Why orbital solar projects need a transparent link budget

Space-based solar power (SBSP) has resurfaced in policy circles every few years since Peter Glaser first proposed orbiting solar farms in 1968. In recent years national laboratories, startup consortia, and defense agencies have begun launching hardware demonstrations that beam microwave or laser energy to ground receiving stations. Despite the headlines, sponsors routinely struggle to defend the basic arithmetic of how many satellites, how much rectenna acreage, and how many hours of storage are required to power a community. Prospectuses often cite heroic conversion efficiencies or gloss over the way diffraction spreads the beam over hundreds of square kilometers. Regulators and community stakeholders therefore lack a repeatable method for checking whether the promised gigawatts would truly arrive on the grid once atmospheric absorption, phase jitter, and curtailment are considered. This calculator is designed for that due diligence stage. It translates laboratory component efficiencies, orbital geometry, and grid planning heuristics into a coherent link budget that shows the average delivered power per satellite, the total constellation required to hit a target, and the supporting ground footprint. With those numbers in hand, planners can finally compare space-based concepts to terrestrial solar, intercontinental HVDC, or even small modular reactors on equal footing.

Unlike traditional satellite link budget spreadsheets, which focus on signal-to-noise ratios for communications payloads, SBSP planning must convert optical or microwave beam power into usable alternating current while keeping community impacts in view. That means accounting for diffraction-limited spot sizes, atmospheric attenuation that varies with humidity and precipitation, as well as the rectenna’s diode and inverter losses. More importantly, SBSP advocates advertise baseload-like availability, yet sunlight access for a geostationary platform varies with the beta angle and eclipse seasons. Even sun-synchronous or medium Earth orbit architectures experience eclipses or regulatory curtailments that create demand for storage. Without modeling these effects simultaneously, investors might approve an architecture that delivers barely half the promised energy. The calculator collects the most influential parameters into a structured workflow and lets users test scenarios instantly.

Core equations and modeling assumptions

The model assumes each satellite produces a fixed electric power level from its photovoltaic or concentrator arrays while in sunlight. That generation is converted by microwave or laser transmitters with efficiency $\eta t$, shaped by phased arrays with pointing efficiency $\eta p$, attenuated by the atmosphere with factor $\eta a$, and finally harvested by a rectenna with conversion efficiency $\eta r$. The instantaneous grid-delivered power from a single spacecraft is therefore

$P{g}_0=P{s}_0·\eta t·\eta p·\eta a·\eta r$,

where $P{s}_0$ is the generating capacity when in sunlight. Because sunlight is not continuous, the average daily contribution scales by the duty cycle $d$. The average delivered power per satellite becomes $P{g}_{\mathrm{avg}}=P{g}_0·d$. The calculator treats the duty cycle as a single percentage that captures eclipse durations, maintenance outages, or regulatory curtailments. If multiple eclipse seasons occur, users can conservatively choose the worst-case value.

The beam footprint is approximated by Fraunhofer diffraction. For a circular aperture of diameter $D$ emitting at wavelength $\lambda$, the half-angle beam divergence is $\theta =\frac{1.22}{D}\lambda$. Over a slant range $R$, the spot radius on the ground is $r=\theta R$ and the illuminated area is $A=\pi r²$. Comparing that footprint to the planned rectenna acreage reveals how much of the beam is intercepted. In many concepts the rectenna fills only 50–60% of the Airy disk to allow safety buffers and wildlife corridors. The calculator reports both figures so communities can visualize the land commitment.

The storage buffer estimate is deliberately simple: if the fleet delivers an average power $P{\mathrm{fleet}}_{}$ and the pause in delivery lasts $h$ hours (due to eclipse, weather curtailment, or grid maintenance), the energy requirement is $E=P{\mathrm{fleet}}_{}h$. Users can expand the buffer to cover longer outages or shrink it if backup generation is available. Although real projects would model battery round-trip efficiency and degradation, the buffer energy is a useful first sizing metric for pumped hydro or thermal storage paired with SBSP.

