# Laser communication for interplanetary probes - aiming is impossible without beacon?

Laser communications are considered very promising for interplanetary probes because of potentially much higher data rates.

I found that laser communication at large distances (over 3 a.u. ~ 450 million kilometers) face big problems. Here https://www.lpi.usra.edu/icegiants/mission_study/Full-Report.pdf - chapter 5.2.Improved Data Return with Optical Communications (page 156). Quote:

The primary problem with operating beyond 3AU is the difficulty of directing the very narrow laser beam carrying downlink optical data to the tracking station on Earth. For distances out to 3AU, this depends on a beacon signal beamed up from Earth with a powerful laser. However, for every doubling of the distance to the spacecraft four times as much beacon power is needed. This presents an environmental hazard and at some point the power absorbed in the atmosphere distorts the waveform so it is no longer even practical. Alternative methods include tracking the Earth in the infrared or referencing nearby stars with star trackers. Research on these alternate approaches are highly desirable.

Bold is myne.

Question:

Is pointing accuracy of a spacecraft can't be reached without a beacon laser from Earth?

I think position and velocity of a spacecraft can be known precizely enough from radio communication. So direction to optical receiver station on Earth (at time of laser signal arrival) can be calculated precizely too. Am I correct here or not?

Or problem with technical accuracy to point our spacecraft? What pointing accuracy is achievable? For Hubble Space Telescope I found (< 2 - 5 mas) http://www.stsci.edu/hst/HST_overview/documents/multidrizzle/ch42.html I suppose it's much less than laser beam width.

Psyche mission will carry Deep Space Optical Communications (DSOC) paiload. https://www.nasa.gov/psyche https://www.lpi.usra.edu/opag/july2014/posters/9-DSOC_OPAG_Poster.pdf

It will use beacon laser from Earth, but communication distances are less than 3 a.u.

• "Alternative methods include tracking the Earth in the infrared or referencing nearby stars with star trackers. Research on these alternate approaches are highly desirable" suggests non beacon methods may be possible, but more research is needed – JCRM Jun 6 '18 at 15:20

Start with the numbers: This answer to another question talks about a small NEO laser system with about $3 \mu\text{rad}$ divergence. From 450Mkm, that's about a 1300km spot on the Earth from just a 125mm transmitting telescope. A 1m one would get that down to a 150km spot: You can't just point at the Earth as a whole and be OK.