Table of contents
1.1 Problem Statement…………………………………………1
1.2.1Metrics of Performance…………………………………1
1.2.2 Sources of test data……………………………………1
1.2.4 Testing and Evaluation…………………………………1
1.2.5 Alternative Approaches………………………………..1
1.2.6 Proposed Method……………………………………..2
1.2.7 Problems that might arise………………………………5
1.3.3 Societal Impact……………………………………….6
1.3.4 Total Addressable Market………………………………6
1.4.1 Virtue Ethics…………………………………………6
1.4.2 IEEE Code of Ethics…………………………………..7
1.4.3 Utility Ethics…………………………………………7
1.7 Gantt Chart……………………………………………….9
2.0 Historical Review…………………………………………11
3.0 Literature Cited…………………………………………..17
List of Figures
Figure 1.2-1, Proposed Retro Directive Method……………………..4
This section is divided into 7 sub-sections, 1.1 Problem Statement, 1.2 Approach, 1.3 Motivation, 1.4 Ethics, 1.5 Standards, 1.6 Validation, 1.7 Gantt chart, 2.0 Historical Review, 3.0 Literature cited
1.1 PROBLEM STATEMENT
Given a microwave wireless power transmission system subject to a constraint that the microwave beam should travel in the desired direction. We seek to find a way to control the direction of the beam using retro directive phased array antenna.
This section is divided into six sub-sections, 1.2.1 Metrics of Performance, 1.2.2 Sources of data, 1.2.3 Prototyping, 1.2.4 Testing and Evaluation, 1.2.5 Alternative approaches, 1.2.6 Proposed Method, 1.2.7 Problems that might arise.
1.2.1 METRICS OF PERFORMANCE
We intend to measure the steering accuracy in order to focus the beam in the desired direction which maintains the efficiency of the transmitting antenna and releases transmit receive frequency from integer constraints.
1.2.2 SOURCES OF TEST DATA:
We intend to build a combination of transceiver architecture and a Phase Lock Loop (PLL) structure. Then simulations are performed to determine the performance of retro directivity beam steering.
1.2.4 TESTING AND EVALUATION
We plan to use a retro directive phase array system using PLL to measure the steering accuracy of the beam. This can be achieved by building transceiver architecture which down converts from 2.9 GHz to 10 MHz and up converts from 10 MHz to 5.8 GHz, maintaining a phase coherency between the channels, along with PLL structure. The schematics were first entered into Protel 2004 layout software and then they are used to lay out on-board components.
1.2.5 ALTERNATIVE APPROACHES
Controlling beam direction precisely is important for two reasons: 1) to amplify the energy transferred to Earth, 2) to limit radiation in undesired directions.
The concept of retro directive beam steering is employed to overcome the obstacles and to direct the microwave beam accurately towards the rectenna present on the ground. Several approaches have been explored till now to achieve accurate beam steering. They are:
VAN ATTA REFLECTORS
The first among them is a passive component called Van Atta reflectors. This reflector achieves retro directivity by transporting phase lag and lead from one side of the receiver to the other side. The general retro directive system has phase conjugate circuits in both transmitting and receiving antenna, which plays the same role as the pairs of antenna in the Van reflectors. The signal transmitted from the target is received and again re-transmitted through the phase conjugate circuit known as a “Pilot Signal.” Here the identical frequency is used for Pilot Signal and returned signal, along with a local oscillator signal with a frequency two times as high as the pilot signal frequency. A standard frequency or phase is required to steer microwave beams in the desired direction. Due to its passive nature, the application of modulation is not preferable.
The second approach is an active component called Heterodyne technique. It was given this name due to its use of lower sideband frequency. By mixing the received signal of known frequency with double transmitter signal frequency, phase conjugation is achieved. Compared to Van Atta reflectors, phase conjugation is slightly different in the Heterodyne method. The steering of the beam is linked to phase gradient. In this process, sign change is used to represent phase conjugation. As it uses active components, the Heterodyne method enables us to provide signal amplification and gain. This is an important feature which is distinct from passive components.
