Inter-satellite laser link update

SpaceX satellite mesh with four
laser terminals on each satellite

Inter-satellite laser links (ISLLs) and electronically steerable flat panel antennas are critical technologies for constellations of low-Earth orbit (LEO) Internet-service satellites. Low-cost antennas are critical for the mass consumer market and ISLLs are required for an effective Internet backbone in space. In an earlier post, we saw that progress is being made on antennas, this one looks at ISLLs.

The figure to the right is taken from a simulation of the first phase of SpaceX's planned broadband Internet service, Starlink. It shows 66 satellites in each of 24 53-degree orbital planes -- a total of 1,584 satellites at an altitude of 550 km. Each satellite has four laser-communication terminals. Two on the front and back and two on the sides. Since the front and back lasers link to satellites in the same orbital plane, they remain at the same place in the sky relative to each other while the side lasers must move to track one another. (To visualize the dynamic nature of the links between the constantly moving satellites, check this clip from the animated simulation).

When Elon Musk introduced his Starlink plan to prospective employees in 2015, he said his goal was to transport "a majority of long-distance Internet traffic" and "about 10 percent of local consumer and business traffic." He pointed out that satellites would have an advantage over terrestrial links since the speed of light is faster in space than through optical fiber and fewer router hops would be needed to reach a distant location.

In addition to mitigating the digital divide by serving rural areas and small organizations, Musk and his competitors at OneWeb, Telesat, Amazon, and Leosat hope to service high-end, high-margin customers like enterprises, governments and maritime, airline and mobile phone companies. ISLLs are necessary for serving those lucrative high-end markets.

Initially, SpaceX proposed five ISLLs for each satellite -- the fifth would have been a link to a satellite in the crossing plane, but last November they cut back to four. The fifth terminal would have been difficult to engineer because while the front, back and side-mounted terminals move slowly relative to each other, this simulation shows that satellites in crossing planes would have been traveling at 7.3 km/second relative to each other. Furthermore, links between satellites in crossing planes would be of short duration. Designing and manufacturing them would have taken time and money.

Furthermore, because of the 53-degree orbit inclination, about half the satellites are moving northeast and half are moving southeast at any time and place. That favors east-west links over north-south links and since most of the lucrative low-latency, long-link traffic is in the northern hemisphere, they could not justify the cost or possible deployment delay. That is not to say they will not deploy them in the future. (Note that the initial five-link constellation was to orbit at an altitude of 1,100, not 550 km. Future plans call for constellations at 1,100 and 335-345 km and there may be ISLLs between all of them).

Tesat laser communication roadmap

But even with 4-links, the terminals are still under development and will be expensive. At least two companies are working on ISLLs, Mynaric and Tesat.

Tesat already markets a laser communication terminal for LEO to ground transmission from CubeSats. Their CubeLCT is 9 x 9.5 x 3.5 cm, has a mass of 360 grams, consumes 8 Watts of power and communicates through the atmosphere to the Earth at 100 Mbps, with a 1 Mbps channel from the ground to LEO. They are developing an ISLL terminal based on that experience and, judging from the diagram shown here, they are pursuing laser communication between the ground, LEO and geostationary satellites.

Mynarc has announced that their ISLL terminal, the MLT-80, will be available in high-volume production this year and both companies are working on faster terminals. A while ago, I suggested that SpaceX would probably develop their own ISLL, but last March, Bulent Altan, a former SpaceX Vice President, joined Mynarc as co-CEO and a few days later Mynaric announced that they had raised $12.5 million from mystery constellation customer. Might the mystery company be SpaceX? Might it be Amazon, which entered the race late and has enough money to pay for terminals or even buy a stake in Mynaric or Tesat? We will know soon because test satellites equipped with Mynaric’s terminals should be launched in late-2019.

The following are selected characteristics of their forthcoming ISSLs:

	Mynaric	Tesat
Link distance	4,500 km	6,000 km
Data rate (full duplex)	10 Gbps	10 Gbps
Target mass	<20 kg	<15 kg
Power consumption	<60 W	80 W

Sources Tesat, Myarnic

The SpaceX simulation shown above was for satellites with 4 ISLLs, but SpaceX launched their first 60 satellites without the ISLLs and, as far as I know, has not said if forthcoming satellites will have them or not. Arthur Sauzay, a French environment and space lawyer has pointed out that SpaceX argued for the allocation of radio frequencies for ISLs in a comment to a recent Whitehouse report on the impact of emerging technologies and their impact on non-federal spectrum demand, but they seem too large, heavy and slow to support a LEO network with long-distance, low-latency links.

OneWeb has decided not to use ISLs in their first constellation and will route traffic through terrestrial gateways. This decision seems to have been at least partially motivated by Russian insistence that satellite traffic passes through gateways within their borders. I imagine China and other nations will impose the same restriction.

Telesat remains committed to ISSLs, but say they will have the flexibility in their network-control system to route traffic coming to a country over satellite or terrestrial links. Erwin Hudson, vice president of Telesat LEO is confident that ISLs will be cheap enough to allow them to compete successfully with terrestrial fiber and 5G, offering fast, 30 ms latency broadband. They also have a $2.8 million contract to study inter-satellite laser links between their constellation and Blackjack, DARPA's 20 LEO satellite constellation and they are collaborating with Google on software, so we might see laser links between Telesat satellites and Google's balloons.

LeoSat is unique in that they are not pursuing the consumer and small organization markets, but are focused exclusively on large, high-end customers. They will provide fast, low latency, encrypted, reliable point-to-point connections to governments at up to 1.2 Gbps with latency under 50 ms and they have over $1 billion in pre-launch customer agreements. ISLLs are mandatory for the markets they are pursuing and since two geostationary satellite operators, Jsat and Hispasat, are investors in LeoSat, they may very well link to them in the future to offer a service similar to the SpaceDataHighway of Airbus and the European Space Agency.

China's Hongyun LEO broadband project is an ISLL unknown. China is doubtless working on laser communication in space, but I have no idea whether or not they will use it in their broadband constellation. Since they say the goal of the project is to serve rural China and they regulate Internet links to the outside world, Hongyun satellites may serve exclusively as "bent pipe" relays between rural locations and China's terrestrial network.

ISLLs will be needed if the Internet backbone in space is to compete with the terrestrial backbone and serve high-value applications. It seems that making cost-effective ISLLs for LEO constellations was harder than Elon Musk and others anticipated, but first production models are now on the horizon and they will improve over time.

For a copy of the PowerPoint presentation I use for teaching this topic click here.