Worked example: powering a remote archipelago

Imagine a coalition of island nations exploring SBSP to decarbonize diesel grids spread across 30 inhabited atolls. They propose a constellation of geostationary satellites each capable of 2,500 MW of direct current from lightweight photovoltaics. Demonstrator payloads suggest microwave electronics operate at 70% efficiency, phase control maintains 90% coherence, and the humid maritime atmosphere transmits 88% of the energy on average. Ground rectennas based on gallium arsenide diodes achieve roughly 80% conversion to alternating current. Eclipse seasons near the equinoxes reduce availability to 86% of each day after accounting for station-keeping thrust arcs. Entering those values with a duty cycle of 86%, a 150 meter transmitting aperture, 12.2 centimeter wavelength (2.45 GHz), and a 36,000 km slant range yields an instantaneous per-satellite grid output of approximately 1,388 MW and an average contribution of 1,193 MW. If the islands demand a guaranteed 5,000 MW average to replace diesel, the calculator indicates that five satellites suffice, delivering 5,966 MW on average. The summary also shows that each spacecraft’s Airy disk radius is about 356 meters, covering 0.4 square kilometers, while the rectenna needs 0.56 square kilometers if the areal power density is 2.5 MW per square kilometer. That mismatch prompts planners to either increase rectenna density with denser diode arrays or accept that some beam power lands on a guard ring that must remain unoccupied.

The CSV export captures every assumption, making it easy for project sponsors to append the scenario to feasibility studies. When the planners adjust the duty cycle downward to 75% to account for regulatory curtailments during migratory bird seasons, the required fleet jumps to seven satellites and the storage buffer rises to 35,000 MWh for a 4-hour outage. Such sensitivity insights often decide whether SBSP remains viable compared with subsea cables. Instead of burying the math in a proprietary presentation, the calculator lets engineers and environmental review boards iterate in real time.

Comparing design levers

The table below highlights how different strategies influence delivered power and land footprint for the same baseline satellite platform. It underscores why beam steering improvements can be as impactful as adding photovoltaic area.

The “Phased-array upgrade” scenario assumes new gallium nitride amplifiers lift transmission efficiency to 80%, which increases average delivered power by 11% but also widens the beam slightly, requiring more land. The “Rectenna diode breakthrough” row keeps transmission hardware fixed but imagines a leap in Schottky diode technology, pushing rectenna efficiency to 90%. That change adds 15% more power while reducing land area because more of the intercepted energy becomes grid power. Combining both upgrades delivers more than 1.5 GW per satellite on average, shrinking the fleet size needed for a given target, though it raises capital costs in orbit and on the ground.

Limitations and next steps

As thorough as the calculator is for early-stage planning, it does not replace a full mission design study. Diffraction-based spot sizing assumes perfect aperture illumination, whereas real phased arrays suffer from grating lobes, panel failures, and beam wander due to vibration. The atmospheric efficiency input hides complex frequency-dependent absorption, rain fade, and ionospheric scintillation that would demand probabilistic models. Likewise, the rectenna areal power density metric compresses soil preparation, wildlife corridors, and community setback rules into one number. In practice, rectennas may be interleaved with agriculture or aquaculture, reducing effective density. The storage buffer ignores round-trip efficiency, degradation, and the economic dispatch strategy for balancing fleets. If policymakers expect to throttle SBSP output to accommodate transmission constraints, they should adjust the duty cycle accordingly or add a curtailment term.

Security and regulatory issues also fall outside the scope of this calculator. International Telecommunication Union filings, space traffic management, and beam safety analyses each spawn their own studies. The model assumes satellites can continuously point at one rectenna, but real deployments may hop among multiple customers, reducing delivered energy per site. Finally, the tool presumes the constellation shares identical spacecraft. In practice, designers might mix large baseload satellites with smaller peaking units or place some platforms in highly elliptical orbits to serve high-latitude markets. Those variations can be approximated by running separate scenarios and summing the results.

Despite those caveats, the link budget output here empowers stakeholders to interrogate SBSP proposals with a common vocabulary. By adjusting efficiencies, apertures, or rectenna densities, engineers can test the value of research investments. Regional planners can compare fleet sizes to terrestrial infrastructure upgrades. Community groups can assess land use implications before endorsing microwave beaming overhead. The ability to export the results as CSV encourages transparent documentation and collaboration with grid operators. As SBSP transitions from concept art to pilot projects, rigorous calculators like this one will determine whether orbiting power plants earn public trust and financing.