PLL ANTENNA ARRAY
The PLL Antenna Array utilizes a control circuitry technology to achieve retro directivity. In this technique, the control circuitry consists of three modes – retro directive mode, conventional phased array receive mode, or transmit mode – and can be converted. In order to limit the steering accuracy, the PLL Antenna Array uses a frequency offset technique with a value of 0.12.
The above methods are previously used for achieving retro directivity and steering of the beam. A drawback of this method is the difficulty in modulation due to the passive nature of reflectors. Another drawback is that the use of same frequencies separation of modulated output from input signal is difficult, so a new method is proposed called retro directive phase array antenna with Phase Lock Loop.
1.2.6 PROPOSED METHOD
The proposed method of retro directivity was created by utilizing several pre-constructed processes. This approach introduced new concepts such as frequency translation and phase scaling. By finding the ratio of two different phase detector gains, we can determine the scaling factor. Frequency translation is demonstrated using the heterodyne technique.
The transmitting frequency of 5.8 GHz and the receiving frequency of 2.9 GHz and IF (Intermediate Frequencies) of 10 MHz is considered. The proposed retro directive system consists of a Phase control loop and it is comprised of a linear phase detector, phase coherent up and down converters, a summer and a voltage controlled oscillator. In SPS, maintaining the steering accuracy of the beam is very important; so phase conjugation is used to derive the performance of beam steering accuracy. To achieve phase conjugation of the proposed method, number of components are used like transceiver architecture, phase detector, signal shape, XOR gate, zero crossing detector, filter, offset and gain adjustment, and VCO.
Transceivers are used to achieve retro directivity of the proposed method. By incorporating phase delay between radiofrequency modulated signal and radiating elements, phase shifting and beam steering can be achieved. RF signal is modulated and then sent to the transmitting elements through phase shifters. As this method converts the phase information to baseband, the transmitted and received signals should be up and down converted.
Phase detectors are used to subtract phase difference between the reference signal and VCO feedback. Phase detector is an XOR which mixes two inputs and results in a pulse train. This detector translates angular domain into voltage. The output from the phase detector is in a negative form and it is fed into the loop summer which indicates a negative feedback loop and it is important to achieve the stability of the loop. The output waveform is not exactly sinusoidal and not even in the digital form to be an input to XOR gate. Therefore TTL logic devices such as flip flops are used. Here the frequency of input waveform is converted into 10MHz square waves and now square waves are divided by a factor of 8. Now the output is fed as an input to the XOR gate. Output from the XOR is filtered using a lag lead network. The designs are verified first with simulations and then they are realized on Printed Circuit Board (PCB) software tools. Schematics are first entered into the Protel schematic editor. The way in which the schematics were entered exploited the multichannel design capabilities of the layout software.
This method doesn’t set steering accuracy limit as other frequency offset methods do. Though this proposed design is more complex than other methods, it can still fit onto a monolithic chip and can employ a silicon circuit design. The unique contribution of this method is that it uses a combination of phase array antennae and Phase Lock Loop. This introduces two concepts known as Frequency translation and Phase scaling which releases the retro directive beam steering from errors and integer constraints.
Even though this method releases beam steering from integer constraints, a little amount of conjugation error is present which limits the steering accuracy of the beam. However in Solar Power Satellite, steering accuracy is mentioned as a performance metric. Here consideration is given to how different array sizes and elements would affect the beam shape, steering accuracy, and measurement requirements.
Rx Pilot Tx
2.9 GHz 5.8 GHz
Down Conversion Phase Coherent
-10MHz + – 10MHz +
Linear Phase Detector
Linear Phase Detector
10 MHz LO
Phase Scaling Summer VCO
Fig- 1.2-1 Proposed Retro Directive Method
We consider different sizes of array elements such as Two element array, Two element SPS Transmitter, and Large array beam steering. Using matlab code we can produce array gain pattern for various elements and can see the effect on the beam shape and steering accuracy. Maximum steering can be obtained for arrays with high directivity and magnitude. In large array beam steering, we considered 100 elements to obtain better steering output. Due to high directivity of SPS transmitting sub array, a large array with many elements is necessary to test the performance of SPS system. For SPS applications the beam steering accuracy should be of order 0.0005 degrees. In order to achieve measurable beam steering, large antenna arrays are required as those proposed for SPS.
