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Laser Inter-Satellite Links (LISLs) in a Starlink Constellation

When we first talked about Starlink back in 2019, we saw in the video that the concept involved laser communication to communicate between the satellites. While the initially launched satellites did not have the laser communication mechanism built in, it looks like they are being added to the newer ones. 

A report from Fast Company in late 2021 said:

One of the next big upgrades in telecom will involve satellites firing lasers at each other—to beam data, not blow stuff up.

The upside of replacing traditional radio-frequency communication with lasers, that encode data as pulses of light, can be much like that of deploying fiber-optic cable for terrestrial broadband: much faster speeds and much lower latency.

“Laser links in orbit can reduce long-distance latency by as much as 50%, due to higher speed of light in vacuum & shorter path than undersea fiber,” SpaceX founder Elon Musk tweeted in July about the upgrade now beginning for that firm’s Starlink satellite constellation.

The first batch of laser-equipped Starlink satellites went up to polar orbits in January, Musk confirmed January 24. Its most recent launch in early September featured version 1.5 spacecraft with the latest laser technology.

SpaceX may be getting the most attention for its use of optical communication, but multiple companies are developing laser systems to deploy on satellites and even in applications closer to Earth.

“Creating that ubiquitous mesh network connectivity is becoming more and more important right now,” said Tina Ghataore, chief commercial officer at Mynaric. That firm, with offices in Gilching, Germany, and Hawthorne, California, builds laser-optical terminals for use in satellites as well as in the air. “The commercial sector is seeing the value of incorporating this tech.”

Lasers can also provide immense bandwidth, thanks to advances in technology that allow more precise control of a beam.

Mynaric has a nice explainer video which is embedded below:

A news article in Space News back in August 2021 said:

SpaceX is adding laser terminals on all future Starlink satellites and is the reason behind a break in launches for the broadband megaconstellation, president and chief operating officer Gwynne Shotwell said.

Shotwell told the Space Symposium Aug. 24 that its decision to add laser crosslinks, enabling the satellites to communicate with each other to reduce their reliance on ground stations, is “why we have been struggling” to launch a Starlink mission since June 30.

SpaceX had been conducting an aggressive launch campaign with its Falcon 9 rocket throughout the first half 2021 before the hiatus, enlarging the Starlink constellation to more than 1,600 satellites in low Earth orbit.

Typically, each Falcon 9 launch for the network has placed 60 Starlink satellites at a time. There were four Starlink launch missions this May alone.

SpaceX has regulatory permission to operate 4,408 satellites at 550-kilometers altitude for global coverage.

Shotwell said the next Starlink launch will be in “roughly three weeks.”

SpaceX launched 10 Starlink satellites with laser crosslinks to polar orbit in January, its first with the capability, so it did not need ground stations over the poles.

By enabling communications from one satellite to another on the same or adjacent orbital plane, a ground station does not have to be in the same satellite footprint as user terminals.

As well as reducing the number of ground stations needed for global coverage, laser crosslinks links can also lower latency because they reduce the number of hops between satellites and ground stations.

A research paper on this topic by Aizaz U. Chaudhry and Halim Yanikomeroglu, Carleton University provides a lot of interesting details. Here is the abstract:

Laser inter-satellite links (LISLs) are envisioned between satellites in upcoming satellite constellations, such as Phase I of SpaceX's Starlink. Within a constellation, satellites can establish LISLs with other satellites in the same orbital plane or in different orbital planes. We present a classification of LISLs based on the location of satellites within a constellation and the duration of LISLs. Then, using satellite constellation for Phase I of Starlink, we study the effect of varying a satellite's LISL range on the number of different types of LISLs it can establish with other satellites. In addition to permanent LISLs, we observe a significant number of temporary LISLs between satellites in crossing orbital planes. Such LISLs can play a vital role in achieving low-latency paths within next-generation optical wireless satellite networks.

Slides on this topic are available here.

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Comments

  1. To how many Starlinks in its vicinity can a satellite communicate?

    How does the switching process work when a satellite leaves the
    local network and a new satellite joins the local network?

    How many laser communication units does a Starlink satellite have?

    How long does it take to switch from one Starlink to the next
    approaching Starlink?

    ReplyDelete

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