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To achieve efficient long-range wireless power transfer (WPT), large antenna systems are necessary spanning lengths of tens to thousands of meters in one dimension. This creates an array in the order of at least hundreds of thousands to billions of elements. This makes the implementation of beamforming control a challenge. Various works focus on iterative optimization or channel estimation to maintain high efficiency in a time-varying environment requiring complex processing capabilities. A simpler alternative is the both-sides retrodirective antenna array (BS-RDAA) system where iterative optimization or channel estimation is not required. In a previous study, it was observed that this system achieves maximum WPT efficiency if the system is marginally stable. Thus, there is a need to regulate the system to maintain marginal stability regardless of the transmission channel conditions. In this paper, we present a plant model for the system and design a control system to drive it to marginal stability. The result is a WPT implementation that is not dependent on complex processing capabilities present in other established high efficiency methods. We also confirmed the ability of the proposed design to maintain maximum efficiency in a dynamic environment through the results of an electromagnetic simulator and a time domain simulator., Comment: 12 pages, 14 figure, this work has been submitted to the IEEE for possible publication

The retrodirective antenna array is considered as a mechanism to enable target tracking of a power receiver for long range wireless power transfer (WPT) due to its simplicity in implementation using only analog circuits. By installing the retrodirective capability on both the generator and rectenna arrays, a feedback loop that produces a high efficiency WPT channel is created. In this paper, we characterize the dynamics of this phenomenon using a discrete-time state-space model based on S-parameters and show that the system can naturally achieve maximum theoretical WPT efficiency. We further confirmed the theoretical analysis through a hardware experiment using a 12-port circuit board with measurable S-parameters mimicking a static wireless channel. The results collected from the hardware experiment show agreement with the proposed theoretical framework by comparing the theoretical efficiency with the measured efficiency and by showing that the collected data points follow the predicted condition to achieve maximum efficiency., Comment: This current version has been submitted to the Space Solar Power and Wireless Transmission on February 19, 2024 for possible publication. Compared to the previous version, this version is a major revision discussing existing works more thoroughly to the proposed idea and also adding more detail to the experiment setup so it can be reproducible

Infrared space interferometers can surpass the spatial resolution limitations of single-dish space telescopes. However, stellar interferometers from space have not been realized because of technical difficulties. Two beams coming from individual satellites separated by more than a few tens of meters should precisely interfere such that the optical-path and angular differences between the two beams are reduced at the wavelength level. Herein, we propose a novel beam combiner for space interferometers that records the spectrally-resolved interferometric fringes using the densified pupil spectroscopic technique. As the detector plane is optically conjugated to a plane, on which the two beams interfere, we can directly measure the relative phase difference between the two beams. Additionally, when an object within the field of view is obtained with a modest signal-to-noise ratio, we can extract the true complex amplitude from a continuous broadband fringe (i.e., one exposure measurement), without scanning a delay line and chopping interferometry. We discovered that this spectral imaging method is validated for observing the solar system objects by simulating the reflected light from Europa with a small stellar interferometer. However, because the structure of the object spectrum may cause a systematic error in the measurement, this method may be limited in extracting the true complex amplitude for other astronomical objects. Applying this spectral imaging method to general astrophysics will facilitate further research. The beam combiner and spectral imaging method are applied to a formation-flying stellar interferometer with multiple small satellites in a Sun-synchronous orbit for observation of the solar system objects in visible and near-infrared. We present an overview of SEIRIOS and the optimized optical design for a limited-volume spacecraft., Comment: Accepted for publication in JATIS, 44 pages, 10 figures

Deci-hertz Interferometer Gravitational Wave Observatory (DECIGO) is the future Japanese space mission with a frequency band of 0.1 Hz to 10 Hz. DECIGO aims at the detection of primordial gravitational waves, which could be produced during the inflationary period right after the birth of the universe. There are many other scientific objectives of DECIGO, including the direct measurement of the acceleration of the expansion of the universe, and reliable and accurate predictions of the timing and locations of neutron star/black hole binary coalescences. DECIGO consists of four clusters of observatories placed in the heliocentric orbit. Each cluster consists of three spacecraft, which form three Fabry-Perot Michelson interferometers with an arm length of 1,000 km. Three clusters of DECIGO will be placed far from each other, and the fourth cluster will be placed in the same position as one of the three clusters to obtain the correlation signals for the detection of the primordial gravitational waves. We plan to launch B-DECIGO, which is a scientific pathfinder of DECIGO, before DECIGO in the 2030s to demonstrate the technologies required for DECIGO, as well as to obtain fruitful scientific results to further expand the multi-messenger astronomy., Comment: 10 pages, 3 figures

LEO-to-GEO intersatellite links using laser communications bring important benefits to greatly enhance applications such as downloading big amounts of data from LEO satellites by using the GEO satellite as a relay. By using this strategy, the total availability of the LEO satellite increases from less than 1% if the data is downloaded directly to the ground up to about 60% if the data is relayed through GEO. The main drawback of using a GEO relay is that link budget is much more difficult to close due to the much larger distance. However, this can be partially compensated by transmitting at a lower data rate, and still benefiting from the much-higher link availability when compared to LEO-to-ground downlinks, which additionally are more limited by the clouds than the relay option. After carrying out a feasibility study, NICT and the University of Tokyo started preparing a mission to demonstrate the technologies needed to perform these challenging lasercom links. Furthermore, to demonstrate the feasibility of this technique, an extremely-small satellite, i.e. a 6U CubeSat, will be used to achieve data rates as high as 10 Gbit/s between LEO and GEO. Some of the biggest challenges of this mission are the extremely low size, weight and power available in the CubeSat, the accurate pointing precision required for the lasercom link, and the difficulties of closing the link at such a high speed as 10 Gbit/s., Comment: 5 pages, 10 figures, 3 tables