1.2.7 PROBLEMS THAT MIGHT ARISE
The difficulty that might arise during the execution of the project is the transportation of equipment from earth to space. After the execution, there might be some risk to living beings due to the microwave beam. This problem can be solved by placing the rectenna site in an isolated place where there is no harm to human life or birds.
This section is divided into 4 sub sections 1.3.1 Novelty, 1.3.2 Contribution, 1.3.3 Societal Impact, and 1.3.4 Total Addressable Market.
Solar power satellite is a huge satellite which is designed to transmit solar energy from space to Earth using three segments: (1) solar energy collector, (2) DC to microwave converter, and (3) a large array antenna. The problem is aroused when transmitting microwaves from space to Earth, because the microwaves get scattered in free space and it will be difficult to control the beam in the desired direction. So our goal is to steer the microwave beam precisely in the desired direction to maintain the efficiency of the transmission system.
A novel approach of retro directive phased array antenna beam steering technology using phase control loop is proposed.
In the Microwave wireless power transmission system, the steering of the microwave beam in the desired direction is important in order to achieve the efficiency of the system. Van Atta Reflector, Heterodyne Technique, and PLL Antenna array are the methods used previously. Due to the passive nature of the Van Atta Reflector, the application of modulation is not preferable. Similarly, due to the use of the same transmit and receive frequencies, isolating the modulated output from the input signal is difficult. We can achieve isolation by creating frequency offset between input and output.
Retro directive techniques are not new; however, the proposed retro directive method uses a combination of Phase array antenna and a Phase Lock Loop structutre which introduces two new concepts of “phase scaling” and “frequency translation”. In frequency translation retro directive system, we consider a linear phase array antenna tuned to receive an incoming pilot signal at some desired frequency (fr). At the same time, it transmits a return signal in the same direction but at a different frequency (ft). The ratio between the transmit (ft) and receive (fr) frequencies is known as “phase scaling factor”, which releases the retro directive transmit-receive frequency ratio from integer constraints and avoids steering errors. In this way, we can be able to steer the beam in the desired direction without any deviation.
1.3.3 SOCIETAL IMPACT
This section describes the impact of the microwave beam on society.
On airplanes and birds:
The impact of the beam is difficult to assess for the objects that fly through the beam. Its effect upon an airplane that flew through the beam would be minimal due to the short time frame within the beam and moderately low power density. However birds get a maximum exposure to the beam because they fly above the rectenna. The solar power satellite beam operates at 2.45GHz in a microwave region and has a wave length near the size of birds. In cold weather, birds in surrounding areas seek the edge of the rectenna array to warm themselves. In order to study the effect of the beam on birds in the summer, the EPA (Environmental Protection Agency) conducted research in 1981. Their report said there is no effect on birds for a power density of 23mW/square cm, but by doubling the power density it did affect the birds.
1.3.4 TOTAL ADDRESSABLE MARKET
Solar Power Satellite is a reliable means of transmitting power wirelessly from space to Earth. In 1981, the estimated cost for the first space based Solar Power Satellite is 11.5 dollars for variable cost and 102 dollars for fixed cost. At present, the cost for launching and operating Solar Power Satellite is estimated to be tens of billions of dollars. According to the NASA/DOE model, they are planning to transmit 6.72 gigawatts of power from space to Earth. The average price for kilowatt per hour is 12 cents. The world’s total population is around 8 billion. If the average power consumption per day is around 12.7 Kw/hr, then the Total addressable market can be calculated by multiplying the total population, average power consumption per day, and average price for Kw/hr. The Total Addressable Market is $1200 billion approximately.
This section consists of 3 sub sections 1.4.1 Virtue Ethics, 1.4.2 IEEE Code of Ethics, 1.4.3 Utility Ethics.
1.4.1 VIRTUE ETHICS
The Solar Power Satellite, which provides an abundant amount of solar energy, has an ability to transfer energy anywhere on Earth. Microwaves are used to transmit power to the earth. Components used in this process are expensive. As we know that microwave power is beamed to the earth, it has some negative effects on the environment, birds, and mankind. These effects can be reduced by placing the rectenna station in an isolated place.
