. The testing of large SPS antennas presents not only the usual difficulty of making accurate RF measurements over a substantial aperture, but also the unusual problems of devising tests that can accurately predict the performance of the antenna under the harsh mechanical, thermal, and radiation conditions in the space environment. New methods of measurement and calibration would therefore need to be developed. Microwave measurements and calibration would be necessary for the evaluation of power, interference, and spurious emissions from the SPS and rectennas.

The proposed antennas – both the transmitting antenna and the rectenna – are expected to be so large that testing them in their entirety will pose significant challenges. Computer simulations can give accurate predictions of the performance of the antennas in terms of gain, beamwidth, and near sidelobes. However, the transmitting antenna can only be accurately tested once in orbit, and to achieve this, special antenna-measurement and calibration techniques will need to be developed.

4. Radio Science Influences and Effects of SPS

4.1 Interaction with the Ionosphere and Atmosphere

To a first approximation, it is generally considered that the interaction of the SPS system with the medium – space, ionosphere, and atmosphere – will be negligible. However, as noted in Section 2.6, SPS subsystems may be affected by accelerated solar-wind particles and solar radiation. Space is a harsh environment, with large temperature gradients and ionising radiation (geostationary satellites are in the solar-wind regime during large geomagnetic storms). On the other hand, currents created by SPS may locally affect the medium [40].

Power loss due to normal atmospheric absorption over the distance from a geostationary orbit to the ground is assumed to be below 2%. In abnormal circumstances, significant departures might be expected when, for instance, the beam encounters scintillations in the ionosphere and rain cells in the troposphere, as explained in the following.

Very few groups have worked on the effects of powerful microwaves on the atmosphere and ionosphere, and the few studies presently available refer to potential effects via the heating of ionospheric electrons or via ionisation of the air. The expertise is limited, but it exists. However, at a time where new observations (transient luminous events, terrestrial gamma-ray flashes) raise new questions about energy coupling between the atmosphere and the space environment [41], studies are needed on all phenomena that may influence the atmospheric electrical conductivity and chemical composition.

In the process of SPS construction, large high-power electric propulsion systems would be needed to move the structures from a low Earth orbit to the geostationary orbit. These would inject heavy ions perpendicular to the Earth’s magnetic field (around the equator). The injection could strongly disturb the electromagnetic environment surrounding the ion engine in the ionosphere and the magnetosphere, through interaction between the heavy-ion beam and the ambient plasmas. Some of these effects are discussed in [1, Section 4.1.3].

A thorough and systematic theoretical analysis of possible ionospheric effects was published under an ESA contract [42]. This analysis indicated several possible relevant effects, but simultaneously stated that “the natural variability of the ionosphere, as well as the fundamental unpredictability of nonlinear effects certainly limit the accuracy with which the performance of SPS systems and their environmental impact can be estimated.”

In principle, radio waves passing through the ionosphere are absorbed due to ohmic heating, i.e., wave energy heats the electrons. This effect is strongest in the ionospheric D and E layers, but the effect is assumed to be small for radio-wave frequencies above 1 GHz, since the heating efficiency varies as the inverse square of the frequency. No ground-based measurements of electron heating by high-power microwaves are available in the GHz range; only theoretical estimates exist for a frequency of 3 GHz [43]. These estimates indicate that an electron-temperature increase from 200 K to 1000 K in the E layer might occur for a power-flux density of 500 W/m². Test microwave injections from a sounding rocket have been carried out in

Japan

[12]. Although ohmic-heating effects were not observed, plasma waves were excited by the injected microwaves. This was in agreement with several theoretical predictions that high-power microwaves may produce plasma instabilities in the ionosphere (e.g., [42]). Several types of such instabilities produce secondary electromagnetic waves, which could be a source of interference to other radio services. The instabilities might also result in additional electron heating and density irregularities, which could have an effect on other radio waves propagating through the region. It is uncertain if the SPS microwave power-flux density would be high enough to cause such effects, or whether these effects could affect the SPS microwave transmissions.

Another problem may be defocusing of the microwave power beam, due to naturally occurring electron-density irregularities causing rapid signal-strength fluctuations (scintillations). This could have severe implications for the beam control described in Section 3.4, but, again, it is not known if this effect is important for the envisioned frequency of the SPS microwave beam. Theoretical considerations show that a 2.45-GHz SPS system would be more strongly affected than a 5.8-GHz system [42]. The effects of defocusing and scintillation on natural irregularities will be there for all power densities. What is uncertain is whether the high SPS power densities would enhance the effect through nonlinear interactions and feedback.

Some effects of powerful microwaves on the stratosphere have been studied both theoretically and experimentally [44]. These investigations have been carried out for a quite different purpose, namely to study the effects of ozone-destroying pollutants in the troposphere, and to create an artificial ozone layer in the stratosphere by high-power electromagnetic waves. The field strength necessary for this is much higher than the values that would be used by an SPS. Therefore, such effects on the atmosphere are not expected.

