PaddleSat Optical Charging Station in Space

This work investigates the feasibility and design trade-offs for a companion spacecraft, or PaddleSat, to charge a host spacecraft by wirelessly transmitting power using a directional laser system. The primary goal of the PaddleSat is to supplement power on a host spacecraft to reduce the requirements for onboard power systems of the host spacecraft or extend mission lifetimes. System performance estimates, link budget calculations, optical transmission hardware and link analysis, design tradeoffs between beam divergence, optical efficiency, and relative orbital control requirements are examined.

💡 Research Summary

The paper presents the concept of a “PaddleSat” – a lightweight, thin‑film companion spacecraft designed to wirelessly transmit optical power to a host satellite in Low‑Earth Orbit (LEO) during eclipse periods. The authors explore the feasibility, design trade‑offs, and performance of such a system, focusing on three main subsystems: power generation and storage, optical transmission hardware, and formation‑keeping with associated RF telemetry.

Power subsystem – PaddleSat is a 6U CubeSat (≈10‑11 kg) equipped with a 1 m² deployable solar array capable of ~340 W peak output. About 175 W is allocated to a high‑duty‑cycle laser transmitter, while a 278 Wh Li‑ion battery supplies power during eclipses and peak transmission. This sizing yields an average expendable power budget of 175‑210 W, sufficient to run a continuous‑wave (CW) 980 nm laser at the required power level.

Optical transmission hardware – The transmitter uses two “paddles,” each comprising a 10‑15 cm diameter lightweight mirror (SiC‑composite or aluminized polyimide) and a high‑efficiency laser‑diode array operating at 980 nm. The wavelength matches the peak quantum efficiency of common silicon (and GaAs) solar cells on the host, maximizing conversion efficiency. The diffraction‑limited divergence for a 15 cm aperture at 980 nm is ~16 µrad; however, realistic beam quality and spacecraft jitter increase the effective divergence to 100‑200 µrad. At the maximum 500 m range this yields a spot diameter of roughly 10 cm, a size that can be reliably captured by a single solar‑cell segment while keeping pointing requirements within achievable limits.

Pointing and attitude control – Precise beam alignment is achieved through a combination of coarse ADCS (reaction wheels, magnetorquers, star tracker) and a fast‑steering mirror on each paddle. The residual pointing error is targeted at ≤20 µrad, which translates to a lateral displacement of only 2 mm at 100 m and 10 mm at 500 m – well below the 10 cm spot size. Relative‑state estimation uses GNSS/S‑band ranging for meter‑level coarse knowledge, supplemented by optical beacons and onboard vision processing to reach sub‑meter accuracy during operations.

Orbit design and station‑keeping – PaddleSat is placed in a near‑circular 500 km orbit matched to the host to minimize differential perturbations. Maintaining a 100‑500 m separation is analyzed using Hill–Clohessy–Wiltshire equations. A 1 m semimajor‑axis error produces ~143 m drift per day, so regular ΔV corrections are required. The authors estimate an annual ΔV budget of 1‑10 m/s, which can be supplied by either a cold‑gas thruster (Isp≈60 s, ~0.09 kg/yr) or an electric micro‑propulsion system (Isp≈1000 s, ~0.006 kg/yr). A safety “retreat” maneuver of 0.5‑1 m/s provides rapid collision avoidance with only a few grams of propellant.

RF command & telemetry link – A short‑range S‑band cross‑link (2.3 GHz) handles command, health monitoring, and arm/inhibit signaling for the laser. At 100‑500 m separation the free‑space path loss is ~94 dB; with a 19.2 kbps BPSK/QPSK telemetry, a required Eb/N0 of ~9.6 dB, and a 10 dB link margin, the necessary transmit power is on the order of tens of milliwatts, imposing negligible load on the spacecraft’s power budget.

System performance and trade‑offs – The authors calculate an overall optical power transfer efficiency of roughly 30‑40 % when accounting for beam divergence, pointing jitter, and receiver capture area. This level of delivered power can substantially reduce the host satellite’s battery cycling, extending mission life or allowing smaller batteries. Key risks identified include maintaining sub‑20 µrad pointing stability, thermal management of the laser diodes, and ensuring safe proximity operations. The paper proposes a multi‑stage acquisition process (coarse GNSS/S‑band, optical beacon acquisition, fine‑loop steering) and explicit safety interlocks to mitigate these risks.

Conclusions – The study demonstrates that a PaddleSat‑type optical charging companion is technically feasible with current small‑satellite components. The design balances mass, power, and precision requirements while keeping the ΔV budget modest. Further work is recommended on long‑duration laser operation in space, degradation of thin‑film optics under radiation and thermal cycling, and an on‑orbit demonstration to validate the end‑to‑end power transfer concept.