1.4.2 IEEE CODE OF ETHICS
According to IEEE code of ethics  we need to accept responsibility in making decisions consistent with the safety, health, and welfare of the public and to disclose promptly factors that might endanger the public or the environment.
The Solar Power Satellite is a gigantic satellite which transmits power wirelessly from space to the earth. Here it transmits power in the form of microwaves. These microwaves are very harmful to the environment and mankind. To ensure the safety of the public and biota, the rectenna stations should be located in a place where there is no effect to mankind.
1.4.3 UTILITY ETHICS
Solar Power Satellite is a tremendous technology which supplies power wirelessly from space to earth using microwaves. Though microwaves are harmful for birds and the environment, it produces a lot of power to many countries. The components used in Microwave Wireless Power Transmission System include Solar Power Satellite, Microwave generator, Transmitting antenna, and Rectenna (Rectifying antenna) present on the ground station. These components are expensive and cannot be easily purchased online or in stores.
Two operating frequencies – 2.45GHz or 5.8GHz – are considered for Solar Power Satellite, where many wireless communications like Wi-Fi, Bluetooth, television broadcast, and radio broadcast signals operate. To avoid interference between these communications, we need to take permission from the Federal Communication Commission (FCC). The microwave transmission composition has a power level below the international safety standard, i.e. at a frequency of 2.45GHz. The solar panels used are standard silicon cells or multi-junction cells made up of gallium arsenide in order to avoid degradation of the atmosphere.
The Solar Power Satellite is a huge satellite used to transmit power from space to earth in the form of microwaves. When microwave power is beamed to earth, it gets scattered in different directions. To control the direction of the beam and to maintain efficiency of the system, we use retro directive phase array antenna. This design is a combination of Heterodyne technique and PLL antenna array. It is the first system that achieves frequency translation and phase scaling. Generally, using Heterodyne technique, frequency translation is demonstrated but restricted by the discrete properties of phase scaling using up converting harmonic mixers. These mixers constrain the factors to positive numbers and the same constraint is set on frequency translation. In this retro directive system, the phase shift between the elements, represented by “φo”, measures the phase shift of the pilot signal with a phase detector gain KPD1. The received phase is referenced into the phase control loop of the transmitting circuit which contains another phase detector of gain KPD2.
To achieve phase control loop of a retro directive system, a phase lock loop (PLL) structure is used. In SPS, steering accuracy is an important factor and phase conjugation is used to derive beam steering accuracy.
To steer the direction of the microwave beam, we use phase array antenna using phase lock loop. We will first verify the designs with spice simulations and then implement using Protel 2004 printed circuit board software tools. Various electrical components are used to create the phase detector and the phase control loop. Here we will design a transceiver which down converts from 2.9 GHz to 10 MHz and up converts from 10 MHz to 5.8 GHz, maintaining a phase coherency between the channels. The transceiver architecture consists of phase detector, signal shape, Zero crossing detector, and XOR. Phase detectors are used to subtract phase difference between the reference signal and VCO feedback. Phase detector is an XOR which mixes two inputs and results in a pulse train. This detector translates angular domain into voltage. The output from the phase detector is in a negative form and it is fed into the loop summer which indicates a negative feedback loop. It is important to achieve the stability of the loop. The output waveform is not exactly sinusoidal and not even in the digital form to be an input to XOR gate, so TTL logic devices such as flipflops are used. Here the frequency of input waveform is converted into 10MHz square waves and now square waves are divided by a factor of 8. Now the output is fed as an input to the XOR gate. Output from the XOR is filtered using a lag lead network. We will also design a phase detector – two phase coherent sinusoidal wave form given as input and we will obtain a DC voltage linear to the phase difference as output and phase control loop- inputs are phase reference voltage, a 10MHz as a reference, and output from voltage controlled oscillator (VCO).
These schematics were first entered into the Protel 2004 PCB layout software and then they were used to lay out on the board components. The way in which the schematics were entered exploited the multichannel design capacities of layout software. Only a single channel is entered once into the schematic, but it can be utilized numerous times without design repetition. We can measure antenna steering accuracy by summing the nonlinearity errors of phase detector and phase control loop. The steering angle (θa) can be defined as the sum of steering error (θe) and ideal steering value (θo).
