How Starlink Actually Works: A Web Page Routed 550 km Up and Back in 26 ms

0:00Internet From the Sky: the phased-array dish

Your internet comes in through a cable. Fibre in the street, a wire into the house, a router. Every website you have ever loaded came through a cable. Starlink has none of that. A flat dish on the roof, and the internet comes down from the sky. How can a satellite 550 kilometres up, moving at 27,000 kilometres an hour, load a web page as fast as the cable did? This video follows one request from a dish to a server and back, and shows every hop. The house in this story is a farmhouse outside Munich. No fibre, no mobile signal. You open a web page that lives on a server in Frankfurt, about 300 kilometres away. One request, and three questions ride on it for the whole video. Which satellite takes it up. How does it get from the satellite down to the server with your page. And how long does the round trip take. Start with the dish. It does not move, and it does not face any one satellite. It is a flat panel of many small antennas, and it forms and steers its beam in software, with no moving parts. Above it, the satellites. Starlink says more than seven thousand eight hundred in orbit, about 550 kilometres up. At that height a satellite moves at about 27,000 kilometres an hour, once round the Earth every 95 minutes. The one over your roof is usable for a few minutes, then it is gone. So the dish is talking to a target that leaves in minutes, all day, and the page still loads. A dish that swings its beam from satellite to satellite, without moving. A satellite that leaves in minutes. One request to deliver.

2:11The Bent Pipe: satellite to ground station to PoP

The request leaves the dish on a beam up to the satellite overhead. The satellite does not aim back at your dish. It paints the ground with beams, each one lighting a patch about 20 kilometres across. Your farmhouse sits inside one patch. Every dish in that patch shares that beam. Which satellite takes it up: whichever one is lighting your patch right now. That is question one. The satellite does not read the request or decide where it goes. It turns it around and sends it straight back down to a Starlink ground station, a site of bigger dishes on the ground. Engineers call this shape the bent pipe: up, and straight back down. The ground station has to be within reach of that same satellite, so within about a thousand kilometres of your dish. For a farmhouse outside Munich, there are several. From the ground station the request goes into a fibre cable, like any other internet traffic, to a building Starlink calls a point of presence. For Germany that building is in Frankfurt. It is where Starlink's network plugs into the ordinary internet. So there is still a cable. It just starts at the ground station instead of your street. How does it reach the server with your page: down to a Starlink ground station, then by fibre to Frankfurt. Question two answered. From Frankfurt it is a normal internet request to the server, and the server sends your page back along the same path in reverse. Now the time. Light takes about two to four milliseconds to go from the dish up to the satellite, and the same again coming down. The whole trip through space, there and back, is usually under ten milliseconds. The fibre from the ground station to Frankfurt costs a few more. Up to the satellite. Down to a ground station. By fibre to Frankfurt. Under ten milliseconds in space.

4:32Who Picks the Satellite: handover

Back to the satellite over the farmhouse. It is moving away. After a few minutes it has slid down towards the horizon and the dish can no longer reach it. By then the next one has moved in high overhead. The dish swings its beam from the old satellite to the new one. The dish does not pick the new satellite. It points at whichever one is lighting its patch, every few minutes, all day. At any moment, more than one satellite is high enough to reach your patch, and each has only so many beams. So who decides which satellite lights the patch next?

5:18Starlink's Network Controller: the 15-second plan

Not the dish, and not the satellite. One system decides for the whole network: Starlink's own network controller. Starlink has not said where it runs, but it is one controller, not one per dish or per satellite. It sees every satellite, every ground station, every patch with dishes in it, and how busy each one is. Out of all that, it writes a plan. For every patch the plan says: this satellite lights you, on this beam, it sends your traffic down to this ground station, and you get this much bandwidth. Every 15 seconds the controller rewrites the plan, and every satellite and ground station switches to it together, on the same tick. The dish never sees the plan. Most ticks, your patch keeps the same satellite. The plan only changes which ground station it goes down to, or your share of the beam. Every few minutes, a tick hands your patch to a new satellite, and the dish re-points. In Germany, researchers found about nine in ten satellite changes landing exactly on a tick. How do we know there is one controller? Researchers ran two dishes at the same time, one in Edinburgh and one in Munich, too far apart to share a satellite. Their delay changed on exactly the same 15-second ticks. One clock for the whole network points to one controller behind it. Other researchers saw video calls drop in quality for a moment on those same ticks. Starlink measures itself on the same beat: every 15 seconds it collects latency from millions of routers. The dish points at whatever satellite lights its patch. The controller decides what lights it, and rewrites the plan every 15 seconds.

7:29Lasers, Then the Total: laser links and latency

The bent pipe needs a ground station within reach of the satellite. Over the farmhouse there are several. A ship in the middle of the Atlantic, or a base in Antarctica, has none. For those, the satellites talk to each other. Each one carries lasers, and it can pass the request sideways to the next satellite, and the next, until one of them has a ground station below. Then it goes down the normal way. Every hop between satellites adds a little time, so the plan uses the lasers only when it has to: when no ground station is in reach, or when the nearest one is overloaded. Straight down is the first choice. For the farmhouse there is a ground station in reach, so the request never touches a laser. Up, down, fibre to Frankfurt. Now the third question: how long does the round trip take. Count it. Up to the satellite, two to four milliseconds. Down to the ground station, the same. Fibre to Frankfurt, a few more. The server sends back the page, and it runs the whole path in reverse. Independent researchers measuring from Europe a couple of years ago got about 40 milliseconds. Starlink's own number, measured more recently across the US at the busiest hour of the day, is about 26. Either way it is in the range of a cable connection, and Starlink's stated goal is a steady 20. Satellite internet is not new. The older satellites sit 36,000 kilometres up, sixty-five times farther away, and their round trip was 600 milliseconds. The height is the whole difference. Where the 26 goes, by Starlink's own list: time in the air, the fibre from ground station to Frankfurt, waiting for a turn on a shared beam, and buffers. The plan works on the third one: how many dishes share each beam. In September, Starship carried up a batch of SpaceX's newest satellites, called V3. Each one carries over a terabit per second, and about ten times the beams of the ones before. One satellite can light far more patches, so the plan has more room to move. Which satellite: the one Starlink's controller puts on your patch, re-planned every 15 seconds. How it reaches the server: down to a ground station in reach, then fibre to Frankfurt. How long: tens of milliseconds, about 26 on Starlink's best count.