In the troposphere, refraction and scintillation effects on the beam (or even those induced by the high-power beam itself) need to be considered. Also, absorption and diffraction by atmospheric gases, aerosols, (water/ice) clouds, and precipitation must be studied. For instance, Recommendation ITU-R P.619 states the following about interference from an SPS: “Using available data on likely harmonic content, it can be shown that – even at the 4^th harmonic – the interfering signal at a distance of 50 km from the rain cell can be comparable with the level of the received signal in the fixed satellite service. At the fundamental frequency, however, direct radiation from the side lobes of the SPS to the terrestrial station will probably exceed the signal due to precipitation scatter.” In addition, two other effects have to be taken into account: beam attenuation and beam diffusion due to rain. As an example, for a cloud temperature of 0° C and a path length under rain of 4 km, the absorption at 5.8 GHz is 0.16 dB, 1.2 dB, and 2.8 dB for precipitation rates of 10 mm/h, 50 mm/h, and 100 mm/h, respectively [45]. Although rain rates of 100 mm/h are rare [1], it has to be stated that the last figure corresponds to a power loss of almost 50%. The beam diffusion at a dBW level can be as large as 4-6 km in diameter for precipitation rates of 50-100 mm/h [45].

4.2 Compatibility with Other Radio Services and Applications

It is assumed that typical SPS systems will use frequency bands around 2.45 GHz or 5.8 GHz. These bands are already allocated in the ITU-R Radio Regulations to a number of radio services (e.g. civilian and military wireless applications), and are also designated for ISM (industry, science, and medical) and applications such as microwave ovens and wireless LANs [46]. It is mandatory that unwanted emissions – such as carrier noise, harmonics, and spurious and out-of-band emissions of the microwave power-transmission beams – are suppressed sufficiently to avoid interference with other radio services and applications, in accordance with the ITU-R Radio Regulations [46]. This is a serious engineering challenge, given the huge disparity between SPS power levels and those of other radio services. Although the intended bandwidth of the SPS emissions is quite narrow – since an essentially monochromatic wave without modulation will be used – spurious and out-of-band emissions generated by microwave power-transmission beams could substantially degrade the performance of other services and applications, even if received only indirectly.

Of particular concern is interference with radio-astronomical observations, which have a protected band (4.9-5.0 GHz) near the envisioned SPS frequencies or their first harmonic. Radio astronomy has historically increased its sensitivity with time, and in the next decade, major initiatives already begun will enhance the sensitivity by 100 fold over existing instruments. All possible measures need to be taken to protect the corresponding observations, since if they cannot be protected, it would not be possible for an SPS to operate legally under the present ITU regulations. Most experts will agree that even a partially operational SPS will constitute a difficult and unwelcome challenge to radio astronomy, and that the coexistence of radio-astronomical observations with an SPS could be extremely difficult. The same applies for measurements by the Earth-exploration satellite services (e.g., a sub-harmonic of 2.45 GHz is close to 1.4 GHz, used for passive sensing of soil moisture and ocean salinity). In 1997, the ITU initiated work towards an ITU-R Recommendation on wireless power transmission [4], which may be relevant to the interference an SPS could cause to other services.

The possibility of spurious emissions related to tube (e.g. magnetron) failure is a serious concern for radio astronomy and many other services. For example, with 10,000 magnetrons of 100 kW output for the microwave transmission, and assuming a mean time to failure of, say, 30 years for these tubes, it is possible that the average failure rate could be one per day at some point in the life cycle.

Furthermore, the passive thermal radiation of the solar cells of a large number of SPS units is expected to make a substantial zone of the sky, centred on the geostationary orbit, unusable for astronomical observations at essentially all frequencies [47]. This would occur even when the microwave transmission of the SPS towards the Earth was not operational.

In addition to this thermal radiation, the huge solar-cell array would act as a broadband antenna for all radio noise created within the SPS (from switching, out-of-band contributions, etc.). Therefore, such RF noise has to be minimised so as not to degrade operations of radio services and applications.

The apparent angular size of a solar-cell array of 10 km² is close to 1 arc minute (somewhat larger than the angular size of Jupiter), and scattering of unwanted radiation in the atmosphere would substantially extend the affected region. This means that even optical astronomy would be affected in an extended region of the sky, particularly if a large number of SPS units were operational. The substantial loss of observable sky resulting from these wideband emissions (optical, UV, infrared, and radio) needs to be carefully considered.

The requirements of spectral purity (a narrowband signal with very low spurious transmission) and the high efficiency of the transmitter will be opposing constraints. They could be difficult to reconcile, since high-efficiency, high-power transmitters have an inherent problem of non-linearity. This needs to be carefully assessed.