For Solar Power Satellite, the maximum steering error (θe) should be of order 0.0005 degrees. Using a highly directive antenna, such as suggested for SPS, requires large antenna arrays in order to achieve beam steering. To accomplish this, careful attention must be given to measurement techniques and accuracy. Steering accuracy and size are considered as performance metrics in Solar Power Satellite. By reducing the size of transmitting antenna, the efficiency of the system is increased.
1.7 GANTT CHART
Fig 1.7-1 Gantt Chart showing 6 Month Time Line
Fig 2.1 shows the Taxonomy of Microwave Wireless Power Transmission from space to Earth
2.0 HISTORICAL REVIEW
Susumu Sasaki et al. explained about the Solar Power Satellite and its operation. Wireless power transmission, one of the important technologies for Solar power satellite, is discussed. Various types of SPS are categorized. In this paper, microwave power transmission for solar power satellite is demonstrated. Microwave technologies are essential to obtain high power conversion and precise beam control. This paper also deals with a roadmap for commercial SPS from research phase to development phase. 
Ralph H. Nansen discussed how the Solar Power Satellite, which is placed in Geo synchronous orbit, transmits solar power from space to earth. The components used are discussed. A ground test program is done for all the components used in the Solar Power Satellite. The important function of this test is to validate the wireless power transmission. The development plan schedule and cost for Solar Power Satellite are also discussed. The important features to signify are beam control, efficiency of steering, reliability, cost, and safety. 
Rugved Bidkar explained an emerging technology called Solar Power Satellite. The concepts of wireless power transmission regarding the solar power satellite and its operation are explained. This paper also discusses the components and parameters of transmitting antenna of the Solar Power Satellite. Not only that, it also shows changes in efficiency of rectenna with different semiconductor materials. The production costs of electricity using coal and solar power satellite are also compared. 
Allam. W. Love proposed a model in which Random phase errors and Pointing inaccuracies are ignored. In this model, the transmitting antenna is designed as a circular aperture. Gaussian aperture distribution is used to achieve unity efficiency of the beam guide. This distribution is shortened at -10dB level at the edge of the transmitting antenna. This paper also describes one parameter distribution which yields better results between narrow beam width, low sidelobes, and beam efficiency. In order to achieve beam efficiency, Klystron reference concept should use a -10db taper over the array. 
Koji Tanaka et al. proposed an electrical demonstration model for the Solar Power Satellite. This model consists of Sun simulator, the satellite segment, and the ground segment. Here the sunlight is simulated by 4 HID models of 400W each, with temperatures around 3000 degrees C. The electrical model generates electricity by means of solar panels which convert DC power into microwaves with a frequency of 5.8GHz. Two types of antenna array are designed: one antenna array with spacing 1λ and the other with 0.7λ. Using phase approximations, beam steering was simulated. 
Shi- Wei Dong et al. explained a new concentric disc solar power satellite which can transmit 1.1 GW power. It is comprised of an annulus solar cell array with diameter of 2.726 km and 1.01 km and a disc transmission antenna with diameter of 1.01km. Here the solar array can collect 5.5 Gw of solar power. The cell annulus is attached with thin film solar cell, both on the upper and lower planes. The disc of transmitting antenna circumrotates is to maintain a planar accuracy, but this accuracy is critical. Then active integrated antenna is employed, where monolithic microwave integrated circuit (MMIC) is integrated on microstrip patch antenna. Construction, generation of power, and microwave power transmission of Concentric Disc SPS are explained in detail. 
Shigeo Kawasaki et al. developed a novel technique of an active integrated antenna for a microwave power transmission system to achieve size reduction of a radar system. The unit cell of the active integrated antenna technique is designed in a laminated structure which is comprised of an amplification layer, a coupling layer, and a radiation layer. In this paper, the circuit method and experimental data of an active integrated amplifier antenna array were demonstrated to realize light and multifunctional spacetenna for SPS2000 system. 
Shu Ting Goh et al. proposed a space craft formation on low earth orbit. Satellites in low earth orbit gather solar energy and transmit power to the receiver station on the ground. Two microwave transmission methods have been introduced. One is the leader space craft which collects power from other space crafts and transmits it to the ground through microwaves and the other is all spacecraft which directly transmits power to ground stations. Simulation results are conducted and several techniques are implemented to provide accuracy of space craft position, Velocity estimation, and oscillator sampling rate. The unique contribution of this paper is improved transmission techniques which can be used for optimal orbit design and high-performance transmission of power. 
