A Starshot Communication Downlink

Abstract: Breakthrough Starshot is an initiative to propel a sailcraft to Alpha Centauri within the next generation. As the sailcraft transits Alpha Centauri at 0.2 c, it looks for signs of life by imaging planets and gathering other scientific data. After the transit, the 4.1-meter diameter sailcraft downlinks its data to an Earth-based receiver. The present work estimates the raw data rate of a 1.02 {\mu}m, 100 Watt laser that is received at 1.25 {\mu}m by a 30-meter telescope. The telescope receives 288 signal photons per second (-133 dBm) from the sailcraft after accounting for optical gains (+296 dBi), conventional losses (-476 dB), relativistic effects (-3.5 dB), and link margin (-3.0 dB). For this photon-starved Poisson channel with 0.1 nm equivalent noise bandwidth, 90% detector quantum efficiency, 1024-ary PPM modulation, and 10^-3 raw bit error rate, the raw data rate is 260 bit/s (hard-decision) to 1.5 kbit/s (ideal) raw data rate, which is 8-50 Gbit/year. This rate is slowed by noise, especially starlight from Alpha Centauri A scattered into the detector by the atmosphere and receiver optics as sailcraft nears the star. Because this is a flyby mission (the sailcraft does not stop in the Centauri system), the proper motion of Alpha Centauri relative to Earth carries it away from the sailcraft after transit, and the noise subsides over days to weeks. The downlink can resume as soon as a day after transit, starting at 7-22 bit/s and reaching nearly full speed after 4 months. By using a coronagraph on the receiving telescope, full-rate downlink speed could be reached much sooner.

• The paper introduces a comprehensive communication downlink model using a relativistic Friis equation and noise analysis to estimate photon-starved performance.
• It employs pulse position modulation to bracket downlink data rates, predicting 8-50 Gbit/year despite significant atmospheric and astrophysical noise.
• The study highlights key uncertainties and future research routes, including advanced coding schemes and improved noise reduction techniques.

Starshot Communication Downlink Analysis

This paper introduces a communication downlink model and link budget for the Breakthrough Starshot initiative, which aims to send a sailcraft to Alpha Centauri (aCen) within a generation. The study assesses the potential performance of a near-infrared laser downlink from the sailcraft to an Earth-based receiver after the sailcraft transits aCen. The paper focuses on the challenges of establishing a reliable communication link under extreme conditions, including relativistic effects and significant noise sources.

Signal and Noise Modeling

The paper details the models used to estimate the received signal strength and various noise contributions. The received signal power is calculated using a relativistic version of the Friis transmission equation, accounting for transmitter power, antenna gains, free-space path loss, atmospheric transmittance, link margin, and relativistic dimming due to the sailcraft receding at 0.2c. The noise model considers noise from the Earth's sky, reflected and re-radiated starlight from aCen A's dust disc, and direct light scattered into the detector by the telescope itself. The noise spectral radiance from aCen A is modeled using a Moffat distribution to account for the scattering of light by the atmosphere and telescope optics.

Channel Characterization and Downlink Performance

Given the photon-starved nature of the communication channel, pulse position modulation (PPM) is considered an attractive modulation scheme. The paper employs both ideal Poisson PPM channel capacity and hard-decision Poisson PPM channel models to bracket the potential downlink data rates. The analysis indicates that direct light scattered by the atmosphere and telescope optics significantly impacts the channel capacity, reducing the ideal channel capacity in the vicinity of aCen A. The hard-decision channel capacity is also affected, with lower data rates observed near aCen A.

Key Findings and Implications

The link budget analysis reveals that approximately 288 photons per second reach the detector, highlighting the photon-starved regime of the communication channel. Under the given assumptions, each Starshot sailcraft could return 8-50 Gbit/year of raw data from its flyby of aCen A. The paper emphasizes the need to address uncertainties related to sailcraft aperture efficiency, available laser power, receiving telescope filter bandwidth, telescope size, PPM downlink coding scheme, and telescope point spread function.

Future Research Directions

The paper identifies several areas for future work, including incorporating a coronagraph in the noise model, accounting for the orbits of the Earth, Sun, and aCen A & B in astrometric calculations, considering multiplexing strategies for multiple sailcraft, and investigating smaller, geographically diverse telescopes to minimize data loss and system cost. Furthermore, future research should focus on high-performance soft-decision schemes to approach the ideal channel capacity.

Conclusion

This paper provides a comprehensive analysis of the communication downlink for the Breakthrough Starshot initiative. It addresses the challenges of establishing a reliable communication link over interstellar distances and offers insights into the potential data rates achievable with advanced modulation schemes and noise reduction techniques. The findings underscore the importance of addressing key uncertainties and pursuing further research to optimize the downlink performance for this ambitious mission.