Japan’s Aerospace Exploration Agency (JAXA) is taking a decisive step toward realizing space-based solar power (SBSP), A system that collects solar energy in orbit and wirelessly transmits it to Earth via microwaves. The OHISAMA satellite, scheduled for launch in 2025, is designed to validate the precision beam control needed for this process.
Unlike traditional ground solar farms, orbital systems receive uninterrupted sunlight and can beam power day and night, regardless of weather. At the core of this program lies a fundamental engineering challenge: aiming a microwave beam from hundreds of kilometers away with pinpoint accuracy.
JAXA’s approach, retrodirective beam control, relies on pilot signals sent from Earth to guide the satellite’s phased-array antennas. This system ensures that the transmitted microwaves always return precisely to their intended receiver, forming the backbone of safe, scalable power transmission from space.
Why Beam Accuracy Matters
Precision for Safety and Efficiency
Microwave transmission in SBSP requires near-perfect targeting. For a 1 GW-class system operating from geostationary orbit (36,000 km above Earth), a pointing accuracy of about 0.001° is needed to keep the beam within the rectenna footprint, a level described in program materials on high-accuracy microwave beam-pointing for SSPS, with deviation on the order of a few hundred meters at the receiver.
Applications Beyond Earth
This level of precision also supports wireless power links from lunar orbit to surface facilities, a pathway highlighted in program materials exploring long-distance microwave transmission for future lunar operations.
Retrodirective Control 101
The Pilot Signal Principle
The control loop starts with a pilot signal transmitted from a ground antenna co-located with the rectenna, which the satellite’s array uses to determine where to send the power beam; the array phase-adjusts toward the pilot’s arrival direction so the outgoing microwave beam coherently returns to the receiver.
Coarse and Fine Alignment
- Amplitude Monopulse Method: Estimates the angle of arrival of the pilot by comparing signal amplitudes across slightly offset elements, giving a robust coarse pointing direction for the main beam.
- REV (Rotating-Element Electric-Field Vector) Method: Sequentially varies element phases and locks where received power is maximized, providing fine alignment and compensating for small deformations or phase errors across the array.
Together, these techniques maintain sub-degree accuracy and adapt to panel deformations, removing the need for heavy mechanical gimbals.
Lessons from the 2015 Ground Demonstration
In March 2015, a kilowatt-class microwave transmission experiment at 5.8 GHz over ~55 m demonstrated active beam control, with ~1.8 kW transmitted and ~320–340 W received when phase correction was enabled (dropping to ~85–95 W when disabled), and 283 amateur radio contacts were completed during the run.
| Parameter | Value |
|---|---|
| Transmit Power | ~1.8 kW |
| Frequency | 5.8 GHz |
| Distance | ~55 m |
| Received Power (Phase Correction ON) | 320–340 W |
| Received Power (Phase Correction OFF) | 85–95 W |
| Achieved Pointing Accuracy | 0.15° RMS |
During the test, the rectenna successfully powered 283 amateur radio contacts, showing consistent energy delivery. When electronic phase correction was disabled, received power dropped nearly fourfold, proof that active beam control is essential.
The experiment validated both the software retrodirective control and the REV feedback optimization, establishing a technical foundation for orbital testing.
Inside OHISAMA’s Phased-Array Antenna
Satellite Overview
OHISAMA (short for On-orbit experiment of High-precision beam control using small Satellite for MicrowAve power transmission) is a ~150 kg-class spacecraft that will orbit at ~400 km altitude.
Its mission objective is to demonstrate on-orbit microwave beam formation and direction to ground receivers in Japan during visible passes, with onboard environment measurement sensors and a release probe included to characterize the ionosphere and support inter-satellite power-transfer tests.
| Parameter | Specification |
|---|---|
| Mass | ~150 kg |
| Orbit | Low Earth Orbit (400 km) |
| Transmission Frequency | 5.8 GHz |
| Output Power | On-orbit microwave beaming experiment (public downlink power not published in official briefs) |
| Ground Receiver | Ground receivers in Japan are used for beam-pattern measurement and validation |
The panel-integrated architecture aligns with power-generation and transmission panels that combine solar cells and microwave antenna functions in a unified module.