William. C. Brown discussed the application of microwave power transmission technology to the solar power satellite. In this he explained five essential parts on how to transmit energy from space to ground. He also mentioned the environmental aspects and problems caused by beam transmission. Apart from these, he compared his study with other approaches like conventional power generation, photovoltaic arrays, and fusion which has some disadvantages over solar power satellite .
Xun Li et al. proposed a novel approach called step amplitude distribution taper for microwave power transmission for solar power satellite. To maximize beam collection efficiency and to reduce side lobe level, the best tapers used are Gaussian and Isosceles trapezoidal amplitude distribution, but due to the smooth nature of (GAD), a large number of amplifiers are needed which is expensive. So to overcome this problem, we are using ITD edge tapering in which internal elements are uniformly excited .
Ronald. J. Gutmann used solar power satellite as a potential source of baseload electric power as a class design project. The concept of SPS has been demonstrated. As it is a class design project, the design group worked on solar collection and conversion, power beam control, and rectenna. The rectenna group evaluated an alternative to the reference SPS system called Hogline rectenna where fundamental considerations are useful in extensive analysis. The purpose of this study was to examine the possibility of employing the dual conversion system in order to increase the efficiency of the SPS system and to decrease the size and mass of the system. 
H. Hayami et al. explained the concept of Solar Power Satellite and its importance in playing a major role in reducing global warming. In this paper, we calculate the CO2 emissions observed from the life cycle of SPS. This cycle consists of production of rocket fuels, solar panels, and construction of rectenna. Base line scenario shows that the emission of carbon dioxide generated from SPS is nearly identical to the emissions emitted from the nuclear power plant, whereas in SPS breeder scenario the CO2 emission is comparatively less. 
William. C. Brown described the advances made in developing the components of microwave power transmission system for Solar Power Satellite, their characteristics, and the process of transmitting power from space to earth. He focused mainly on the architecture of transmitting part and explained how the phase and amplitude of magnetron directional amplifier output is controlled by phase and feedback loops .
William. C. Brown explained the feasibility in adapting the microwave technologies used for transmitting microwave beam efficiently from space to earth using space based solar power satellite. He proposed a new kind of electron tube called Amplitron (Continuous cathode cross field amplifier) which can produce an efficient microwave input signal. This paper also deals with transformation of free space microwave transmission system from laboratory technology to 10Gw .
Golap Kanti Dey et al. explained the microwave power transmission and the triple junction photo voltaic cell for the Solar Power Satellite. This paper discusses that target detection and beam control can be achieved by using a retro directive system which is made of transmitting, receiving antennas, and phase conjugate circuit. FRIIS transmission equation is used to receive the power from the transmitting antenna. As microwave beams are harmful to animals and human life, dam, lake, and offshore sites are considered as best locations for receiving microwave power. 
Dai Suke Sato et al. proposed an approach opposite to those of SPS concepts called “Tethered SPS.” This approach is very simple and technically feasible. It uses a unique PV/Micro wave conversion hybrid panel which has a sandwich structure with three functional layers: solar cells, DC-RF converter layer, and power transmission layer. High efficient multi junction solar cells are attached to the surface of power generation and transmission layers, where the generated power is converted into microwaves by DC-RF converter. Now the microwave power is transmitted to the transmission layer by phase array antenna. Concepts of panel design and thermal resistance measurement is explained and future research works to be addressed is presented. 
Ken-ichiro Maki et al. proposed a breadboard model for the Solar Power Satellite which consists of a panel structure. This panel consists of three layers assigned to transmitting antenna, microwave amplification, and thermal radiation. The specifications required for the breadboard model such as panel size, thickness, antenna elements, frequency, and output power are presented. This paper also deals with radiation characteristics which are investigated by experiments using antenna panel. These characteristics are measured in four types of formation. The beam steering performance is examined by measuring radiation pattern in the radio anechoic chamber. 
