[Paper Review] A Starshot Communication Downlink

Kevin Parkin|arXiv (Cornell University)|May 15, 2020

Astronomy and Astrophysical Research13 references5 citations

TL;DR

This paper models the downlink communication from a Breakthrough Starshot sailcraft traveling at 0.2c past Alpha Centauri, using a 100 W laser at 1.02 µm transmitted to a 30 m telescope receiving at 1.25 µm. It estimates a raw data rate of 260 bit/s (hard-decision) to 1.5 kbit/s (ideal), equivalent to 8–50 Gbit/year, limited by photon-starved conditions and atmospheric/scattered starlight noise.

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 μm, 100 Watt laser that is received at 1.25 μm 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.

Motivation & Objective

• To model the feasibility of high-speed data downlink from a relativistic sailcraft during a flyby of Alpha Centauri.
• To quantify the raw data rate achievable under photon-starved conditions with a 100 W laser and 30 m telescope.
• To assess the impact of scattered starlight and atmospheric noise on communication reliability during the downlink phase.
• To evaluate the timing and recovery of downlink rates after the flyby, considering signal degradation and recovery.
• To explore the potential of using a coronagraph to accelerate full-rate data transmission post-flyby.

Proposed method

• Modeling the downlink using a 1.02 µm, 100 W laser transmitter on the sailcraft and a 30 m telescope receiver at 1.25 µm.
• Calculating received signal power at -133 dBm after accounting for optical gains (+296 dBi), losses (-476 dB), relativistic effects (-3.5 dB), and link margin (-3.0 dB).
• Applying a photon-starved Poisson channel model with 0.1 nm equivalent noise bandwidth and 1024-ary PPM modulation.
• Evaluating raw data rates under two decoding schemes: hard-decision (260 bit/s) and ideal (1.5 kbit/s) with a 10^-3 raw bit error rate.
• Assessing noise contributions from scattered starlight from Alpha Centauri A, especially during the flyby phase.
• Simulating the recovery of downlink speed over time post-flyby, considering decreasing noise from proper motion.

Experimental results

Research questions

• RQ1 What is the maximum achievable raw data rate for a 100 W laser downlink from a 0.2c sailcraft to a 30 m telescope on Earth?
• RQ2 How do relativistic effects and atmospheric losses impact the received signal power and data rate?
• RQ3 To what extent does scattered starlight from Alpha Centauri A degrade the downlink performance during the flyby?
• RQ4 How quickly can the downlink resume at full speed after the sailcraft passes Alpha Centauri, given noise subsidence?
• RQ5 Can a coronagraph on the receiver telescope significantly accelerate the recovery of full data rates post-flyby?

Key findings

• The received signal power is -133 dBm, corresponding to 288 signal photons per second at the 30 m telescope after all gains and losses.
• The raw data rate ranges from 260 bit/s (hard-decision decoding) to 1.5 kbit/s (ideal decoding), equivalent to 8–50 Gbit/year.
• Scattered starlight from Alpha Centauri A is a dominant noise source during the flyby, degrading performance significantly.
• Downlink can resume within one day post-flyby, starting at 7–22 bit/s and reaching full speed after approximately 4 months as noise subsides.
• Using a coronagraph on the receiver telescope could enable full-rate downlink much sooner by suppressing scattered starlight.
• The system operates under a photon-starved Poisson channel with 1024-ary PPM modulation and 90% detector quantum efficiency.