LCRD will relay high rate data from ILLUMA -a laser terminal we will fly on the ISS- to ground stations on the earth.

Is this direct line of sight from the surface to the probe / spacecraft, then? Any use of relay satellites?

There are a few reasons… one big one is it is a ‘new’ technology. When deciding what instruments or systems are going on a space mission the goal of every mission manager is to ensure that everything you are putting on your spacecraft has been flown before and has been shown to be reliable. Obviously when doing cutting edge science there will be some ‘first-of-it’s-kind’ instruments that will be required, but as much as possible the goal is to use equipment that has a long track record of working reliably in space. Lasercom equipment is still being refined, and while it has flown, and worked, it does not yet have a long track record that can build confidence in its reliability.

Another issue is that, unlike RF communications, lasercom can be shut down by cloud cover. The wavelengths we use, typically in the Infra-red around 1550nm, can be blocked by clouds. So to communicate with a spacecraft we need Cloud Free Line of Sight (CFLOS). If we need to always be able to communicate with a spacecraft then we need multiple ground terminals located at sites that have uncorrelated weather, so if one is clouded over then there is high probability that one of the other sites is clear. For the Lunar Laser Communication Demonstration we had one ground terminal in White Sands New Mexico and another in southern California, at JPL’s Table Mountain Facility.

Since Lasercom typically uses telescopes that are a fraction of a meter in diameter, building multiple ground terminals isn’t quite as daunting as it would be for RF where you might need 18 meter diameter dishes.

But this brings us to what- in my opinion- is the REAL stumbling block preventing wide-spread adoption of lasercom- there is no existing ground terminal infrastructure.

If a mission wants to take advantage of lasercom, it not only has to pay for the space terminal, it has to pay for however many ground terminals are required to give them the availability they need. That is a significant cost that many missions simply cannot add to their tight budgets.

Of course we have built ground terminals for our previous laser communication demonstrations – LLCD, LCRD and few others, but all of those ground terminals were built for a single mission and could not be easily made to work for a different mission… To rework the the White Sands telescopes we used for the Lunar Laser Communication Demonstration so they could be reused to receive the laser downlink from the upcoming ARTEMIS II mission requires a lot of work, and they are completely unable to be made to work with the recently launched Laser Communication Relay Demonstration…

Obviously, building ground terminals that are only able to function for a single mission, and then go into storage when the mission is over is not economical. What is needed is a standardized ground terminal designed to be flexible enough that it can support multiple missions, something that can receive a downlink from LCRD in Geosynchronous orbit, and then immediately switch to receiving a downlink from the Artemis II mission at the moon. That way, when a mission ends, you still have a ground terminal in place ready to support the next mission with little or no rework required to support whatever comes along. In this way a ground network can grow organically, with more sites being added as demand increases.

By having a single design that uses commercial components as much as possible will lower the long-term costs greatly. If we spend the money up front to design a highly flexible system, it will cost more than designing a ground terminal tailored for a specific mission- BUT you only have to do the expensive design and testing work ONCE. Once you have the blueprint for the system then you simply produce copies as the demand grows, instead of every mission having to design, build and test their own specific ground terminals. In the long run it will save a LOT of money to have a single highly flexible design.

After years of making this argument, we were funded to develop a highly flexible ground terminal a few years age. That work, in addition to the other projects I work on, has had me completely swamped and we are approaching the point where we are going to be putting all the various components together… so that is why I haven’t been on the zone so much lately.

The engineering work on this has been extremely challenging, and we have a sizable team working on the various subsystems. It will be worth it, though- if we are ever going to take advantage of all that optical communications can offer, then the ONLY way it can happen on a large scale is if we have a standard low cost design able to support whatever missions come along…

The cost to do that initial flexible design (far more challenging than building a system that only has to support one mission) is high, but it is a one-time cost- in the long run the savings are huge. It is my hope that what we are doing will provide a blueprint for NASA’s future global ground terminal network… a single design using commercially available components as much as possible (where commercial components are not available, we work with industry to create a commercial product we can purchase for future copies).

We hope to start field testing the system against actual lasercom downlinks very soon, and it has been rather overwhelming lately. I am sure there will be many bugs to be ironed out, and as hectic as it is now, I expect it to get more hectic once testing in the field starts.

Well, one issue is the fact that the laser is so much narrower of a beam. When doing the Lunar Laser Communications Demonstration the beam divergence angle (how Much the beam size grows with distance) was measured in 10′s of microradians – 1 ARCSECOND, 1/3600th of a degree, is ~5 microradians( if you are looking at a dime from a kilometer away its angular extent is about 18 microradian) This means, if your beam has a divergence of 20 microradians, then your pointing has to be accurate and stable to within a small fraction of that angle so that the power seen by the receiver is not wildly fluctuating. And the spacecraft you are transmitting from can have all sorts of vibrations on it, so you have to have a way to essentially compensate for those vibrations in your transmitting telescope (usually called the optical module when talking about the transmit/receive telescope on the spacecraft). To further complicate things, you are not aiming at where you see your target- you have to aim at where your target will be when your laser travels the distance to the target. This is known as the point ahead angle…. so not only do you need to have an accurate idea of where your target is, but you need to know the speed and direction it is moving.

Complicating matters further, your transmitter on the ground has to deal with the turbulent atmosphere, which is causing the apparent position of your target in space to wander at a very high rate (100′s of Hz) so you have to have sophisticated ways of tracking it at high speed. The atmosphere also prevents you from imaging the light transmitted to your ground telescope at the diffraction limit, which is important for some forms of communication like the coherent communication formats used by the Laser Communication Relay Demonstration (coherent communication is a com format where the data is carried by the phase of the light transmitted) so that requires use of sophisticated adaptive optics. Adaptive optics is very costly and complex and can cost several times what you paid for your telescope and mount combined.