William. C. Brown discussed the principles and components used for beamed microwave power transmission. Choice of frequency is also examined. An application is explained regarding the transportation system from low earth orbit to geo stationary orbit whose power is supplied by a microwave beam originating at the earth’s surface. Principles of electric propulsion were outlined and environmental considerations are also examined. 
Ying Wang et al. explained analysis of antenna transmission efficiency. In this paper, circular aperture of 5.8GHz and beam efficiency of 93.8% is considered as an example and performed simulation. Using microwave theory and radiation field integration methods, efficiency of beam is calculated. Not only that, connections between rectenna diameter and beam efficiency are discussed. 
Takaki Ishikawa, Naoki Shinohara described one of the beam correction methods called Position and Angle Correction (PAC) to maintain the flatness of the transmission antenna surface. Using the concept of PAC, the researchers sent a pilot signal from the rectenna site on the ground to measure the phases of the signal on every panel. The gradients of panel modules were estimated using Direction of Arrival method. Schematic diagram of panel module with 3,4 pilot signal phase measurement points is explained. PAC technique has an issue with panel gradient estimation; it corrects the transmission microwave phases only when the panel module gradients are low. So, to modify this, an improved PAC method is proposed. Using these methods, the phases of the power transmission microwaves can be corrected without the impact of DOA uncertainties. 
S. K. Saha et al. proposed a novel approach of antenna array with beam steering technology which can steer the beam in a desired direction, even in the presence of array imperfections. Conventional beam forming and optimal beam forming processors are explained. Conventional beam forming, known as delay and sum, has weights of equal magnitudes and needs information about the direction of interference sources and this beam forming doesn’t maximize output SINR. To limit this, an optimal processor is used to increase the SINR (Signal to Interference Noise Ratio) output. Using this proposed method, we can cancel directional interferences and can steer the beam in the desired direction. 
Takaki Ishikawa et al. explained the phase array system which consists of 256 antenna elements and 5bit digital phase shifters which are used for beam forming. The parameters required for phase array system are presented. Here the researchers measured H-plane radiation patterns in an Advanced Microwave Energy Transmission Laboratory. In this paper, flat topped beam pattern is formed, which requires accurate output phase control to estimate the calibration accuracy. 
Kozo Hashimoto describes the Solar Power Satellite which generates DC power by solar cells and transmits it to earth through microwaves. This paper also discusses the frequency to be allocated for the Solar Power Satellite. Generally used frequencies for wireless power transmission experiments are 2.45GHz and 5.8GHz. However the 2.45 GHz band is widely used for wireless LAN and the 5.8 GHz band is used for dedicated short-range communications (DSRC); interference occurs between them and some of the potential interference for the Solar Power Satellite is discussed in this paper. So researchers should seek permission from International Telecommunication Union (ITU) in order to operate at available frequency. 
Christopher T Rodenbeck et al. explained the concept of Solar Power Satellite which is used to transmit electrical power continuously to the rectenna site by means of a microwave beam. Retro directive control is used in order to maintain the operating efficiency. This paper discusses a limitation which has not been addressed previously, known as “Beam Pulling”. To explain this effect, we considered two different retro directive arrays. One array is composed of four isotropic elements and the other array is composed of four microstrip patch antennas with spacing of 0.5 wavelength. The beam transmitted from the first array is precisely steered in the desired direction, but the beam transmitted from the second array steers 2 degrees closer than prescribed. So, to eliminate this, they have to design a retro directive array with an infinite number of elements. 
Christopher T. Rodenbeck et al. explained an alternative architecture for retro directive control of the SPS transmitter called phased array architecture. This architecture achieves maximum operating efficiency and reduces the environmental concerns. This method uses the 5.8 GHz transmit beam and the 2.9 GHz pilot beam which directly conjugates the phase of the received signal and then retransmits the conjugated signal. This architecture uses a second harmonic retro directive transceiver to steer the harmonic signal to the direction of the pilot beam and allows the 2.9 GHz transmit beam to guide the 5.8 GHz. 
