Starlink, the low earth orbit satellite constellation from SpaceX, has hit the minimum configuration to provide broadband. So far 482 satellites have been launched, just above the 400 needed for a minimum service. 800 will provide a ‘moderate’ broadband service, and the company aims to have as many as 30,000 satellites in orbit, launching 60 at a time.
“For Starlink, we think of our satellites more like servers in a data centre than special one-of-a-kind vehicles,” said Matt Monson, Director of Starlink Software at SpaceX. He has given some more details of the satellite technology. “Each launch of 60 satellites (above) contains more than 4,000 Linux computers. The constellation has more than 30,000 Linux nodes and more than 6,000 microcontrollers in space right now,” he said. The full ‘megaconstellation’ would have over 360,000 microcontrollers and over a million nodes.
These ‘servers in the sky’ generate huge amounts of data. “We’re currently generating more than 5TB a day of data,” said Monson. “We’re actively reducing the amount each device sends, but we’re also rapidly scaling up the number of satellites (and users) in the system. Doing the detection of problems onboard is one of the best ways to reduce how much telemetry we need to send and store so we only send it when it’s interesting.”
There is less redundancy in these LEO satellites than larger geostationary systems that need to last twenty years or more. “We’ve designed the system so that satellites will quickly passively deorbit due to atmospheric drag in the case of failure,” said Monson. “We still have some redundancy inside the vehicle, where it is easy and makes sense, but we primarily trust in having system-level fault tolerance: multiple satellites in view that can serve a user. Launching more satellites is our core competency, so we generally use that kind of fault tolerance wherever we can, and it allows us to provide even better service most of the time.”
“The Starlink hardware is quite flexible,” said Monson. “It takes a ton of software to make it work, and small improvements in the software can have a huge impact on the quality of service we provide and the number of people we can serve. We’ve spent a bunch of time making it easier, safer, and faster to update our constellation. We tend to update the software running on all the Starlink satellites about once a week, with a bunch of smaller test deployments happening as well. By the time we launch a batch of satellites, they’re usually on a build that already older than what’s on the rest of the constellation.”
The updated software in the satellite also needs to be tested. “There are some things that we need to be absolutely sure of (commanding, software update, power and hardware safety), and therefore deserve to have specific test cases around. But there’s also a lot of things we can be more flexible about — for these things we can take an approach that’s more similar to the way that web services are developed,” he said.
“We can deploy a test build to a small subset of our vehicles, and then compare how it performs against the rest of the fleet. If it doesn’t do what we want, we can tweak it and try again before merging it. If we see a problem when rolling it out, we can pause, roll back, and try again. This is a hugely powerful change in how we think about space vehicles, and is absolutely critical to being able to iterate quickly on our system.”
However the scale of hundreds of satellites presents challenges.
“We’ve definitely found places where our test cases had holes. Having hundreds of satellites in space 24/7 will find edge cases in every system, and will mean that you see the crazy edges of the bell curve. The important thing is to be confident about the core that keeps the hardware safe, tells you about the problem, and then gives you time to recover.”
“We’ve had many instances where a satellite on orbit had a failure we’d never even conceived of before, but was able to keep itself safe long enough for us to debug it, figure out a fix or a workaround, and push up a software update.”
“And yes, we do a lot of custom ASIC development work,” he said.
The US regulator, the FCC, this week voted to allow Starlink to apply for the next round of bidding for a global low latency broadband network service. Demonstrations and a pilot service over the next few months will show low the latency is and what kinds of download speeds can be achieved with the current satellite constellation.
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