Astronomical Radio Quiet Zones (RQZs) are currently in the process of being implemented in isolated areas in, e.g., Australia, China, and South Africa. This is being done to ensure the regulatory protection of next-generation giant radio telescopes against detrimental manmade radio interference over wide frequency ranges, based on interference threshold levels recommended by the ITU. Currently, regulatory control over the RQZs applies only to ground-based transmissions. However, for the zones to be effective, it is important that they are not exposed to harmful levels of emissions from space. Even when an SPS is operating entirely within its permitted frequency range, with no out-of-band transmissions, the power transmitted within its sidelobes may still be harmful to the operation of broadband radio telescopes in RQZs (and elsewhere). An additional challenge will therefore be to devise solutions to prevent unwanted interference from the SPS into such facilities. These solutions may include aspects of antenna design, location of the SPS, and deployment of mitigation techniques at the radio-astronomy sites.

4.3 Microwave Power Transmission Effects on Human Health

A variety of environmental considerations and safety-related factors should continue to receive consideration because of public concerns about radiowave exposure [48]. Above the centre of the rectenna, the SPS power-flux density will be considerably higher than the currently permissible safety levels for human beings. The ICNIRP (International Commission on Non-Ionising Radiation Protection) and Japan both apply limits of 50 W/m² and 10 W/m² for 2.45 GHz and 5.8 GHz, respectively [49]. The latter level is equal to the power-flux density at the perimeter of the rectenna site [1, Section 4.3]. The corresponding exposure limits for IEEE standards have recently been revised, and they are now closer to the ICNIRP limits (see [50] for details).

Since established safety limits for microwave exposure are exceeded in an area around and above the rectenna during normal operation of the SPS, access would need to be carefully controlled to ensure that environmental safety and health standards are maintained. Under normal operating conditions, the SPS microwave downlink will need to be monitored continuously to ensure that the tightly tuned phased-array techniques and beam control are functioning correctly. Should there be a loss of control, beam-defocusing techniques to disperse the power would need to be applied.

It should be noted that there are currently insufficient data on specific microwave power-transmission effects on human health, and that standards for this particular application are not sufficiently developed. Taking into consideration the importance of this field, more studies are urgently needed regarding human health and its bioeffects (see also more details in [1, Section 4.3]).

5. Radio-Science Issues for Further Studies

The list of issues below is most likely not complete. Depending on the outcome of the questions addressed, other issues may come up. Again, the list is limited to issues of URSI’s scientific domain.

• Can the exposure level of the microwave density at the perimeter of the SPS receiving rectenna site be adequately controlled to avoid exceeding the safety level fixed by international standards?

• What is the impact of rectenna operation on (i) biological systems, such as human beings, birds, insects, and plants, etc.; (ii) airborne vehicles, such as airplanes; and (iii) other electric/electronic equipment and telecommunication networks?

• Can SPS operations be made safe by a precise control of the high-power beam using a pilot signal from the Earth, also taking into account the time delay of the signal?

• The influences of atmospheric refraction, beam defocusing, and of absorption and diffraction by atmospheric gases, aerosols, clouds, and precipitation have to be further examined. Are there other effects caused by the SPS power beam on the environment (magnetosphere, ionosphere, troposphere, etc.) that have not yet been explored?

• What is the impact of SPS electromagnetic emissions – both intended and unwanted (harmonics of the microwave frequency, unexpected and harmful radiation resulting from malfunctions) at microwave frequencies and other related frequencies – on telecommunications, remote sensing, navigation satellite systems, and radio-astronomical observations? What actions can be taken to suppress this unwanted emission? Constraints imposed by the Radio Regulations of the International Telecommunication Union must be taken into account.

• How will reflections of sunlight from the huge satellite structure affect optical-astronomical observations, and how will passive thermal emissions affect radio-astronomical observations?

• What are the consequences of long-term exposure to solar-wind particles, and solar radiation of solar cells and other solid-state devices, for the reliability and costs of SPS systems, taking maintenance and possible replacement into account?

• Will an SPS lead to congestion at the geostationary orbit and to interference with communication satellites?

Even if it is beyond URSI’s scientific domain, the economics of SPS systems have to be examined by competent organisations, since the cost advantage is a crucial issue for the feasibility of the whole SPS concept.

Only some parts of these questions can be addressed by laboratory work, simulations, or system analyses. Tests of the large structures (solar-cell arrays, transmitting antenna, mirrors) in space are mandatory. After successful testing, launching a pilot SPS unit as an operational demonstrator project – presumably with broad international consensus – may be a suitable way to assess the remaining questions. However, before being considered for launch, even for such a pilot unit, all concerns, such as the impact on communications, radio astronomy, Earth observations, and bio-hazards, must be fully addressed.

6. Acknowledgements

This white paper is based on detailed reports on SPS systems prepared by the URSI Inter-Commission Working Group on SPS (SPSICWG) [1]. The URSI Board of Officers is indebted to the members of this Working Group and to R. M. Dickinson, D. Farley, R. Gendrin, and L. Summerer for their help in evaluating this paper.

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