Yuichiro Ozawa et al. explained the Solar Power Satellite concept. For SPS, the steering of the microwave beam is an important factor to maintain the efficiency of the transmission system. Here the beam direction is controlled by adjusting the phase difference. For SPS, it is necessary to control the output phase of antenna within 5 degree accuracy. So the researchers developed a phase control system which consists of Phase Lock Loop (PLL). The PLL is used for stabilizing the phases of output to maintain high degree accuracy. Beam control experiment configuration is discussed, which explains that the beam direction can be controlled by using a command. 
Peter E. Glaser explained the status and overview of Solar Power Satellite and its significant usage of power for global basis. This paper also discusses the economic considerations and environmental issues related to SPS. Apart from these, he proposed shielding and radio receive filters. By using this, we can reduce interference with other satellites when transmitting microwave beam from space to earth. 
Geoffrey A. Landis examined new methods in order to make the Solar Power Satellite feasible. The sites served by the same satellite can be at most 80 degrees by assuming that the allowable zenith angle at the receiver is 45 degrees. This paper discusses the difficulty in system design, i.e. distributing the power. This problem can be solved by mounting the phase microwave transmitter array to the solar array. A Space Segment Model is introduced to assess the effect of technology on performance, size, and cost. 
Rong Wang et al. proposed a novel approach for SPS application based on the concept of artificial perfectly matched layer. Conventional rectenna technology consists of a conventional rectenna and dc filter. However, it is not suitable for SPS applications, because it suffers from back scattering and non-ideal aperture efficiency. To overcome this, an approach based on PML is designed. In this approach, conventional antenna is replaced with MM pattern with a rectifying diode and dc filter is used to transfer the rectified power to ground. Design and fabrication measurements have also been studied. 
B. Sandhya Reddy et al. proposed a microstrip patch array antenna to reduce the weight of the Solar Power Satellite. It is operated at a frequency of 5.8 GHz. Foam is used as a substrate material, because it provides low dielectric constant and loss tangent. The proposed method consists of four rectangular linearly polarized 2X2 array with a spacing of inter elements which is 0.75 times the free space wavelength. This spacing is considered in order to yield a better result among sidelobe level, antenna gain, and aperture efficiency. Using Moment of Method (MOM) simulation tool, simulation results are carried out for Voltage standing wave ratio (VSWR), resonant frequency, and gain of array antenna. 
Thorat Ashwini Anil et al. explained the Solar Power Satellite which transmits electrical power from space to earth. Solar Power Satellite is situated at Geo synchronous orbit at a distance of 36,000km. It consists of three functional steps. Two processes, photovoltaic conversion and solar dynamic conversion which is used for converting solar power into electricity, are discussed. Merits and demerits of solar power satellite are also discussed in this paper. 
Makinde K et al. discussed the concepts of Wireless Power Transfer and Solar Power Satellite. The process of converting solar power into electricity is also explained. Microwave Power Transmission is considered as one of the important alternative technologies for power transmission. Components of Solar Power Satellite such as Microwave generator, Transmitting antenna, Rectenna, and their uses are discussed in this paper. Biological impacts of wireless power transmission are also discussed. 
Ajay Krishnan Selucca et al. explained the Solar power Satellite and the history of wireless power transmission and their types. Conversion process of solar power into electricity is explained. Advantages and Disadvantages of SPS are discussed. Comparison of laser over microwaves is presented. Some of the major challenges related to SPS are presented. Future commercial applications of wireless power transmission are discussed. 
Paul Jaffe et al. explained the history of Solar Power Satellite. Few models of Solar Power Satellite are proposed by Japan, and NASA such as SPS 2000, Sun tower, and Modular symmetric concentrator. This paper also discusses the retro directivity concept in order to control the direction of the microwave beam. He proposed a prototype model for SPS called sandwich module and discussed about the metrics of performance. 
Shyma.S et al. explained the largest application of wireless power transmission called Solar Power Satellite and the parameters required for the transmitting antenna of SPS such as frequency, diameter, efficiency, and rectenna diameter. He also explained the type of antenna used for Microwave Power Transmission. Some recent technologies employed for retro directive beam control are discussed. 
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Figure 1.2-1. Proposed Retro Directive Method. Adapted from “Retro Directive Phase Lock Loop Controlled Phased Array Antenna for a Solar Power Satellite System” by Samuel John Kokel, 2004, pp. 34.
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