# 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.

Can't find beam width for already flown or planned missions. LADEE lunar probe had experimental LCDC laser communication paiload. https://en.wikipedia.org/wiki/LADEE#Lunar_Laser_Communication_Demonstration

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
– user20636
Jun 6 '18 at 15:20

## 1 Answer

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.

So how to do better? Easiest is to get a bright dot at the target to lock into. (See that prior answer for one method of using that, though this situation is a bit more complicated due to transit time) So why not send a laser back to the spacecraft to make that dot?

It's because a small space telescope can send a tight beam through vacuum, but a ground-based telescope has to first send it's beam through air. Air is complicated in both space and time. That makes stars twinkle even though the beam is going only a couple km to reach you after going through air: After a few hundred million km of travel there's really no point in trying to send a tiny beam to a target. You have to send a broad beam. And to do that, while still having it useful, you need a lot more power.

You can build an optical system for the satellite to track Earth itself, but that needs further research i.e. will be hard. You're trying to precision image something with really bright and dim parts. You can use a big telescope, but that requires having another big telescope, or sharing the one you already have. Nothing that's thought to be impossible, just a lot of details to sweat and hard work to do.

• Wouldn't it be possible to transmit from the distant spacecraft to a spacecraft in Earth orbit? The equipment onboard that Earth orbiting spacecraft may be limiting, but the data could be stored on board before a radio retransmission to the ground (NASA has developed such a protocol called the Delay Tolerant Network, or DTN). If such is possible, the distant spacecraft could quickly get back to its science while the Earth orbiter would transmit to the ground over some time. Jun 7 '18 at 2:09
• @ChrisR Certainly possible, but needs more work. Both the Laser Communications Relay Demonstration (LCRD) (nasa.gov/mission_pages/tdm/lcrd/overview.html) and Space Based Relay Study (SBRS) (nasa.gov/directorates/heo/scan/engineering/studies/…) groups have done a little work on it, though it's not their main focus. The question is whether that kind of development and infrastructure is cost effective or not. Jun 7 '18 at 3:59