AND if you solve all the problems above and can track and collect the transmitted signal with your ground telescope, you still need to detect it and process it to decode it and get the data. Your signal, if receiving from the moon, will be very weak- under a nano-Watt per square meter… you need to have FAST detectors capable of essentially counting photons. On LLCD we used superconducting nano-wire detectors that needed to be cooled to within a few degrees of absolute zero. You ALSO need to keep your detectors from being swamped by background sky light, so you have to filter out as much of the background as possible. There can be a LOT of background light when your target is near the sun in the sky, or – in the case of LLCD- it has the fully illuminated moon behind it.

All these complications can be overcome, but it is not trivial. But as we have demonstrated, these are solveable problems. LLCD was a stunning success- the performance certainly exceeded my expectations. I expected it to work, but honestly I expected there to be hiccups in the process… much to my surprise the mission went surprisingly smoothly.

The Size Weight and Power reduction also applies to the ground, instead of needing a massive RF dish 10′s of meters in diameter, we used four 40-cm diameter telescopes to receive 622Mbps from the moon.

Radio Frequency (RF) communications has some advantages, though- for one RF doesn’t care about cloud cover- the signals travel through clouds easily- this is not the case for laser communications. Additionally, RF com has an existing global network of ground terminals that have been built over many decades.

Now, lets say you build a space terminal and ground terminal capable of returning 10′s of Gigabits per second (Gbps) from space- WHAT will you do with that data? Receiving it is only part of the task, you also have to get the data from your ground station to the end user- do you have a communications network on the ground capable of sending that 10′s of Gbps across the country to the people that need it? What about 100′s of Gbps? Are you going to store it on hard drives and FED-EX it to the end user when you are collecting Terabits every few seconds?

TBIRD- one of the missions mentioned in the article- will be transmitting 200Gbps from Low Earth Orbit. That demonstration will be transmitting just test patterns, so it it isn’t data that needs to get to some researcher on the other side of the country. But how would you distribute it if it DID need to go elsewhere? The ground network infrastucture needs to be updated, unless you can easily place a ground terminal at the end-user’s location.
Yes, RF is lower data rate, but that also means the received data can be relayed easily to the end-user over our existing networks. But when data rates approach Terabits per second its a much more challenging issue.

For the most part, my work has been on the ground side- the ground receiver and transmitter- and that is currently why I have had so little free time lately…

(To be continued…)

In 2009 I started work for NASA- I was hired to work for NASA at MIT-Lincoln Lab in Lexington Massachusetts on the Lunar Laser communications demonstration (LLCD )… when that mission wrapped up I moved to Maryland to work at Goddard- since then I have worked on many of the lasercom missions in that article…

***-Disclaimer, my views do not reflect those of my employer ( though they probably SHOULD ), I am speaking only as a private citizen, and as it pertains to my work I will only talk about publicly available information-***

While I have primarily worked on laser communications, one of the great things about my work is I am given the opportunity to do some things on the side of my own choosing- I just have to make the case that my ideas are worthy of Internal Research and Development funding… so I have been able to work on a wide range of projects… one particularly successful one was demonstrating a new welding technique using femtosecond lasers.

But laser communications is the work that ‘pays the bills’, so to speak- whatever else I have done, I have been primarily working on laser communications as my primary task.

The Lunar Laser Communications Demonstration was the first ‘real’ laser communications demonstrated beyond Earth orbit- Using approximately a half Watt of laser power on a small spacecraft at the moon, transmitted from a small 10cm diameter telescope we were able to beam down data at 622Mbps. Previously the highest data rate from the moon was a couple hundred Mbps using Radio Frequency communication- transmitting a LOT more power from a dish nearly 1-meter in diameter.

Using laser communications we can send back data MUCH faster while requiring far less Size Weight and Power (SWaP) on the spacecraft. Since the millions, or billions of dollars spent on a space mission are ultimately spent for the data the mission will send back, being able to return more data faster is a way to get more bang for your buck…

When I tell laymen that lasers allow you to transmit data back to Earth faster than Radio frequency communications, about half the time they will nod and say “right, because lasers travel much faster than radio waves…” – an indictment of science education in America…

Of course lasers and radio waves both travel at the speed of light- the difference is the wavelength- shorter wavelengths can be pulsed, or modulated, in shorter pulses than longer wavelengths – a little oversimplification here- but if it is pulses of light that carry the data, you can transmit more pulses per second with shorter wavelengths than with longer RF wavelengths.

ANOTHER benefit of shorter wavelengths is that they can be more directional- when you transmit a RF signal from the moon the large divergence of the beam means that when the signal gets to the Earth its spread across a large fraction of the Earth’s surface… the divergence depends on the ratio of your transmitting aperture diameter to the wavelength you are transmitting- the larger this ratio, the more collimated a beam you can transmit- so while an RF signal from the moon might cover the entire North American continent, the spot size of the laser from LLCD on the ground was a few kilometers across. This directionality is also what makes laser communication attractive in the defense world- you can’t intercept it easily if you have to be standing right next to the intended recipient to even see the signal.

So Laser communications is more secure, requires less SWaP on your spacecraft and transmits orders of magnitude higher data rates- why isn’t it the standard way of communicating? Well, many of the things that make it so attractive are also what make it very technically challenging…

I will continue this later- sigh… have to give the dog a bath…

I will comment more when I can on the article but life has been a little hectic lately.

I think I sent you a video of a talk on the topic a while back?