Modular Panel Design
The spacecraft employs power-generation and transmission panels that integrate solar conversion and phased-array transmission functions in a compact module, reducing harness mass and avoiding bulky gimbals. Each panel:
- Measures roughly 50 × 50 cm and weighs under 9 kg.
- Contains integrated amplifiers and phase controllers.
- Generates phase-controlled microwave output guided by ground pilot signals.
Instruments and Experiments
OHISAMA carries instruments such as:
- Environment measurement sensors (Langmuir Probe, Plasma Wave Analyzer, Impedance Probe) to characterize ionospheric conditions during beaming.
- Release probe (6U-class) carrying a rectenna to support inter-satellite power-transfer tests.
- Distributed ground receivers to measure beam shape, coherence, and intensity during passes over Japan.
Challenges Ahead
Atmospheric and Ionospheric Effects
While 5.8 GHz microwaves penetrate cloud and rain, ionospheric electron fluctuations can perturb the phase and scatter the beam. OHISAMA’s environment measurement sensors collect in-situ data to refine beam-control models for operational conditions.
Scaling from 55 m to 400 km
Transitioning from controlled ground tests to full orbital operation increases complexity across every subsystem. At 400 km altitude, the satellite must maintain beam lock on a moving ground receiver while traveling 7.6 km/s. This demands real‑time sensing of the pilot signal, rapid phase computation across thousands of antenna elements, and continuous correction for Doppler shifts caused by orbital motion.
The phased‑array control must also respond to thermal expansion and contraction of structural components as the spacecraft cycles between sunlight and shadow, preserving phase coherence within microseconds. Even slight delays or misalignments can lead to major energy losses, making adaptive, autonomous control algorithms essential.
Long‑Term Path to Gigawatt Systems
Extending OHISAMA’s 1 kW demonstration to a 1 GW commercial platform requires breakthroughs in multiple areas. Modular array assembly techniques must support kilometer‑scale phased antennas built from hundreds of thousands of panels. Efficient thermal management systems will be needed to dissipate waste heat from high‑power amplifiers in a vacuum.
In addition, autonomous inspection, repair, and calibration will be vital for maintaining alignment over decades of operation. Engineers are exploring robotic assembly, laser metrology for structural monitoring, and redundant power routing architectures to keep large arrays functioning reliably in orbit.
Scaling OHISAMA’s 1 kW beam to 1 GW would require advances in modular array assembly, thermal management, and autonomous maintenance of kilometer-scale structures in orbit.
(Speculative cross-industry insight): The algorithms developed for OHISAMA could inform Earth-based microwave links, adaptive radar systems, and wireless energy transmission for high-altitude drones, illustrating how orbital beam control research could influence terrestrial applications.
Broader Implications: A New Path for Clean Energy
If successful, OHISAMA will be among the earliest orbital demonstrations of a directed microwave link to a ground array, following a prior orbital test that detected a beamed energy signal on Earth.
By merging space engineering, microwave electronics, and renewable energy science, Japan’s program demonstrates how cross-disciplinary coordination can address both energy security and climate objectives. With continuous sunlight and precision control, space-based solar power may one day deliver uninterrupted, zero-emission electricity to Earth’s grid.
Conclusion
JAXA’s OHISAMA mission represents the culmination of decades of methodical progress in wireless power transmission. Program briefs on high-accuracy microwave beam-pointing control summarize how today’s retrodirective methods (monopulse + REV) grew from that groundwork.
By 2025, OHISAMA’s on-orbit validation will provide the data needed to design scalable systems capable of transmitting megawatts, and eventually gigawatts of solar energy from space.
While commercial deployment may still be years away, the principles proven by OHISAMA could transform. How humanity thinks about clean energy, from fixed installations on Earth to dynamic, orbiting power plants illuminating the planet below.
